Method of irrigation affects sour skin rot of onion by yurtgc548


									Method of irrigation affects
sour skin rot of onion
Beth L. Teviotdale      o    R. Michael Davis       R    John P. Guerard
Dennis H. Harper

                                                                                              Onions irrigated by sprinkler too late in the
Sour skin rot of onion was con-               present in decayingplant tissue, and moved      season tend to develop sour skin rot. Affected
trolled successfully by substitut-            in irrigation water. It has not been reported   leaves turn light brown and scales discolor and
                                              on onion seed. Bacteria, includingP. cepacia,   separate from healthy portions (above).When
ing furrow irrigation for overhead            are readily transported in water, and many      harvested, infected onions are rotted and dis-
sprinkler irrigation. Fresh weight            bacterial diseases depend upon rain or          colored within (above left).
of bulbs and percentage soluble               sprinkler irrigation for their spread.
solids were not affected by the                   Our dry California climate serves as a      replications of four irrigation regimes: fur-
irrigation method or the volume of            natural control for many bacterial diseases,    rowirrigationallseas0n;sprinklerirrigation
                                              but sprinklerirrigationmaycreateconditions      until bulbing and furrow irrigation for the
water delivered.                              that favor disease. Careful management of       remainder of the season;sprinklerirrigation
                                              sprinkler irrigationcan limit the infectionof   to30dayspastbulbingandfurrowirrigation
                                              crops subject to bacterial disease. The ob-     for the remainder of the season; and sprin-
Sour skin is a serious bacterial disease of jective of this study was to determine the        kler irrigation all season.
processing onions in California’s Central influence of the irrigation method and the                  hs
                                                                                                  In t i study, bulbing, which occurred in
Valley. The bacteria (Pseudomom cepacia) amount of water applied by sprinkler irri-           late May inboth years, is defined as the time
enterplantsthroughwoundsinyoungleaves gation on the incidence of sour skin rot and            when the bulb diameter is about twice that
and progress down the Ieaf to rot the corre- yield in processing onions.                      of the neck. Each week included two &hour
spondingscales in thebulb.The infectedleaf        South Port White Globe onions were di-      sprinklersessionsand onefurrowirrigation.
develops a watery rot in the onion neck, rect-seeded at the University of California          The final irrigation came in the first week in
which t r s light brown and can be re- West Side Field Station, Fresno County, in             July each year.
moved easily from the plant. In advanced January 1987 and 1988. When the plants                   The total seasonalwater applicationsfor
stages, infected scalesdiscolor and separate developed three to four leaves in mid-April      treatments 1 through 4, including furrow
from healthy portions of the bulb. Severe of each year, the fields were divided length-       irrigations, were 24.9, 24.3, 21.9, and 19.3
outbreaks may result in the rejection of en- wise into four sections of 16 beds each.         inches in 1987 and 38.0,37.2,36.5, and 36.0
tire lots of onions at the processing plant.  Sprinklers were placed between the center       inches in 1988.Variations in pump pressure
    Although details of the surp.ival and two rows of each sectionand gated pipe was          and an increasein the number of irrigations
spread of P. cepacia are not well understood, laid transverselyacrossthe field, creating 16   because of weather conditions accounted
the pathogen is known to be soilborne or sections. These sectionsaccommodatedfour             for the differencesin totals for the two years.

                                                                               CALIFORNIA AGRICULTURE, SEPTEMBER-OCTOBER 1990            27
    Plots were centered between risers and      correspondingbeds in the furrow-irrigated           The fresh weight of bulbs was signifi-
located in the second beds on both sides of     plots were recorded in 1987.Identical mea-       cantlylower in two experimentsduring 1989
sprinkler lines and on the corresponding        surements were taken from 10 feet per plot       inwhichonionswere irrigatedusing3hz-inch
two beds in the furrow-irrigated sections.      in 1988.The percentageof soluble solidsin a      nozzles than with either 7/6- or %-inch
Both beds in each sectionwere divided into      sample of about 30 macerated onions taken        nozzles (table 2). No sigxuficant differences
10-foot subplots, which were either inocu-      from each plot was measured with a re-           in the fresh weight of bulbs were found
lated or noninoculated.The inoculum con-        fractometer.                                     among treatments in the 1987or 1988, or in
sisted of a water suspension of P. cepacia,         The incidence of sour skin was sigrufi-      one of the 1989 experiments. Percentage
approximately1millionbacteria per ml, and       cantly less in the two treatments employing      soluble solidswas not affected sigruficantly
was applied to runoff using a compressed-       furrowirrigationformost or all of the season     by the variable amountsof water in any trial.
air sprayer during two regularly scheduled      than in the two treatments irrigated by
sprinkler irrigations in the first and third    sprinkler for most or all of the season (table   Conclusions
weeks of June in both years.                    1).The highest percentage of rot occurred in  Method of irrigation has a substantial im-
    Diseaseincidencewas evaluated in mid-       plots irrigated by sprinkler all season. Rot  pact on the incidenceof sour skin of onions.
July. All bulbs in the subplots were cut just   found in uninoculated checks was attrib-      Season-longoverhead irrigationprovided a
below the shoulder, and disease incidence       uted to natural spread and infection.         favorable environment for sour skin rot,
was expressed as the percentage of bulbs            There were no sigruficant differences in  whereasfunowirrigationresultedinalmost
with internal discoloration. Pseudomonas        fresh weight of bulbs or percentage soluble   complete control of the disease. Only treat-
cepaciawas recovered in culturefrom repre-      solids among the four irrigation systems in   mentsin which sprinklerirrigationwas used
sentative diseased bulbs.                       eitheryear. Yields ranged from2.8to 3.4 and   after bulbing resulted in high levels of the
    Yield data were collected in August 1987    5.9 to 6.6 pounds per bed foot in 1987 and    disease. The f n l four or five sprinkler irri-
and July 1988. Fresh weights of all bulbs       1988,respectively.Percentagesoluble solids    gationswere accompaniedby a 150to 300%
from20feet of the third (noninoculated)  bed    varied from 22.5 to 23.7 in 1987and 13.6 to   increase in disease. Where sour skin is a
on both sides of the sprinkler line and the     15.0 in 1988.                                 potential problem, a change from sprinkler
                                                                                              to furrow irrigation-at least from bulbing
                                                Volume of overhead irrigation                 to the end of the season -is advisable.
                                                We tested the influence of variations in the      The possibility that disease incidence
                                                amount of water applied by sprinklers on could be lowered by reducing the volume of
                                                disease incidence and yield by altering the water delivered was not substantiated by
                                                nozzle orifice size in five commercialonion our experiments. Varying the amounts of
                                                fields. The experiment was conducted in water with different sizes of nozzle orifice
                                                fieldsof fresh-marketF d processingonions did not reduce the incidence of sour skin in
                                                in Kern County in 1987 and 1988, respec- plotsirrigatedwitheventhesmallestvolume
                                                tively, and in three fields of processing on- of water. Thus, growers opting for season-
                                                ions in 1989 in Fresno County. Plots were long overhead irrigation may not be able to
                                                established between two adjacent sprinkler reduce the volume of sprinkler irrigation
                                                lines 16 beds apart.                          enoughto reducediseasewhile maintaining
                                                    Two opposite pairs of sprinklers, form- optimum yields.
                                                ing a square,were fitted with nozzleshaving       Percentage soluble solids of the bulbs
                                                orifices3hz,Va, or %inch in diameter.These was not affected by differing amounts of
                                                nozzles delivered 0.10,0.20, and 0.26 inches water delivered by the three nozzle sizes.
                                                water per hour, respectively.                 However,yield asmeasuredbyfreshweight
                                                    There were four replications of each of bulbs was reduced in two experimentsin
                                                treatment (each orifice size). Irrigation 1989 where the 3%-inch-diameter orifice
                                                schedulesvaned, but most were every 4 to 7 sprinklers were used. This may also have
                                                days and 2 to 6 hours long.                   been true for the third 1989experiment, but
                                                    One of the center beds in each plot was the test for significance failed by a slight
                                                inoculatedwithp.cepacia as describedabove margin.
                                                during regularly scheduled irrigations in         The 3hz-inch nozzle size offers no advan-
                                                June. Plots were inoculated twice in 1987 tage for disease control, and may be delete-
                                                and 1988 and once in 1989.Al bulbs in the rious to yield.The ?a-inch nozzle appearsto
                                                center 10 feet of each inoculated bed in the be a better choice than the %-inch nozzle
                                                1987KernCountytrialandloobulbsineach becausethe formerdeliverssigruficantlyless
                                                plot in the othertrialswerecutand evaluated water, but does not reduce the yield of fresh
                                                for disease in mid-July. Fresh weights of bulb weight or the percentage soluble solids
                                                bulbs in 10 feet in each treatment were de- content of bulbs.
                                                termined and soluble solidswere measured
                                                as described above.                           Beth L. Tm'otdaleis ExtensionPlant Pathologist
                                                    The volume of water delivered by the and DennisH.Harper is StaffResearch       Associate,
                                                three nozzle orifice sizes increased with in- KearneyAgricultural Center,Parlier;R.Michae1
                                                creasing orifice diameter. However, there Davis is ExtenswnPlantPathologist,Department
                                                was no correlation between the amount of o Plant Pathology, University of California,
                                                water deliveredand the percentageof rotted Davis;andJohnP. Guerard isFarm Advisor, UC
                                                onions. The incidence of sour skin was low Cooperative Extension, Kern County. This re-
                                                infourtrials (lessthanlo%inalltreatments), search was supported by a grant from the
                                                but high in the other t i l (63%in all treat- American Dehydrated Onion and Garlic Asso-
                                                ments).                                        ciation.


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