Processing tomato growers fight soil-borne disease
ByBob Johnson Processing tomatogrowers face thediffi- cult jobof tryingtostayaheadofalong-last- ing, soil-borne disease that can devastate entire fields of susceptible varieties. While there are finally enough plants resistant to Fusarium wilt race 3 patho- gen to meet demand, some growers are still asked to plant susceptible varieties because they produce fruit with qualities desired by their cannery. Other growers contendwithground infestedwitha relat- eddisease, Fusariumfalciforme, forwhich there are no truly resistant varieties. “The supply of race 3-resistant varieties is now fairly good,” said Brenna Aegerter, University of California Cooperative Extension farm advisor in San Joaquin County. “If you look at the top 10 variet- ies in terms of loads from 2020, three of themwere resistant to race 3. If you look at the number of loads of the top 50 vari- eties, one-thirdof those loadswereof race 3-resistant varieties.” Fusariumwilt infects susceptible toma- toes through the roots and plugs the wa- ter-conducting tissue, causing the plants to wilt and die or survive to produce a subpar crop. Aegerter continues to study strategies for managing this disease in conjunction with Cooperative Extension plant pathol- ogyspecialistCassandraSwett atUCDavis. The challenge for growers is no lon- ger finding plants that can produce in ground with the latest race of Fusarium
wilt, as plant breeders have caught up with the demand. “The host resistance is quite good,” Aegertersaid.“Youcanstillseeanoccasion- al affected plant, but not enough to justify chemical treatment or avoiding the field.” The problemnow is that some growers areasked togrowsusceptiblevarieties and cannot find enough ground that is free of wilt race 3. “A cannery may still request a particu- lar nonresistant variety for its desired fruit chemistry characteristics, whichmay not be race 3-resistant, so we will continue to see problem fields plantedwith nonresis- tant cultivars,” Aegerter said. While the normal strategy would be to plant these nonresistant varieties in dis- ease-free ground, the increase in tree and vine planting in the Central Valley has made that ground harder to find. “Rotation out of tomato is certain- ly done in some cases,” Aegerter said. “However, with the increase in perma- nent crops, there is limited open ground for growing tomatoes, so some growers do not have the option of avoiding infest- ed fields while still meeting their desired acreage of cannery contracts.” When there is enough ground to rotate out of tomatoes, that strategy may still be ineffectivebecause thediseasecansurvive in thesoil for yearswithout host plantsand infect the next susceptible variety. “Evenaftermultipleyearsof rotationout of tomatoes,westill seeFusariumwiltwhen
tomatoesarereplanted—socroprotationis not as effective as we might hope for this particulardisease; the fungus isquite long- lived in the soil, or is reproducing onother crops or onweeds,” Aegerter said. Theremaybeonemoretool,however, for reducingdiseasedamageininfestedground plantswithsusceptiblevarieties.Trialsshow thatchemicalcontrolscanreducecroploss. For the past four years, Aegerter, in co- operation with Swett’s lab, has studied applying the fumigant K-Pamthrough the buried drip line at least three weeks be- foreplantingor fungicides at planting and found they giveat least enoughprotection tomore than pay for themselves. “If we use an average increase of 8 tons inyield, andassumeacommoditypriceof $80per ton, then the economic advantage of the fumigation is $640,” Aegerter said. “As thecostsof fumigationarearound$150 to $400 per acre, we can conclude that on average, in fields with soilborne disease pressure, the costs of drip fumigation are likely to be repaid in increased yield.” Fumigants and fungicides are not nor- malmanagementstrategies,butalast resort where resistant varieties or long-termrota- tionout of tomatoes arenot viableoptions. “Chemical control shouldnotbeconsid- ered the first line of defense against these diseases, but rather the focus should be on resistant or tolerant tomato varieties and avoiding infested fields when feasi- ble,”Aegerter said. “However, insituations where resistant or tolerant varietiesarenot
an option, or when the disease tolerance is not sufficiently high, chemical control might be an option to consider.” Some promising news is that the fu- migant and fungicides appeared to also provideat least some relief fromFusarium falciforme, a relatively recently identified disease for which there are no fully resis- tant tomato varieties. This disease, which has been identified in at least 100 commercial tomato fields, causes vinedecline from45 to60daysafter planting and severely reduces yield. InaSan JoaquinCounty trial, chemicals run through the buried drip line provided some protection against yield loss due to Fusarium falciforme. “We have observed some efficacy of K-Pamand fungicides toreduce incidence ofFusariumfalciformeand, yes, thiswould be the first effective tool for this disease,” Aegerter said. Swetthasspent yearsstudyingFusarium falciforme and screening processing to- mato varieties for their ability to produce acrop inground infestedwith thisdisease. “Some growers are usingmore tolerant cultivarswithreportedsuccess,”Swett said. These plants are not resistant to Fusarium falciforme—there are no resis- tant varieties—but showtheability todelay symptoms long enough for the plants to set a crop. ( Bo b J o hn s o n i s a r e p o r t e r i n Sacramento. He may be contacted at bjohn11135@gmail.com.)
CIMIS REPORT | www.cimis.water.ca.gov
CALIFORNIA IRRIGATION MANAGEMENT INFORMATION SYSTEM
For the week July 29 - August 4, 2021 ETO (INCHES/WEEK)
YEAR
3.0
THIS YEAR
2.5
LAST YEAR AVERAGE YEAR
2.0
1.5
1.0
0.5
0.0
MACDOEL II (236)
BIGGS (244)
DAVIS (06)
MANTECA (70)
FRESNO (80)
SALINAS-SOUTH (214)
FIVE POINTS (2)
SHAFTER (5)
TEMECULA (62)
IMPERIAL (87)
THIS YEAR LAST YEAR AVG. YEAR % FROM AVG.
1.36 1.83 1.65 -17
1.77 1.68 1.63 9
2.13 2.12 1.96 8
2.21 2.03 1.78 24
1.81 1.87 1.72 5
1.88 1.80 1.77 5
2.05 2.09 1.91 7
1.42 1.35 1.26 12
1.55 1.44 1.54 2
2.07 1.98 1.98 5
W eekly reference evapotranspiration (ETo) is the rate of water use (evapotranspiration—the sum of soil evaporation and crop transpiration) for healthy pasture grass. Multiplying ETo by the appropriate “crop coefficient” gives estimates of the ET for other crops. For example, assume ETo on June 15 is 0.267 inches and the crop coefficient for corn on that day is 1.1. Multiplying ETo by the coefficient (0.26 inches x 1.1) results in a corn ET of 0.29 inches. This
information is useful in determining the amount and timing of irriga- tion water. Contact Richard Snyder, UC Davis, for information on coefficients, 530-752-4628. The 10 graphs provide weekly ETo rates for selected areas for average year, last year and this year. The ETo information is provided by the California Irrigation Management Information System (CIMIS) of the California Department of Water Resources.
For information contact the DWR district office or DWR state headquarters:
SACRAMENTO HEADQUARTERS: 916-651-9679 • 916-651-7218
NORTHERN REGION: Red Bluff 530-529-7301
NORTH CENTRAL REGION: West Sacramento 916-376-9630
SOUTH CENTRAL REGION:
SOUTHERN REGION:
Fresno 559-230-3334
Glendale 818-500-1645 x247 or x243
14 Ag Alert August 11, 2021
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