Lettuce Mosaic Virus causes Internal Rib Necrosis

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					           LETTUCE MOSAIC VIRUS:
                                 STELLA M. COAKLEY                -       R . N. CAMPBELL   *   K . A. KIMBLE

                   reported here (in two
                                                         ~   ~~       ~          ~

      articles) were conducted to deter-      Lettuce has been the most important winter vegetable crop in the Imperial Valley for
 mine the role of lettuce mosaic virus        the past four years, averaging about 42,000 acres grown, and an average gross value
 (LMV) in the development of internal         of about  $35 million. Lettuce grown for harvest in midwinter (December 15 to March
 rib necrosis (IRN) and rusty brown dis-      15) is planted from about September 10 to November 5. Plantings during October are
coloration (RBD) . This virus has been        largely of the cultivar Climax. During the winter harvest season of 1968-69, a disorder,
consistently isolated from IRN-affected
                                              later known an internal rib necrosis (IRN), occurred throughout the Imperial Valley in
lettuce heads since 1969. Three cultures
                                              epidemic proportions just prior to harvest and caused serious losses in the field and in
of lettuce mosaic virus were inoculated to
                                              transit. Persons associated with the lettuce industry recalled the disease from past years
lettuce plants grown in the greenhouse
or growth chambers so they were known         but it had never been ranked as a major disease until 1969. N o valid estimate was
to be free of other viruses. Lettuce plants   made of the losses from internal rib necrosis during the 1968-69 season but i t was
were of the susceptible cultipar Climax,      common for growers to abandon entire fields. Since then IRN has appeared sporad-
as well as nonsusceptible cultivars, Van-     ically in lettuce planted for midwinter harvest but not in epidemic proportions. In
guard and Calmar. After inoculation the       addition, a postharvest disorder (then called rusty rib) now called rusty brown                          dis-
plants were kept in growth chambers           coloration (RBD), occurred on lettuce during storage and transit at 35OF. In February
equipped to provide a 12-hour “day.”          1969, 90% of the lettuce shipments arriving in New York had RBD. The cause (or
Temperaturvs were set at 70O F day-           causes) of IRN and RBD has not been identified but environmental conditions (freezing
50’ F night in some trials o r at 6 5 O F     temperature and rainfall), aqua ammonia damage and a synergistic reaction between
day-45’ F night in other trials. As the
                                              beet western yellows virus and lettuce mosaic virus have been speculated as causes.
plants grew, the lower leaves were exam-
                                              Two papers presented here report the results of cooperative research by personnel
ined for IRN. When the plants formed
heads and matured, they were harvested,       of the California Agricultural Experiment Station, the California Agricultural Extension
examined again for IRN, assayed for           Service, and the US Department of Agricultural to determine the causes of IRN and RBD.
LMV infection, and stored at 33-35O F
for up to two weeks to allow RBD to
                                              dish-brown. No IRN o r RBD developed                 Other trials were conducted to deter-
   The results of nine trials were similar    on Climax plants that were inoculated             mine if other viruses might causc IRN
and have been combined (see table).            but failed to become infected with LMV           and/or RBD on Climax lettuce. Inoc-
Plants of various ages were inoculated.       or on noninoculated Climax plants. Ad-            ulations with alfalfa mosaic virus, cu-
Thirty-day-old plants developed severe        ditionally, none of the Calmar or Van-            cumber mosaic virus and beet western
mosaic symptoms with mottled leaves           guard plants developed IRN o r RBD                yellows virus separately did not cause
and severe twisting of the leaves. These      whether they were infected by LMV ( 2 0           IRN or RBD. Similarly, inoculations of
plants developed some IRN but usually         plants) or not (40 plants).                       beet western yellows virus together with
failed to head. Plants of this age were                                                         LMV did not cause more severe IRN
                                                  These results show that LMV alone
therefore not used in most trials. Plants                                                       than that produced by LMV alone.
                                              causes IRN in growing plants and that
that were 45 or 60 days old at inocula-
                                              it is an important factor in the induction           Since LMV causes IRN and RBD, the
tion generally did not show pronounced
                                               of severe RBD in stored heads. They also         most direct method of preventing these
symptoms of infection by LMV but a
                                               confirm that IRN and RBD occur only              diseases is to control LMV. Thus, these
high proportion of these plants devel-
                                              in the cultivar Climax. The Iettuce heads         results strongIy support the continuation
oped I R N and RRD. Careful inoculation
                                              produced in these growth chamber ex-              and improvement of the seed indexing
and dissection records showed that IRN
                                              periments did not reach the same degree           program curently being used in Imperial
Generally started to form on leaves that
                                              of maturity or head firmness that is              County. Breeding a variety to rFplace
were about one-half to one inch long
                                              encountered in field-grown lettuce. Thus,         Climax can be accomplished hut this will
inside the young head at the time of
                                              the mild discoloration similar to RBD             require several years.
inoculation, hut it did not start to form
                                              but occurring on field-grown, mature to
anti1 about 21 days after inoculation
                                              overmature heads without LMV infec-                 DEVELOPMENT OF INTERNAL RIB NECROSIS (IRN)
   Microscopic examination of these                                                                 AND RUSTY BROWN DISCOLORATION (RED) ON
                                              tion was not observed (see next article).          CLIMAX LETTUCE INFECTED WITH LETTUCE MOSAIC
plants showed that the pattern of dead           Treatment of other plants with aque-                             VIRUS (LMV)
cells was the same in the experimental        ous ammonia induced a discoloration of                                                   Results
plants as in plants collected from the        the midrib that superficially resembled                                    No. w i t h   No’ w‘th   No’ with
                                                                                                   Age a t      No. of
Imperial Valley. In the case of IRN there     IRN. Microscopic examination showed,               Inoculation    heads      LMV         LMV’
                                                                                                                                        IRN        L RED’
wwe groups of dead cells between the          however, that this discoloration was in                30           18        13             7           4
beins at the midrib, whereas RBD caused       the veins. It is clear that ammonia dam-               45           15        13            12          12
                                                                                                     60           38        28            28          23
death of epidermal cells that became red-     age is distinct from IRN.                         Noninoculated     42         0             D           0

6                                                                                    CALIFORNIA AGRICULTURE, SEPTEMBER, 1 9 7 3

                                                                                                                                        was installed over two beds planted to
                     INOCULATED W I T H LETTUCE MOSIAC VIRUS, BRAWLEY, CALIFORNIA 1972                                                  Climax lettuce and irrigation was not
                               Internal rib necrosis*                        Location of                  Rusty brown discoloration+
                                                                                                                                        tlont, until the shelters were completed.
                                LMV-            Plants
                                                                        Internal rib necrosis?
                                                                                                               LMV-           Plants
                                                                                                                                        One shelter s e n d as a control and was
                               infected         free of                                                      infected        free of    entered once to thin the plants. Each of
        Treatment               plants           LMV             Wrapper       Cap          Internal          plants           LMV
                                                                                                                                        the remaining three shelters were entered
        Inoculated:                                                                                                                    once to thin the plants and two more times
          50 days              40/40            o/o                4            27               40          39/39            o/o      to inoculate selected plants with lettuce
          70 days              38/38            0/3               29            33               38          37/37            0/3
                                                                                                                              o/o      mosaic virus ( L M V ) at 50. 70 and 90 days
          90 days              40/40            o/o               34            35               40          38/38
        Noninoculoted:                                                                                                                  before harvest. At harvest time the plants
          Control                                                                                                                      were evaluated for internal rih necrosis;
              shelter           8/14            0/33               0             0               8           10/14            0/33     they were then bagged, shipped to Dab is.
                                                                                                                                       individually assayed on Chenopodium
              shelters         15/18            0/23                2            3               15           14/18           0/23
                                                                                                                                       quinoa for lettuce mosaic I irus infection,
          *   Number o f heads with internal rib necrosis/number of plants i n category.
          t   Number of heads with internal rib necrosis showing i n wrapper leaves, cap leaves or within the head.                    stored for two weeks a t 35 F, and eval-
          $ Number of heads with rusty brown discoloration/number of plants i n category as determined after storage                   uated for head firmness and rusty hrown
        for 14 days a t 34°F.
                                                                                                                                       discoloration. The cages were not insect
                                                                                                                                       proof. At harvest on February 28 and
                                                                                                                                       29, 1972, green peach aphids that are
        Field studies of                                                                                                               effective bectors of LMV were found in
                                                                                                                                       two shelters.
               INTERNAL RIB NECROSIS                                                                                                       T h e results (see table) are of interest
                                                                                                                                       despite the failure to exclude aphids. All
               AND RUSTY BROWN                                                                                                         but three of the plants inoculated at 50,
                                                                                                                                       70 o r 90 days wrre infected with LMV;
               DISCOLORATION O F THE                                                                                                   all the LMV-infected plants dewloped
                                                                                                                                       IRN and RBD. Because of the aphid
               LETTUCE CULTIVAR CLIMAX                                                                                                 infestation some of the noninoculated
                                                                                                                                       plants became infected with LMV and
                                                                                                                                       many of these debeloped IRN and RBD.
                  STELLA M. COAKLEY                          *   R. N. C A M P B E L L           *    K . A. K I M B L E               It is likely that not all of these plants
                        H. J O H N S O N , J R .        *    K. MAYBERRY                  T. W. WHITAKER                               devdoppd IRN and RBD because they
                                                                                                                                       were inoculated b y aphids within one or
                                                                                                                                       two weeks of h a n e s t . This is supported
                                                                                                                                       Iiy the fact that most of the IRN ob-
        B       Y T I r E END O F T H E
             son Climax was known to be the
                                           1970 harvest sea-

        lettuce cultibar most severely affected by
                                                                              (b) Nine different lots of Climax seeds pro-
                                                                           vided by various seed companies, as well as the
                                                                            original release, produced plants that had
                                                                                                                                       seried in these plants was on younger
                                                                                                                                       leaves within the head. w h u e a s most
                                                                           equally severe IRN. A few plants were free of               inoculated plants had IRN in the older
        internal rih necrosis. Investigations i n
                                                                           symptoms of IRN. These plants were saved,
        1970-71 were planned to test additional                                                                                        wrapper and cap leaies as \tell. None
                                                                           transported to the greenhouse and seed was
        commercial varieties and all possible                              obtained from them. The seed from these ap-
                                                                                                                                       of the plants that were free of LhIV de-
        seed sources of Climax, including seed                             parently disease-free plants was tested for IRN             \eloped either IRnT or RBD.
        from the original release of Climax for                            susceptibility during the 1971-72 growing sea-                 Since aphids had infested the plants
        their IRN reaction to determine if some                            son. All were susceptible to IRN, indicating that           in two chambers and might h a l e intro-
                                                                           these particular plants were escapes rather                 duced other iiruses. especiallJ beet west-
        change had occurred since the cultivar
                                                                           than resistant or tolerant to IRN.                          ern yellows virus ( B W Y V ) , additional
        was relcased. Seed was planted on four
        dates: Octoher 7, October 14, October                                 Experimental plots were designed in                      assays for BWYV wrre done from some
        24 and October 28. T h e results of these                          the 1971-72 growing season to field test                    plants. Eight plants with severe IRN
        tests were surprisingly uniform and                                whether lettuce mosaic virus could cause                    were selected from each of three shelters.
        clearcut:                                                          IRN and RBD, as had been demonstrated                       These plus eight plants from the control
          (a) Only Climax showed the typical internal                      under controlled greenhouse conditions                      shelter were assayed but only one IRN-
        rib necrosis (IRN) symptoms. The other cultivars                   (as reported in the companion article in                    affected. aphid-infested plant yielded
        tested-Merit,       Calmar, Greenland, Green Bay,                  this issuc) . Four shelters were construct-                 BWYV. Thus, the trial gibes strong con-
        Golden State        D, GL   4 0 7 and Francisco-were               cd a n d c m r r e d with tobacco cloth to                  firmation that LMV alone causes IRK
        uniformly negative for IRN.                                        cxcludc aphids. Each 12-ft-long shelter                     and RBD in Climax.

        CALIFORNIA AGRICULTURE,                             SEPTEMBER,        1973                                                                                                7
                                 F                             unknown, but judging from their re-
 A N D RUSTY BROWN DISCOLORATION I N CLIMAX                    sistance to RBD i n other tests it seems
PLANTS INOCULATED WITH LETTUCE MOSAIC VIRUS                    likely that susceptibility to the mild dis-
   (37 plants from each of 3 inoculation dates were            coloration represents an undesirable char-

                  analyzed) r = 0.989
                                                               acteristic of the variety Climax, which
                                                               is accentuated by LMV infection.
                                                                  A double-wall plastic shelter covering
                                                               two lettuce beds, 12 ft long, was con-
                                                               structed to evaluate the effect of mini-
                                                               mum tcmprrature on the incidence of
                                                               IRN. Large vents on each end serxed to
                                                               keep the inside temperature near ambi-
                                                               ent during daytime and also allowed free
                                                               access to all insects. A thermostatically        VICTOR V O T H      .   R. S. BRINGHURST
                                                               controlled electric heater was installed to
                                                                                                                 HOWARD J. BOWEN            *   TOM MOCK
                                                               keep the minimum temperature ahoi e
                                                               40° F, during the night when the vents
                                                               were closed. The shelter was installed
                                                               over young plants on Nokember 30,1971,
                                                               and at the same time several plants were       Water placement is very important to
                                                               inoculated with lettuce mosaic virus. At
                                                                                                              strawberry production. Under the condi-
  1          I          I                    1         I   I   maturity, on January 25, 1972, the plants
             I                              i          4   5                                                  tions of this experiment, plants on a con-
                            H t A U F I RMNL I S * *
                                                               from the shelter were evaluated for IRN.
                                                                                                              ventional 40-inch bed with standard fur-
                                                               Of the 41 plants within the shelter all hut
 * 0 = no leaf discoloration                                                                                  row irrigation required the greatest
   5 = severe discoloration                                    two exhibited typical symptoms of IRN.
 t 1 = immature, soft head                                     This experiment confirms that tempera-         amount of water and yielded the least
   3 = optimum market maturity
   5 = hard. overmature head                                   tures below 40’ F are not required for the     fruit, whether figured on a per-plant or
                                                               development of IRN symptom in Climax.          per-acre basis. In contrast, the experi-
            When the heads were stored for two                    This work demonstrates that IRN and         mental 60-inch midbed furrow-irrigated
         wecks for RBD to develop, additional                  RBI) are limited to the variety Climax         plants yielded significantly more fruit on
         tests wvre done. In one test half of the              among the varieties which are commonly         a per-plant or per-acre basis, even though
         LMV-inoculated heads were placed in                   grown in the Imperial Valley, California.      there was a 35% increase in plant popu-
         pairs in a large plastic bag and a n                  It also confirms the results of carefully      lation. Apparently surface application,
         additional virus-free Climax head col-                controlled experiments which have shown        with the water constantly moving down-
         lcctcd from a commcrcial field was also               that lettuce mosaic virus is the cause of                                            f
                                                                                                              ward, improves plant performance. O the
         placcd i n the hag to determine if a                  IRN and RRD, and it emphasizes the             new application systems investigated here,
         volatile compound produced in the LMV-                nccpssity for the control of lettuce mosaic    only the use of porous pipe placed under
         infected heads could cause RRD on the                 virus through a clean-seed program. A
         virus-free head. Although RBD devel-                                                                 the soil surface was of questionable value
                                                               clean seed program in which the only           from the point of view of plant perform-
         oped on all 60 LMV-infected heads, none
                                                               lettuce seed lots certified for planting are   ance.
         of thc 30 healthy heads developed RBD.                those with zero mosaic in 30,000 seeds
         In addition, there was a direct correla-
                                                               was instituted in Imperial County in
         tion between head firmness (i.e. matur-               1971. This has given excellent control of
         ity) and RRD severity with thc more ma-               LMV in subsequent seasons. A simiIar
         ture heads developing more severe RRD                 program has effectively controlled LMV
          (see graph).
            Climax lettuce heads collected from a
         commercial field were assayed and found
                                                               for 10 years in the summer lettuce crops
                                                               of the Salinas Valley.                         T    H E MILD CLIMATE of coastal Cali-
                                                                                                                   fornia contributes to high straw-
                                                                                                              berry yields, although urbanization is
         to he free of LMV. When these heads                      Stella M . Coakley is Research Assist-      reducing the available land in the best
         were stored for three weeks or more, they             ant; R. N . Campbell is Professor; and         areas. These experiments were designed
         often developed a mild discoloration that             I<. A. Kimblp is S t a f Research Associate,   to study the production efficiency of
         was similar to KBD. This mild discolora-              Department of Plant Pathology, Univer-         novel irrigation methods, but also in-
         tion also was more severe on the firmer               sity of California, Davis. Hunter John-        volved bed shape and plant placement.
         heads hut it never hecame as severe as                son, Jr. is Extension Vegetable Specialist,
         RRD caused by LMV; it would rate less                 University of California, Riverside; K .       Drip system
         than 1 on a 0-5 scale of increasing RBD               Mayberry is Farm Advisor, Imperial
         sew-rity. The similarity of this discolora-           County, and T . W . Whitaker is Research         Promising resuIts were obtained in
         tion and RBD suggests that LMV infec-                 Geneticist, USDA, U . S. Horticultural         1967-68 at San Diego in preliminary
         tion accclerates physiological processes              Field Station, La Jolla. This research was     experiments with the “drip” irrigation
         that normally do not produce effects on               supported in part b y a National Science       system utilizing plastic pipe positioned
         Climax lettuce until the lettuce is overnia-          Foundation Scholarship and b y a grant         between the plant rows on top of the
         turc. This work was done only with Cli-               from the lmperid Valley Vegetable              conventional 40-inch, double-row bed
         max so the reaction of other varieties is             Growers Association.                           with orifices every two feet. Water con-

         a                                                                                       C A L I F O R N I A AGRICULTURE,   SEPTEMBER,     1973