Vol. 41_ No. 4. Can. Plant Survey Sept. 1961 STRAwBERRY VIRUSES

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Vol. 41_ No. 4. Can. Plant Survey Sept. 1961 STRAwBERRY VIRUSES Powered By Docstoc
					  218                           .
                  Vol. 41, No. 4 Can. Plant                      Survey Sept. 1961

                                      STR A w BERRY V I R U S E S ~

                   F r a n c e s C.             and Randal E.

                                             Introduction

               The majority of s t r a w b e r r y v i r u s e s do not produce, on c o m m e r c i a l
v a r i e t i e s , symptoms that a r e sufficiently c h a r a c t e r i s t i c to allow positive
identification in the field.                                  to indicator plants is n e c e s s a r y
for the detection and identification of these v i r u s e s ,
               When, as often happens, m o r e than one v i r u s is present, the
separation of the components of the mixture can sometimes be achieved
by making use of differences in their vector relationships o r differences
in t h e i r stability when the plants a r e grown at high t e m p e r a t u r e s .
               Before entering upon a detailed description of the various v i r u s e s ,
it w i l l be useful to make some g e n e r a l comments on the indicators and
techniques we have 'used in o u r studies in B r i t i s h Columbia.

                                         INDICATOR PLANTS

         S e v e r a l s t r a i n s of the wild s t r a w b e r r y , F r a g a r i a vesca L., a r e used
a s indicators. No one of these s t r a i n s is adequate f o r the detection of all
the known s t r a w b e r r y v i r u s e s but, by using discrimination, a minimum
number of indicators may be selected for a complete indexing program,

                         East               clone of F r a g a r i a vesca (E M C)

          This clone was originally selected a t E a s t            England, because
of i t s sensitivity to mottle viruses. This sensitivity is now known to be due to
the presence of latent-A v i r u s with which the clone is infected. The E a s t
           clone is a good indicator f o r mottle viruses, for veinbanding, and
for              but it is l e s s sensitive to mild yellow-edge than s o m e of the
other indicators. It is a poor indicator for crinkle because latent-A is
apparently a s t r a i n of, and affords partial                       the crinkle
virus.

                                                          ve

               The        F r a g a r i a vesca is a r u n n e r l e s s s t r a i n grown f r o m seed,
 The seedlings a r e not entirely uniform in t h e i r reaction to any given v i r u s
 isolate. They a r e satisfactory indicators for mild yellow-edge and for s e v e r e
 s t r a i n s of mottle o r                   but a r e poor indicators for mild s t r a i n s of
\mottle and veinbanding. They a r e not sensitive to                                   virus.



   Contribution' No, 31 f r o m the Canada Agriculture R e s e a r c h Station,
   Vancouver, B. C.
           Pathologist and Director, respectively.
                         Vol. 41, No. 4. Can. Plant Dis. Survey Sept. 1961

                             F r a z i e r ' s runnerinn         seedling       C

                                                           --
                   This is a runnering s t r a i n of F. vesca, selected by N. W. F r a z i e r of
        Berkeley, California. This clone was developed f r o m an Alpine seedling and
                                                                               --
        it is a s s u m e d to be a hybrid of Alpine and E a s t Malling F. vesca. It is a
        good indicator for both crinkle and mild yellow edge. It is not sensitive to
        latent-C, and is l e s s sensitive to mottle and veinbanding than is the E a s t
        Malling clone of F. vesca.
I   .
                                  Miller' s virus- free                 vesca

                      This clone w a s selected by P a u l W, Miller at Oregon State College.
                                                                             --
        It probably a r o s e f r o m a seedling of the E a s t Malling F. vesca. It is f r e e
        of latent-A virus, and symptoms on this indicator a r e s i m i l a r to those on
        F r a z i e r ' s U C 1 clone.

                                        s latent- free F r a g a r i a vesca (E M K)

                     This is a clone developed by J.P. Fulton of Arkansas, f r o m a plant
        of the E a s t Malling clone which he had f r e e d of latent-A by heat treatment.
        Symptoms of mild yellow edge and latent-C on this indicator a r e               to
        those on the East Malling clone, Symptoms of mottle and veinbanding a r e
        s i m i l a r to those on          s U C 1 and M i l l e r ' s clone.

                                                TECHNIQUES

        Grafting
                  Runner grafting, described by H a r r i s and King                      h a s been widely
        u s e d for the experimental t r a n s m i s s i o n of s t r a w b e r r y viruses. If this
        technique is used in indexing c o m m e r c i a l stock, a non-grafted s i s t e r plant
        should be used for propagation because of the possibility of t r a n s m i s s i o n
        of a latent v i r u s f r o m an infected indicator.




                 In runner grafting, young stolons f r o m the donor and indicator plants
        a r e grafted together a s shown above. A slanting cut, about t h r e e q u a r t e r s of
        a n inch long, is made in each of the stolons. The cut is made toward the
        stolon tip of the donor plant, and toward the stolon base of the indicator plant.
        The cut s u r f a c e s a r e fitted together and bound with raffia, scotch tape, or,
        preferably, with a self- sealing e l a s t i c bandage.
  2 20              Vol. 41, No. 4. Can. Plant D i s . Survey Sept. 1961

        The leaf i n s e r t method of graft indexing, described by Bringhurst
and Voth       h a s s e v e r a l advantages: no r u n n e r s a r e r e q u i r e d so that
indexing can be done at any time of year; fewer donor plants a r e r e q u i r e d
for replicated indexing; and t h e r e is no danger of contaminating the donor
plant with a latent v i r u s f r o m the indicator.




              In leaf i n s e r t grafting, a leaf is detached f r o m the donor plant. The
side leaflets a r e removed and, i f the leaf is l a r g e , most of the center leaflet
cut off. The petiole is cut to wedge shape, half to t h r e e q u a r t e r s of an inch
long. On the indicator a center leaflet is                          f r o m a young leaf and
the petiole is s p l i t to correspond to the wedge on the donor leaf. The donor
leaf is i n s e r t e d snugly into this s p l i t and bound a s f o r runner grafting. A t
l e a s t two leaflets should be i n s e r t e d in e a c h indicator. Survival of the
i n s e r t e d leaves for three weeks o r              indicates successful graft union.
              It should be emphasized that                  of these techniques, even where
the union r e m a i n s sound for                weeks, will a s s u r e t r a n s m i s s i o n f r o m an
infected plant. Only repeated indexing w i l l give any a s s u r a n c e that a given
plant is virus - free.

Aphid t r a n s m i s s i o n
              The use of vectors in indexing is usually confined to those c a s e s where
the presence of a complex of v i r u s e s is suspected and it i s desirable to
s e p a r a t e the components.
              V i r u s e s that a r e aphid- transmissable may be s e p a r a t e d f r o m those
that a r e not by the use of aphids. F u r t h e r m o r e , v i r u s e s that differ in their
persistence may be separated f r o m mixtures by varying the acquisition and
t r a n s f e r feeding periods, and by making s e r i a l t r a n s f e r s over a period of
s e v e r a l days. Finally, certain aphid species may t r a n s m i t one v i r u s
efficiently than another and, by using selected vectors, individual v i r u s e s may
be separated f r o m a mixture.
              The m o s t important v e c t o r s of the s t r a w b e r r y v i r u s e s a r e m e m b e r s of the
genus Pentatrichopus. Because of the difficulty of distinguishing between species
of this genus, we have used clonal lines of aphids for m o s t of o u r vector work
in B r i t i s h Columbia.
              A colony of virus- free aphids is easily established if two facts a r e born
in mind: first that the young aphids do not acquire the v i r u s directly f r o m a
viruliferous mother; and second that the nymphs do not start to feed f o r
s e v e r a l hours a f t e r they a r e born. However, moving a newly born nymph
without killing it is difficult. Our method of obtaining non- viruliferous colonies
is as follows: a n adult is t r a n s f e r r e d to a detached leaf and observed at i n t e r -
v a l s of one o r two hours. A s each nymph i s born, the leaf piece on which it
                         41, No. 4. Can. Plant Dis. Survey Sept. 1961                           221

r e s t s is cut out and t r a n s f e r r e d to a f r e s h leaf f r o m virus- free plant. The
nymphs move off the wilting leaf pieces before they s t a r t to feed and so will
establish a clone that is certain to be virus- free.

Heat treatment
            A number of strawberry viruses can be elminated by heat treatment.
Mottle is the most heat-labile and is usually inactivated by growing infected
plants a t              for 2 weeks, a period which most varieties will withstand.
Other v i r u s e s a r e eliminated only a f t e r such prolonged treatment that survival
of the treated plant becomes uncertain,
             Strawberry varieties differ in their ability to survive heat treatment,
but in addition to inherent varietal differences, the condition of the plant, and
the way it is handled during treatment, also affect survival. Plants should
have well developed roots, even to the extent of being                        pot bound.
Humidity should be low; 40 to 50% is adequate. Soil moisture should be kept
to the minimum n e c e s s a r y to prevent wilting; overwatering, o r allowing the
pots to stand in water, will reduce survival. Evaporation from unprotected
clay pots may reduce soil temperature as much a s                      below the air
temperature, and attempts to c o r r e c t this by wrapping the pots in aluminum
foil o r plastic, o r by using plastic pots, hasten plant mortality. According
t o Posnette and                 (22) removal of older leaves f r o m the plants before
treatment also shortens the period plants w i l l survive.
             It may be that plants will survive longer at a fluctuating temperature than
at a constant temperature. The standard heat treatment is at a constant
temperature of 100              We have had considerable                 however, using a
heat chamber in which the daily temperature fluctuates from                    to
P l a n t s continue to grow actively in this chamber, and                                 for
3 o r 4 months.         One plant survived 8 months of treatment. W have succeeded
                                                                           e
in inactivating five viruses under these conditions. We have repeatedly
eliminated mottle and latent-A v i r u s e s and, in a few instances, crinkle,
witches' broom, and
             The length of treatment required for virus elimination can be reduced
by propagation of cuttings from treated plants. Posnette and Jha (24)
describe propagation by slicing the crown of                  treated plant into d i s c s 0.5
or 1             thick, and planting the d i s c s in sand o r peat. By this method they
developed plants f r e e of crinkle virus after only 2 o r 3 weeks treatment
whereas, in previous experiments, they had found a                       heat treatment
n e c e s s a r y for the inactivation of this virus. Unfortunately, heat treatment
decreased the proportion of cuttings that became established and, in one
t r i a l where 5 2 cuttings were made f r o m plants treated for 2 weeks, only 5
 survived.
             We use another method of propagation. Small axillary buds a r e
excised and rooted in sand under m i s t during, o r very shortly after, heat
treatment of the parent plants. By this method we have developed plants
f r e e of latent-A virus after 5 weeks treatment, whereas this v i r u s is not
eliminated from the parent plant by a 3-month heat treatment. Plants a
 year o r more old a r e the most suitable for this type of treatment. On such
 plants, axillary buds frequently develop on the older part of the crown and
 can be removed during treatment a s they r e a c h the desired stage of develop-
    222             Vol. 41, No. 4,              Plant          Survey Sept. 1961

                  The s m a l l e r the bud excised, the m o r e likely it is to be v i r u s - f r e e
    but buds weighing l e s s than 20                do not r o o t readily. F u r t h e r m o r e , the
    development of a t l e a s t a partially expanded leaf on the excised bud a p p e a r s
    to be n e c e s s a r y f o r survival. The length of heat t r e a t m e n t h a s no a d v e r s e
    effect on rooting. W e have established plants f r o m                     excised a f t e r
    t r e a t m e n t periods of 4 months.
                  The apparent r e c o v e r y of plants during heat t r e a t m e n t may be
    deceptive. P l a n t s of the m o r e heat- tolerant varieties, t r e a t e d under optimum
    conditions, will continue to grow during treatment. On plants infected with
    v i r u s e s which cause symptoms, new leaves formed during t r e a t m e n t may be

I
    symptomless so that, with the death and removal of older leaves, the t r e a t e d
    plants appear normal. After treatment, the re- appearance of symptoms may                                  .
    take much longer than expected on those plants in which the v i r u s was almost,
    but not quite, inactivated. P l a n t s developed f r o m excised buds may r e m a i n
    symptomless even longer than the parent plants, and the s m a l l e r the excised
              the longer the new plant w i l l r e m a i n symptomless.
                  Posnette and               (22) r e p o r t ,that some plants infected w i t h yellow
    edge virus remained symptomless for m o r e than a year a f t e r treatment., and
    then symptoms reappeared,
                  We have found a s i m i l a r , although l e s s extreme, delay in the r e -
    appearance of symptoms. In studying the elimination of latent-A v i r u s
    by heat therapy, we used t e s t plants infected with veinbanding and latent-A
    v i r u s e s , a combination which c a u s e s s e v e r e symptoms on c o m m e r c i a l
    varieties, and f r o m which elimination of latent-A is indicated by loss of
    symptoms f r o m the treated plant. Some of the plants developed f r o m
    excised buds appeared n o r m a l a s long a s 1 2 weeks before symptoms again
    appeared. All the plants that appeared n o r m a l a t the end of that time,
    however, were still n o r m a l 3 y e a r s l a t e r , and it is a s s u m e d that latent-A
    was completely eliminated f r o m these,
                  The reappearance of crinkle symptoms was s i m i l a r l y                       on
    plants developed f r o m excised buds. One plant, developed f r o m an axillary
    bud, appeared n o r m a l for 4 months before crinkle symptoms reappeared,
    although the parent plant f r o m which this bud was taken again showed symptoms
    3 weeks a f t e r treatment.
                  W i t h such experience it follows that, when latent v i r u s e s a r e involved,
    r e s u l t s should be interpreted with a g r e a t deal of caution. Indexing should be
    repeated over a long period of time before any t r e a t e d plant is pronounced
    v i r u s - free.

                    NOTE ON SOIL-BORNE VIRUSES INFECTING STRAWBERRY

                   A number of soil-borne viruses, whose principal hosts a r e other plants,
    have been shown to infect s t r a w b e r r y in Britain and Europe. With the exception
    of tobacco n e c r o s i s virus (6 and 11) t h e r e a r e no s i m i l a r r e p o r t s f r o m North
    A m e r i c a , although the extent of survey has admittedly been limited.
                   Summarizing the situation in Britain, L i s t e r (15) r e p o r t s that all the
    i s o l a t e s of soil- borne v i r u s e s infecting s t r a w b e r r y that were collected in
    England have proved to be s t r a i n s of a r a b i s mosaic virus, whereas a l m o s t
                    Vol. 41, No. 4. Can. P l a n t Dis. Survey Sept. 1961.                                    223

     a l l collected in Scotland have been s t r a i n s of tomato black r i n g and r a s p b e r r y
                 viruses.
               The soil- borne v i r u s e s a r e not discussed in this review.


                                     MOTTLE VIRUS, Thomas

              What is described h e r e as s t r a w b e r r y mottle v i r u s is evidently a
     group of v i r u s e s o r virus s t r a i n s which have s i m i l a r vector c h a r a c t e r i s t i c s
     and s i m i l a r r e s p o n s e s to heat therapy but which produce a very wide range
     of symptoms on F r a g a r i a vesca.

     SYNONYMS: Virus I (mild crinkle), P r e n t i c e and H a r r i s (28)
               Type I, Demaree and Marcus (3)
                                 component of yellows", Mellor
                    and Fitzpatrick (18)

     GEOGRAPHIC DISTRIBUTION:                      World wide.

     SYMPTOMS: On c o m m e r c i a l v a r i e t i e s there a r e no reliable symptoms but
        t h e r e is          evidence that some s t r a i n s d e p r e s s vigor.
                On F r a g a r i a vesca symptoms range f r o m an a l m o s t undetectable

                          .
        mild mottling to s e v e r e stunting accompanied by various types of leaf
        distortions
                The first symptoms appear on the youngest leaf 10 to 20 days a f t e r
        inoculation. The petiole of this leaf is short, and                         two leaflets
        are                 and s m a l l e r than normal. These leaflets become mottled,
        crinkled, cupped, o r otherwise distorted depending on the s t r a i n of the
        v i r u s present. Subsequent leaves show the symptoms that a r e c h a r a c t e r -
        i s t i c of that particular strain, although there may be minor fluctuations
        t h e i r intensity depending on growing conditions and season of the year.

     COMPLEXES WITH OTHER VIRUSES:
               Mottle v i r u s e s combined with mild yellow edge v i r u s produce xanthosis
       o r yellows in susceptible c o m m e r c i a l v a r i e t i e s and in F. vesca.--
               Mottle and latent-A v i r u s e s combined cause no diagnostic symptoms in
       c o m m e r c i a l varieties, but         is depressed even f u r t h e r than by mottle
       alone
              Similarly, mottle and veinbanding combined cause no symptoms on the
       c o m m e r c i a l varieties although again there is a g r e a t e r depression of vigor
                                                         --
       than with either virus alone. In F. vesca, the mottle symptoms usually
       m a s k the veinbanding although the combined effect is g r e a t e r than that of
       e i t h e r virus alone,

     TRANSMISSION is mainly by aphids of the genus Pentatrichopus: P. fragaefolii
        (Cock.), P. thomasi R i s Lambers, and P. thomasi s s p .          R i s Lambers
        a t the       Coast; and P. minor          in E a s t e r n North America.




I,
2 24               Vol. 41, No. 4. Can. P l a n t Dis. Survey Sept, 1961

              Aphids can acquire the v i r u s f r o m an infected plant within a n hour but
       vector efficiency i n c r e a s e s with longer feeding periods. The aphids usually
       lose their v i r u s charge within 6 hours of leaving the source plant. T h e r e
       a r e differences in the efficiency of clonal                    aphids, and the milder
       s t r a i n s of the v i r u s a r e sometimes difficult t o transmit.
              Other aphids that have been r e p o r t e d a s vectors of mottle           are:
       Acyrthosiphon malvae ssp.                       (Theob.), Amphorophora            (Kalt.),
       Aphis gossypii Glover, Macrosiphum pelargonii (Kalt.),
                    ),          ascalonicus Doncaster, M. ornatus Laing,
       Sander son, and Pentatrichopus                           (Walk.).
HEAT INACTIVATION:
      Mottle v i r u s e s can usually be eliminated by growing infected plants at
                        4
        for 10 to 1 days.

DETECTION A N D IDENTIFICATION:
       The E a s t         s t r a i n of F r a g a r i a v e s c a (E  is the m o s t sensitive
   indicator f o r the mottle viruses.         This s t r a i n is        with s t r a w b e r r y
   latent-A v i r u s                       the symptoms of mottle.
       Where the presence of other v i r u s e s is suspected, the plants should be
   heat- treated to eliminate the mottle v i r u s e s and             the                       of
   any accompanying viruses.

REFERENCES: 3,               16, 18,

ILLUSTRATIONS:
      Fig. 1. P r i m a r y symptoms of mottle in E a s t Malling F. v e s c a
            .
      Fig. 2 Chronic symptoms of mild mottle in E a s t                F. vesca
      Fig. 3. Chronic symptoms of moderately s e v e r e mottle
              Malling F. vesca --
      Fig. 4. Chronic symptoms of s e v e r e mottle in E a s t Malling F. vesca
                             ( s e e also Figs. 5 and 6)
                                                                                     --

           STRAWBERRY LATENT-A VIRUS, F r a z i e r and Posnette

SYNONYMS: Strawberry latent virus, s t r a i n A., F r a z i e r
   Note: According to F r a e i e r and Posnette (9) this v i r u s affords partial
   protection against the s t r a w b e r r y crinkle virus'es and consequently can
   be considered a s a s t r a i n of these. However, because of its importance,
   it is t r e a t e d h e r e as a s e p a r a t e virus.

GEOGRAPHIC DISTRIBUTION:
          Little is known of the n a t u r a l distribution of s t r a w b e r r y latent-A virus.
  It w a s originally found by F r a e i e r in the s t r a i n of E a s t Malling F r a g a r i a
   v e s c a that P r e n t i c e and H a r r i s had selected for its unusual sensitivity t o
  the mottle viruses. F r a z i e r also found a s i m i l a r v i r u s in a number of
   clones of          californica growing in widely separated localities in the Coast
   Range mountains of California, and in a number of n u r s e r y colonies of
   apparently n o r m a l plants of the Marshall variety.
    .
    -

I-'
 Vol. 41, No. 4. Can. Plant   Survey Sept, 1961   225
                           .
              Vol. 41, No. 4 Can, Plant                      Survey Sept. 1961                         227

         Wherever e l s e it has been found, it is assumed to have been introduced
     into stock that has been indexed by runner grafting to E a s t        F. vesca.              --
SYMPTOMS: Strawberry latent-A v i r u s c a u s e s no symptoms, either on com-
   m e r c i a l v a r i e t i e s o r on vesca. Its presence is detected by its influence
   on the symptoms of                     o r veinbanding viruses.

COMPLEXES WITH OTHER VIRUSES:
         Latent-A and mottle v i r u s e s                       d e p r e s s the vigor of c o m m e r c i a l
                                                                      --
  v a r i e t i e s but cause no tangible symptoms. In F. vesca, latent-A i n c r e a s e s
  the s e v e r i t y of the symptoms of any particular mottle s t r a i n to those of a
  m o r e s e v e r e strain.
          Latent-A and veinbanding combined cause s e v e r e symptoms on com-
                                        --
  m e r c i a l v a r i e t i e s and on F. v e s c a ( s e e veinbanding virus).

TRANSMISSION is by grafting only; no vector has been found.

HEAT INACTIVATION:
          S t r a w b e r r y latent-A v i r u s will survive plants heat- treated for 3
   months. It can be eliminated, however, by propagation of                          buds
   excised f r o m heat- treated plants. We have developed plants f r e e of this
   v i r u s f r o m buds weighing up to 85 mg., excised a f t e r 5-weeks treatment,
   and f r o m l a r g e r buds a f t e r longer treatment. Virus- free buds were ex-
   c i s e d up to 3 weeks after the end of the treatment.

DETECTION AND IDENTIFICATION:
         S t r a w b e r r y latent-A v i r u s can be most easily detected by grafting to
  an indicator that               c a r r y i n g veinbanding virus. The complex of the two
  v i r u s e s w i l l produce the leaf distortion shown in Figs.          and 10.

REFERENCES: 4,                  16, 19.

   LUSTRATIONS:
       Fig. 5. M i l l e r ' s latent- free F. vesca with mild
       Fig. 6. M i l l e r ' s latent- free       vesca with mild mottle and
                   latent -A
                                   (see a l s o Figs. 8,   and 10)
41, No. 4. Can. P l a n t Dis. S u r v e y Sept. 1961   229
                   41, No. 4. Can. P l a n t Dis. Survey Sept. 1961                        23 1

                          VEINBANDING VIRUS. F r a z i e r

SYNONYMS: None, but the complex of a s t r a i n of veinbanding and s t r a w b e r r y
   latent-A v i r u s in E a s t                             vesca w a s by P r e n t i c e
   a s v i r u s 5, o r s t r a w b e r r y leaf c u r l virus (27).

GEOGRAPHIC DISTRIBUTION:
      O c c u r s on both the Pacific and Atlantic Coasts of North America.
  It does not appear to occur naturally in G r e a t Britain (Prentice' s isolate
   was f r o m a plant imported to England f r o m the U.S.A.).

SYMPTOMS: On c o m m e r c i a l varieties: T h e r e a r e no diagnostic symptoms
   although some s t r a i n s may d e p r e s s vigor slightly.
       On F r a g a r i a vesca: Symptoms appear 4 to 6 weeks a f t e r inoculation
  a s discontinuous chlorotic s t r e a k s along the m i d r i b s and some secondary
   veins. Vigor is moderately reduced, The clarity of the veinbanding
   symptom fluctuates greatly and, particularly following transplanting o r
   application of f e r t i l i z e r , infected plants sometimes produce a s e r i e s of
   symptomless leaves.

COMPLEXES WITH OTHER VIRUSES:
         Veinbanding and latent-A v i r u s e s combined cause s e v e r e symptoms
  both on c o m m e r c i a l v a r i e t i e s and on the indicators. On
  v a r i e t i e s the leaves a r e twisted due to the shortening of portions of the
  veins. Chlorosis o r purpling develops along the veins and d a r k purple
  lesions appear on the petioles. Vigor is much reduced. On F, vesca,         --
  the symptoms a r e s i m i l a r but t h e r e is l e s s purpling of the veins and
  m o r e twisting of the leaves.

TRANSMISSION is by s e v e r a l aphid species representing a number of differ-
   e n t genera, Vectors r e p o r t e d to date a r e :
   Aulacorthum solani (Kalt.),
   pelargonii (Kalt.), M. r o s a e
   o r n a t u s Laing,               (Sulz.), Pentatrichopus fragaefolii   ),
   P. tetrarhodus              P. thomasi Ris Lambers, and P. thomasi'spp.
               Ris

        Virus- vector relationships a r e s i m i l a r to those of the mottle viruses.
The aphids can acquire the virus f r o m a source plant in 30 minutes but
efficiency i n c r e a s e s with longer feeds. P e r s i s t e n c e in the vector is relatively
short, being usually l e s s than 6 hours. There a r e differences in the efficiency
of clonal lines of aphids, and there is evidence that some aphid s p e c i e s w i l l
          some s t r a i n s of veinbanding but not o t h e r s (19).




     vector r e f e r r e d t o by Mellor and F o r b e s (19) a s Aphis rubifolii (Thomas)
 has since been identified a s Aphis idaei van d e r Goot.
232                        .
             Vol. 41, No. 4 Can. Plant Dis. Survey Sept.

HEAT INACTIVATION:
        Veinbanding is one of the m o r e heat- stable v i r u s e s and its inactivation
  by heat has not previously been reported. We have found that it survived
  in plants t r e a t e d for periods up to 8 months.          have one plant, however,
  f r o m which veinbanding has been eliminated. This plant a r o s e f r o m an
  axillary bud a f t e r the parent plant w a s apparently killed by a 6-month heat
  treatment. Indexing this               18 months a f t e r the end of the t r e a t m e n t
  still failed to demonstrate the presence of the virus,

DETECTION AND IDENTJFICA
          Since it is difficult to detect the                             in the presence of
   s t r a w b e r r y mottle virus, plants to be indexed should first be heat- treated
   to remove the mottle viruses. The presence of veinbanding can then be
   demonstrated by t r a n s m i s s i o n to indicators c a r r y i n g latent-A virus, o r
   to Alpine - -    F. vesca.

REFERENCES: 5) 7, 9, 16, 19.

ILLUSTRATIONS:
      Fig. 7. Veinbanding in Alpine F. vesca
      Fig.     Veinbanding in                            --
                                           latent- free F. vesca
      Fig. 9. Veinbanding in E a s t Malling           --
                                                    vesca showing the
                  c u r l and twisting of the leaflets c h a r a c t e r i s t i c of the
                  veinbanding-latent A complex.
      Fig. 10. Veinbanding-latent A complex in


                  MILD YELLOW E D G E VIRUS, Prentice

SYNONYMS: Virus 2, P r e n t i c e (25)
           P e r s i s t e n t component of                           and
          Fitzpatrick (18)

GEOGRAPHIC DISTRIBUTION:                Probably world wide.

SYMPTOMS: On c o m m e r c i a l varieties: Because of the ubiquity of the s t r a w -
   b e r r y mottle viruses, mild yellow edge seldom o c c u r s alone under field
   conditions. It is virtually symptomless alone, causing little reduction
   in vigor and a t most v e r y slight chlorosis, mainly a t the m a r g i n s of the
   leaves.
          On F r a g a r i a vesca: Symptoms appear on plants of                 U C 1
   clone o r on Alpine seedlings 4 to 6 weeks after inoculation, but may take
   much longer to appear on plants of the E a s t Malling clone. Symptoms
   vary, depending on the s t r a i n of the indicator and on the s t r a i n of the
   virus. P l a n t s of the E a s t Malling clone a r e usually reduced in vigor
  with marginal chlorosis and slight cupping of the leaves, However,
  these symptoms a r e                     difficult to distinguish, even with
  healthy controls' for comparison, Plants of                     s U   1 clone, o r
  Alpine seedlings, a r e              indicators for this virus. Symptoms on
Vol. 41, No. 4. Can. Plant D i s . Survey Sept. 1961   233
           Vol. 41, No. 4. Can. Plant Dis. Survey Sept. 1961                   235




   these indicators consist of premature reddening and yellowing of the
   older leaves, followed by scorching and their death.

              WITH OTHER VIRUSES:
       Mild yellow edge and mottle viruses combined cause xanthosis or
                                                                .
   yellows in susceptible commercial varieties and in F vesca. The
   economic importance of this disease in Europe and                     North
   America, and its l e s s e r importance in E a s t e r n North America a r e
   probably due to differences in the susceptibility of the different varieties
   grown in these a r e a s r a t h e r than to the geographic distribution of the
   virus.

TRANSMISSION:
       By aphids of the genus Pentatrichopus: P. fragaefolii, P. thomasi,
   and P. thomasi s s p .
               require acquisition and transfer feeding periods of 1 to 2 days
   each for transmission of this virus, and remain infective for 10 to 12
   days after leaving the source plant.

HEAT INACTIVATION:
       Mild yellow edge is not readily inactivated by heat therapy. Posnette
   and           (22) r e p o r t that plants were not cured by treatment a t
             for periods up to 26 days and although some plants remained
   symptomless for m o r e than a year, they eventually relapsed. They did,
   however, obtain one runner plant, propagated immediately after treatment
   of the parent plant for 16 days, which was apparently virus- free two y e a r s
   later.
                                                                           .
                                                                           -
                                                                           .
                                                                           I
                                                                                                            -
                                                                                                            -


236             Vol. 41, No. 4.               Plant Dis. Survey                1961

              We have not eliminated this virus.           Axillary buds, removed a f t e r
      t r e a t m e n t of the parent plant a t             for periods up to 3 months,
      w e r e a l l infected.

DETECTION AND IDENTIFICATION:
         Mild yellow edge can be separated f r o m the xanthosis complex by
  allowing aphids to feed on source plants for s e v e r a l days, and then
  t r a n s f e r r i n g them t o a s e r i e s of plants at daily intervals. The first
   plant o r plants in the s e r i e s w i l l become infected w i t h any non- persistent
   viruses                 mottle and veinbanding) that may be present, whereas only
  mild yellow edge will be transmitted to plants further along the s e r i e s .
         The best diagnostic c h a r a c t e r s a r e probably the progressive redden-
  ing, yellowing, scorching, and dying of the older leaves on                            s
  U         1 clone o r on Alpine seedlings, and the ability to produce typical
  xanthosis when combined with mottle virus in a susceptible' variety such
  a s Marshall.

REFERENCES: 3, 9, 16, 18, 19, 22, 25.

ILLUSTRATIONS:
      Fig. 11.                          1 F. vesca with       yellow edge, showing
                           the                scorching and dying of older leaves.
          Fig. 12.     Right: E a s t Malling F. vesca with mild yellow edge
                       Left: E a s t Malling      --
                                                vesca, control plant.


                        CRINKLE ,VIRUS Zeller and

SYNONYMS: F r a g a r i a virus 2, Zeller and Vaughan
          Virus 3, Prentice (26)
          S t r a w b e r r y virus 4, J. Johnson
          Marmor fragariae,

GEOGRAPHIC DISTRIBUTION: Pacific coast of North America, and G r e a t
   Britain.

SYMPTOMS:               c o m m e r c i a l varieties: The m o r e s e v e r e s t r a i n s of crinkle
   v i r u s seriously reduce the vigor of plants of susceptible v a r i e t i e s such a s
   Marshall. Yellow spots appear on the leaves, varying in                                   f r o m pin-
   points to l a r g e chlorotic a r e a s which cause leaf distortion. Leaflets a r e
   unequal in size and there is marked marginal                                 of the distorted
   leaves. Very mild s t r a i n s of the v i r u s may cause only s m a l l chlorotic
   s e c t o r s and, occasionally, unequal size of leaflets. On v a r i e t i e s l e s s
   susceptible than Marshall, symptoms may be so mild that indexing is
   r e q u i r e d for diagnosis,
          On F r a g a r i a vesca: On plants of                   U C 1 clone o r on Alpine
   seedlings, symptoms appear about 4 weeks a f t e r graft inoculation.
   plants of the E a s t                    clone, symptoms a r e much slower to develop,
Vol. 41, No. 4. Can. Plant Dis. Survey Sept. 1961   237
                                             i.
                 Yol. 41, No. 4. Can. Plant D s Survey Sept. 1961                                     239

    presumably because the latent-A v i r u s affords p a r t i a l protection against
    infection by the crinkle virus. Symptoms a r e s i m i l a r on the 3 indicators.
    Pinpoint chlorotic spots appear which may, in s e v e r e c a s e s , cause leaf
    distortion. Lesions frequently appear        petioles and stolons, s o m e t i m e s
    causing, the angular bending shown in Fig. 15. Very mild s t r a i n s may
    cause only petiole lesions       occasionally, the backward bending of a
    center

COMPLEXES WITH OTHER VIRUSES:
      Crinkle and mottle v i r u s e s together seriously reduce the vigor of
  plants of the Marshall variety; crinkle, mottle, and mild yellow edge

                                                          .
  together cause v e r y s e v e r e degeneration; and if latent-A is a l s o p r e s e n t
  the degeneration is even
  even reduce the vigor and
                                        extreme These latter complexes will
                                               of v a r i e t i e s such as Northwest,
  which a r e highly tolerant of the mottle-mild yellow edge complex, and
  will cause speckling and mild chlorosis.

TRANSMISSION:
          The virus- vector relationships of the crinkle v i r u s and its s t r a i n are
   not c l e a r l y understood. Vaughan                    and Prentice (26) r e p o r t t r a n s -
   mission of crinkle by the s t r a w b e r r y aphid without any particular diffi-
   culty. More r e c e n t attempts by F r a z i e r and Posnette in England
   and               and F o r b e s in B r i t i s h Columbia          have m e t with indifferent
   s u c c e s s o r downright failure. Naturally infective aphids, taken f r o m
   diseased plants in the field, transmitted the v i r u s                   but no t r a n s m i s s i o n s

                      .
   were obtained by aphids r e a r e d in the laboratory and
   experiments
                                                                                   in controlled

          The m o s t that can be said at the p r e s e n t time is that the s t r a w b e r r y
   aphids, Pentatrichopus
                                               .       certainly, and               P. t h o m a s i
   also, a r e the principal v e c t o r s According to P r e n t i c e and-Woollcombe
   (29) the aphids can acquire the virus within 24 hours, but an incubation
   period of 1 2 to 16 days is r e q u i r e d before they can t r a n s m i t . The i n s e c t s
   r e t a i n their ability to t r a n s m i t for s e v e r a l days.

HEAT INACTIVATION:
       Posnette and               (22) r e p o r t elimination of crinkle v i r u s by t r e a t -
  ments at                      for 50 days. They found some s t r a i n s e a s i e r to
  inactivate than others. Posnette and Jha (24) developed crinkle- free
   plants f r o m s t e m cuttings taken a f t e r 2 o r 3 weeks heat t r e a t m e n t of the
   parent plant. We have developed a crinkle- free plant f r o m an
  bud excised f r o m an infected plant a f t e r 8-weeks heat treatment. The
   parent plant, however, remained infected.

DETECTION AND IDENTIFICATION:
         On c o m m e r c i a l varieties crinkle symptoms may be confused with other
  d i s o r d e r s . F o r example, the complex of veinbanding and latent-A v i r u s e s ,
  o r infestations of the shallot aphid (Myzus ascalonicus) o r of two-spotted
  mite, produce symptoms that may be confused with crinkle. However, i f
  the symptoms a r e caused by the veinbanding-latent-A complex, and no
240          Vol. 41, No. 4. Can. Plant Dis. Survey Sept. 1961

      crinkle is involved, grafting to F. vesca will give the distinctive symptoms
      of this complex (see                         If the trouble is from the shallot
      aphid, close examination early in the season w i l l reveal the aphids, and the
      plants w i l l recover completely a s new leaves a r e formed during the summer.
      If symptoms a r e due to mite infestation, closer examination w i l l reveal the
      mites, and leaves formed after elimination of the mites will be normal.
          On F. vesca the leaf symptoms of crinkle virus a r e nearly indistinguish-
      able from those caused by some strains of mottle virus on E a s t Malling F.
      vesca. This was the source of much of the early confusion between
              crinkle" (i.e. mottle) and bona fide crinkle. \Mottle, however, does
      not cause the petiole and stolon lesions.



ILLUSTRATIONS:
      F i g . 13, Marshall with severe crinkle.
      Fig. 14. F r a z i e r ' s U C 1 with crinkle, showing leaf symptoms and
                     petiole lesions.
      Fig. 15.                   U C 1 with crinkle, showing lesions which cause
                     sharp bending of the stolon.


                                   VIRUS,            (16)

SYNONYMS: The complex of latent-C and latent-A viruses in E a s t Malling
   F r a g a r i a vesca was described by Demaree and Marcus (3) a s  2
   symptoms       .
GEOGRAPHIC DISTRIBUTION: Eastern North America.

SYMPTOMS: On commercial varieties latent-C virus causes no symptoms,
   but it decreases vigor of a t least some varieties.
         On Fragaria vesca      the E a s t Malling clone,       s latent-free
   clone, and on some seedlings of the E a s t Malling clone, latent-C causes
   a shock symptom consisting of severe epinasty of the young leaves. This
   is followed by a proliferation of crowns with very small leaves. On Alpine
      --
   F. vesca,               s U   1 clone, Miller's latent-free clone, and on some
   seedlings of the E a s t Malling clone, latent-C causes no symptoms, but it
   de c r e a s e s vigor.

                   is by graft only; no vector has been reported.

HEAT INACTIVATION:
        A l l attempts to free plants of this virus by heat therapy have so far
   failed.
Vol. 41, No. 4. Can. Plant Dis. Survey Sept. 1961   24 1
Val, 41, No. 4, Can. Plant Dis. Survey Sept. 1961   24 3
                  Vol.           No. 4.                Plant              S u r v e y Sept. 1961                         245

         C           AND                        A TION:
             After                               e l i m i n a t e d by heat t h e r a p y , l a t e n t - C          be
                     by graft- inoculation of E a s t                               F. v e s c a .    On this i n d i c a t o r
                     s y m p t o m s could only be confused                             t h o s e of a c o m p l e x of
     w i t c h e s ' b r o o m a n d veinbanding. However the witches' b r o o m v i r u s ,
              or without veinbanding, causes c r o w n p r o l i f e r a t i o n on c o m m e r c i a l
     Varieties a n d on all the                              i n d i c a t o r s p e c i e s , while l a t e n t - C d o e s
     not c a u s e c r o w n p r o l i f e r a t i o n on c o m m e r c i a l v a r i e t i e s , on Alpine, on
     M i l l e r ' s l a t e n t - f r e e clone, o r on                        s U C 1.




ILLUSTRATIONS:
      F i g . 16. P r i m a r y s y m p t o m s of latent- C i n East Malling F. v e s c a .
      Fig. 17. C h r o n i c s y m p t o m s of latent- C i n East              vesca.                   --
                               WITCHES' BROOM 'VIRUS, Z e l l e r (32)

SYNONYMS: S t r a w b e r r y v i r u s 2, J. Johnson
          F r a g a r i a v i r u s 3, Z e l l e r
          Blastogenus f r a g a r                     y
          Nanus                        Holme s

GEOGRAPHIC DISTRIBUTION: N o r t h A m e r i c a

SYMPTOMS: On                                 v a r i e t i e s : P l a n t s a r e dwarfed,                in
  a p p e a r a n c e , with m u l t i b r a n c h e d c r o w n s , and e r e c t , spindly p e t i o l e s
   supporting                leaves,
         On F r a g a r i a v e s c a s y m p t o m s a r e                    t o t h o s e on t h e commercial
  v a r ie t i e s

TRANSMISSION:
          Z e l l e r ( 3 2 ) r e p o r t e d t r a n s m i s s i o n by the s t r a w b e r r y aphid,
   P e n t a t r i c h o p u s sp. Mellor a n d F o r b e s (19) h a v e been unable to obtain
   t r a n s m i s s i o n with either P. f r a g a e f o l i i o r P. t h o m a s i .

H E A T INACTIVATION:
           We have eliminated this v i r u s from f e w p l a n t s of the B r i t i s h
     S o v e r e i g n v a r i e t y by growing them at 95-115" F for 8 o r 10 weeks.

D E T E C T I O N AND IDENTIFICATION:
            On c o m m e r c i a l v a r i e t i e s the f i e l d s y m p t o m s are d i s t i n c t i v e a n d there
     is u s u a l l y n o difficulty in identifying the d i s e a s e .
                 - -
             On F. v e s c a the s y m p t o m s a r e similar to c h r o n i c s y m p t o m s of l a t e n t - C
     v i r u s . However, l a t e n t - C d o e s n o t produce witches' b r o o m in c o m m e r c i a l
     v a r i e t i e s . Since, in e x p e r i m e n t s with P r e m i e r known t o be c a r r y i n g l a t e n t -
     C, t h e r e w a s       protection a g a i n s t witches' b r o o m , it                         be a s s u m e d t h a t
     the two v i r u s e s a r e n o t r e l a t e d .
.
    I
.




Vol. 41, No.   Can. P l a n t Dis. Survey Sept. 1961   24 7
                  Vol. 41, No. 4. Can. P l a n t Dis. Survey Sept. 1961
                                                                                                            ,


    ILLUSTRATIONS:
          Fig. 18.                    broom in              Sovereign
          Fig. 19,                    broom in E a s t
                                                                   --
                                                                 F. v e s c a


                             ASTER YELLOWS VIRUS,

    SYNONYMS: Western a s t e r yellows virus
              Callistephus v i r u s
              Chlorogenus                 var. californicus, Holme s.

    GEOGRAPHIC DISTRIBUTION: California,

    SYMPTOMS: Phyllody of the flowers of infected s t r a w b e r r y plants is s i m i l a r
      to the symptom typical of this v i r u s on other plants. P l a n t s eventually die.

    TRANSMISSION is by                         s fascifrons, Colladonus
       geminatus, and C. montanus) but with considerable difficulty.

    HEAT INACTIVATION:                  information.

    DETECTION AND IDENTIFICATION:                    See symptoms.

    REFERENCES:          9, 10, 12.


                        GREEN PETAL VIRUS, Posnette

    SYNONYMS: None. F r a z i e r and Posnette (8) state that variations in symptoms
       on s t r a w b e r r y plants suggest that two d i s e a s e s may be grouped under the
       name " g r e e n petal." They distinguish these as (a) green petal caused by
       the v i r u s inducing phyllody in clover, and (b) bronze leaf w i l t caused by
       the clover witches' broom virus. They a l s o suggest that green petal v i r u s
       may be r e l a t e d t o a s t e r yellows virus.


.   GEOGRAPHIC DISTRIBUTION: England, E a s t e r n Canada.

    SYMPTOMS: The flower symptoms a r e the m o s t c h a r a c t e r i s t i c of this disease,
       and these may appear while the foliage still r e m a i n s normal. Sepals a r e
       enlarged; petals dwarfed and pale green,                 flowers a r e s t e r i l e ; o t h e r s
       f o r m a small, hard, green receptacle which fails t o ripen and f r o m which
       the achenes stand out, appearing unusually large. Dried inflorescences a r e
       a useful symptom in field diagnosis.
              Leaves formed' a f t e r infection a r e dwarfed, slightly cupped, with main
       veins and margins yellow, The old leaves first turn dull yellowish o r olive
       green and then bright r e d in August and September. The whole plant may
       collapse and die in                      o r there may be t e m p o r a r y r e c o v e r y in
               Vol. 41, No. 4. Can. P l a n t          Survey Sept. 1961

    September with the formation of secondary crowns with minute leaves.
        stolons a r e s h o r t and thickened, and the young runner plants e x t r e m e l y
    dwarfed.

TRANSMISSION is by graft; not by the s t r a w b e r r y aphid. The v i r u s h a s been
   t r a n s m i t t e d f r o m s t r a w b e r r y to clover by leafhoppers.

        INACTIVATION:          No information.

DETECTION AND IDENTIFICATION: See symptoms.

REFERENCES:         2, 8, 9, 13, 21.

ILLUSTRATIONS:
      Fig. 20. Flower symptoms of g r e e n petal v i r u s in c o n t r a s t to n o r m a l
                       flowers, on field plants of the variety Senator
      Fig. 21. Flower                of g r e e n petal virus.
      Fig. 22, F r u i t symptoms of green petal virus.
 ..
..    .   .   . .




                    Vol. 41, No. 4. Can. Plant Dis. Survey Sept. 1961   251
                 Vol. 41, No. 4.              Plant         Survey Sept. 1941                     253

                                               O F STRAWBERRY VIRUSES

                                                                                   Inactivation
    Virus           Symptoms                                  Vectors                by heat
                    Commercial             F. vesca clones
                    varieties                Alpine UC 1 E M K
    Mottle                            S         S       S     S    aphids N P           A
    Latent-A                                                   -   unknown              B
    Ve inban ding                               S             S    aphids N P           C
    Mild yellow edge                            S       S     S                         C
    C r inkle                                   S       S          aphids P             B
    Latent-C                                                   S   unknown              D
    Witches' broom                              S       S      S   aphids               B
    A s t e r yellows                                                afhoppe r s
    Green petal                                                    leafhoppers
    Key to Symbols
    Symptoms: S indicates diagnostic symptoms;               - indicates no diagnostic symptoms
    Vectors:      N P non- persistent in vector;            P persistent in vector
    Inactivation by heat: A readily inactivated.
                          B inactivated only by prolonged heat treatment, o r by
                            taking cuttings f r o m heat t r e a t e d plants.
                          C inactivation in a single instance,
                            ,has not yet been inactivated.

                                             Summary

                 The detection and identification of latent v i r u s e s in                  straw-
    b e r r y plants usually r e q u i r e s t h r e e steps: preliminary indexing to determine
    whether the mottle v i r u s e s a r e present, heat t r e a t m e n t to eliminate mottle,
    and re-indexing the heat- treated plants to detect any heat- stable v i r u s e s . F o r
    the re-indexing, t h r e e indicators a r e desirable:' E a s t               F. v e s c a to show
    veinbanding o r latent-C (or mottle), U C 1 o r Alpine to show                   yellow edge,
    and one of the latent- free F. v e s c a clones infected with veinbanding, to show
    latent-A. When the r e s u l t s of indexing a r e negative the ' t e s t s           be repeated
    s e v e r a l t i m e s before any plant is pronounced virus- free.
.                                         A ckn ow     men t s
             Most of the information given in this review is either f r o m published
    data o r f r o m our own experience. Much of the information on crinkle virus,
    however, was supplied by D r . C.D. Schwartze of the Western Washington
    Experiment Station, Puyallup, Washington, and the illustrations of the
    symptoms of green petal virus were provided by M r . C.O. Gourley (Fig.
    of the Canada Agriculture R e s e a r c h Station a t Kentville, Nova Scotia, and
    Dr. R.O.                (Figs. 21 and 22) of the Canada Agriculture R e s e a r c h
    Laboratory a t Ste. Anne de la Pocatiere, Quebec.
254                  Vol. 41, No. 4. Can. Plant Dis. Survey Scpt. 1961

                                    L i t e r a t u r e Cited

1.    BRINGHURST, R . S.          V. VOTH. 1956. S t r a w b e r r y virus t r a n s m i s s i o n
            by grafting excised leaves. P l a n t Dis. Reptr. 40: 596-600.
2.    CREELMAN, D. W. 1961. A s u m m a r y of the prevalence of plant d i s e a s e s
            in Canada in 1960. Can. P l a n t Dis. Survey 41: 101.
                   J.B. and C.P. MARCUS. 1951. V i r u s d i s e a s e s of s t r a w -
            b e r r i e s in the United State s , with special r e f e r e n c e to distribution,
            indexing, and insect v e c t o r s in the east. Plant Dis. Reptr. 35:
                527-537.
4.    FRAZIER, N. W. 1953. A latent v i r u s of F r a g a r i a vesca. P l a n t Dis.
            Reptr. 37: 606-608.
      FRAZIER, N. W. 1955. S t r a w b e r r y veinbanding virus. Phytopathology
                     307-312.
6.  FRAZIER, N. W. 1955. Tobacco n e c r o s i s v i r u s on s t r a w b e r r y . P l a n t
           Dis. Reptr. 39: 143-147.
7. FRAZIER, N. W. 1960. Differential t r a n s m i s s i o n of four s t r a i n s of
           s t r a w b e r r y veinbanding v i r u s by four aphid vectors. P l a n t
           Dis. Reptr. 44: 436-437.
8. FRAZIER, N. W. and A. F. POSNETTE. 1957. T r a n s m i s s i o n and
           host- range studies of s t r a w b e r r y ' green petal v i r u s . Ann.
           App. Biol. 45: 580-588.
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