Welcome to the July 2002 edition of Washington State University’s Vegetable Pathology Extension Team newsletter, the third edition of the 2002 growing season. This newsletter follows the team’s July conference call when team members discussed current vegetable diseases occurring in the state, their diagnoses and control. We thought it would be fun to focus the July issue on ONIONS! Did you know that onions are Washington’s third-highest value vegetable crop? Approximately18,000 acres of storage onions and approximately 800 acres of non-storage onions were harvested in Washington in 1999. For more onion statistics, visit the WSU Vegetable Pathology Team’s website at:http://mtvernon.wsu.edu/path_team/onion/andhttp://mtvernon.wsu.edu/path_team/Overview/. If you have questions or comments regarding vegetable diseases or WSU’s Vegetable Pathology Team, contact Debbie Inglis (firstname.lastname@example.org) or Lindsey du Toit (email@example.com).
Onion Smut Detected
Onion smut has been detectedthis year in the south Columbia Basin (Franklin County). Although this disease is not new to the state, previous reports have been limited to western WA and the following counties in eastern WA: Walla Walla, Yakima, Spokane, and Kittitas. The large acreage planted to onion in Washington justifies grower awareness of the disease and appropriate measures to minimize spread of the pathogen.
The fungus responsible for smut on onions (Urocystis colchici) can survive many years as a saprophyte in infested soil. Symptoms of smut appear in the seedling stem as it emerges, and blisters of black powdery spores result. Seedlings either die at emergence or produce distorted bulbs with smutty lesions. Seed treatment for next year’s crop is highly advisable. Consult PICOL for products registered for use in WA at:http://picol.cahe.wsu.edu/labels/Labels.php
PLACES TO FIND INFORMATION ABOUT ONIONS
Onion Field Day Scheduled The WSU Onion Field day is scheduled for Aug. 30 at 9 am near Quincy. For information and directions, contact Gary Pelter at firstname.lastname@example.org
Onion Cultivar Demonstration and Storage Trial Results Available Since 1984, Gary Pelter, WSU Extension Agent, has carried out annual demonstration trials and storage trials of onion cultivars in the Columbia Basin. Results of the demonstration trials include information on maturity, yield, and bulb size (# of bulbs >4″, 3–4″, 2.25–3″, <2.25″, and # of defects). Results of the onion storage trials include information on incidence of neck rot and other storage rots, # of sprouted bulbs, # of bulbs with single centers, bulb firmness, scale quality, and uniformity of bulb shape. For copies of the results from 1984 to 2000, contact Gary Pelter at (509) 754-2011 ext. 413 or email@example.com. Results of the 2001 trials can be accessed at the Grant-Adams area website:
2001 Onion Storage Demonstration Results – http://grant-adams.wsu.edu/
2001 Onion Varietal Demonstration Results – http://grant-adams.wsu.edu/agriculture/index.htm
Onion World: Voice of the Onion Industry
For information on the latest issues affecting the onion industry, subscribe to Onion World: Voice of the Onion Industry by contacting the Executive Office of Columbia Publishing, 417 North 20th Avenue, Yakima, WA 98902; tel. (509) 248-2452 or fax (509) 248-4056. Onion World is published eight times a year, and the annual subscription fee is $15.
An estimated $50 million crop loss affecting about 60% of Georgia’s sweet onion crop this year was probably caused by a combination of warm fall temperatures that promoted bolting, heavy spring frosts that allowed Stemphylium leaf blight to develop followed by an outbreak of sour skin (Pseudomonas=Burkholderia cepacia), and unusually hot/humid weather in April. Growers harvested only 2 million bushels of what should have been a 5 million bushel crop.
Onion Crop Profile Under Development
A Crop Profile for onion in Washington is being developed by Gary Pelter and Erik Sorensen, WSU Extension Agents and members of the WSU Vegetable Pathology Team. The Onion Crop Profile will be available in August or September. A Crop Profile is a condensed production story of an individual agricultural commodity. It includes information on production; cultural practices; pest, disease, and weed problems and management practices; and, IPM or alternative strategies for managing pests, diseases, and weeds. The purpose of a Crop Profile is to provide an overview of the importance of that commodity, identify production/crop protection concerns, and suggest alternative management opportunities. For further information on Crop Profiles, access the following website:http://www.tricity.wsu.edu/~cdaniels/CropProfiles.pdf. For questions on Crop Profiles completed for Washington State, contact Catherine Daniels, WSU Pesticide Information Coordinator, at firstname.lastname@example.org or (509) 372-7495.
2002 National Allium Research Conference To Be Held in Pasco
The 2002 National Allium Research Conference will take place December 12-14 in Pasco, WA at the DoubleTree Hotel. The conference includes two days of oral and poster presentations, followed by a tour of the onion industry in the Pacific Northwest (production, storage, and processing facilities). The deadline for submitting papers/posters has been extended to September 1, 2002. For information contact Wendy Peay at (509) 547-0701,email@example.com or visit the conference website at http://narc2002.wsu.edu.
International Allium White Rot Conference Recently Held The 7th International Allium White Rot Conference was held at Harris Ranch, CA on June 4-8, 2002. The meeting included sessions (oral presentations and interactive discussions) on the history, geographic distribution, and importance of white rot caused by Sclerotium cepivorum; the biology and epidemiology of this fungal pathogen; and, management practices for white rot, includingfungicides, sclerotium germination stimulants, biological control, and host resistance. The meeting was attended by delegates from Canada, Mexico, the Netherlands, New Zealand, and the US. Proceedings from this conference will be posted later this year on the website of the Canadian Phytopathological Society (http://www.cps-scp.ca/conference.html)
Note: A white rot quarantine is enforced by the WSDA for Grant, Adams, and Franklin Counties.
INFORMATION ABOUT ONION DISEASES & PESTS
Onion Disease Compendium Available
APS Press offers the “Compendium of Onion and Garlic Diseases” edited by Howard F. Schwartz and S. Krishna Mohan. The book includes an introduction to the genus Allium; extensive information about infectious biotic diseases and noninfectious abiotic conditions of onion and garlic, including diagnosis, epidemiology, and disease management; and, 100 color images of biotic and abiotic problems on onion and garlic to assist with diagnoses. The cost of the compendium is $49.00. To order call 1-800-328-7560 (ISBN 0-89054-170-1) or view: http://store.yahoo.com/shopapspress/41701.html
Some Additional References on Diseases of Allium Crops
Onion Diseases: A Practical Guide for Seedsmen, Growers, and Agricultural Advisors.B.K. Gabor, Editor. For copies of this handy reference, contact your local Seminis dealer. This book includes brief descriptions of the major biotic and abiotic diseases of onions, with excellent color photos accompanying each description.
Diseases and Pests of Vegetable Crops in Canada. 1994. R.J. Howard, J.A. Garland, and W.L. Seaman, Editors. The Canadian Phytopathological Society and the Entomological Society of Canada, Ottawa, Ontario, Canada. For copies contact the ESC at (613) 725-2619 or by fax at (613) 725-9349. This extensive reference covers diseases and insect pests of a wide range of vegetable crops, including diseases of greenhouse vegetables, with color photos of the pests and diseases.
Onions 1993. D.A. Bender. Chapter 12, In: Nutrient Deficiencies & Toxicities in Crop Plants. W.F. Bennett, Editor. The American Phytopathological Society Press, St. Paul, MN. To order a copy of this book, visit the APS Press website at:http://www.shopapspress.org/41515.html or call 1-800-328-7560 (ISBN 0-89054-151-5).
2002 Onion Research Projects in Washington
Research projects on onion diseases, pests, and production issues are being carried out in the Columbia Basin by a number of faculty and extension agents:
Doug Walsh, WSU Entomologist, is investigating control of thrips (western flower thrips and onion thrips) in onion and onion seed crops in the Columbia Basin.
Lindsey du Toit, WSU Vegetable Seed Pathologist, has projects on Botrytis neck rot/scape blight of onion/onion seed crops: a) a survey of seed crops to determine theBotrytis species prevalent in this semi-arid region and the level of infection on the harvested seed; b) the importance of seedborne Botrytis spp. of onion as measured by the rate of seed-to-seedling transmission; c) a fungicide efficacy trial for onion seed crops; d) survival of Botrytis allii in culled onions.
Gary Pelter, WSU Extension Agent, has an onion demonstration trial as well as weed control trials for onion seed crops located in the northern Columbia Basin.
For further information on any of these projects, contact the individual researchers: Doug Walsh at (509) 786-2226 or firstname.lastname@example.org, Lindsey du Toit at (360) 848-6140 email@example.com, and Gary Pelter at (509) 754-2011 ext. 413 or firstname.lastname@example.org.
2001 Field Research Reports on Onion Diseases
The 2002 volume of Fungicide and Nematicide Tests(results of 2001 published in 2002) include two reports of field research done on onions: 1) Mohan, S.K. and Bijman, V. P. Evaluation of fungicide sprays for control of downy mildew in onion, Cayon County, Idaho, 2001, Report No. 57:V052; and, 2) Langston, Jr., D. B. Evaluation of spray programs for control of foliar pathogens (Botrytis & Alternaria), 2001, Georgia, U.S., Report No. 57:V051. For access, review http://www.apsnet.org/online/FNtests/vol57/top.htm
Important Diseases on Onion in Washington and Their Management
Fusarium oxysporum f. sp. cepae
Host plants: onion, other cultivated Allium spp.
Favorable conditions: 77-82°F optimum, limited infection <59°F. Can infect directly, but injury to stem plate, roots, or bulbs by insects promotes infection.
Symptoms: curving, yellowing, dieback of leaves starting at tips. Infected roots brown, flattened, transparent, may be hollow. Vertical cut through bulbs reveals watery brown outer layer of basal plate. Rot progresses from stem plate through storage leaves. White mycelium on stem plate. Infected plants pulled easily because of stunted, decayed root system. Infection anytime during season but bulbs may only display symptoms after harvest.
Management: Plant resistant cultivars; rotate out of onions for 4 years; minimize injury to bulbs; store bulbs at 39°F.
Host plants: onion, fruits, other vegetables
Favorable conditions: field, transit, or storage. Temperatures >86°F in field or >75°F in storage. Free moisture on onion for 6 hours.
Symptoms: Can resemble onion smut. Black discoloration at neck on injured or necrotic leaf tissue, on injured or diseased roots, on bruised or split outer scales. Clusters of black spores along veins and between outer papery scales. Entire surface of bulb may turn black with all scales infected and bulb may shrivel. Secondary soft rot bacteria may turn bulb soft/mushy. May be no visible external symptoms, but central portions of bulb gray to black when cut in two. Discoloration may extend from neck into central fleshy scales.
Management: dry bulbs promptly and adequately after harvest; store bulbs at 35-55°F and low humidity; avoid bruising bulbs during handling; plant treated seed – consult PICOL for products registered for use in Washington at:http://picol.cahe.wsu.edu/labels/Labels.php
Botrytis leaf blight
> Host plant: Allium spp. (garlic highly resistant, chive immune)
Favorable conditions: prolonged moist conditions, moderate temperatures (53-75°F); uncommon disease in Columbia Basin.
Symptoms: primarily on leaves. Small white or tan spots (2 mm) with light green halo. Spots resemble insect feeding damage, mechanical damage, herbicide injury – halo is diagnostic. If lesions expand (5 x 7 mm), halo may disappear. Blighting of leaves and tip dieback. Severely affected fields appear blighted as leaves killed prematurely – bulbs may be small.
Management: remove/destroy cull piles and debris (inoculum sources); rotate out of Allium crops for >2-3 years; avoid extended overhead irrigation; protective fungicide applications may provide some control – consult PICOL for products registered for use in Washington at:http://picol.cahe.wsu.edu/labels/Labels.php
Bacterial soft rot
Host plants: most Allium spp., other vegetables (carrot, celery, potato, etc.)
Favorable conditions: mechanical injury, bruises, insect injury, & sunscald promote infection. Mature bulbs infected in field, transit, or storage, if necks not cured before harvest, or grading and storage procedures not followed. Necks infected through injured or dying leaves. Warm, humid conditions (68-86°F) favor infection. Heavy rains before harvest. Infection continues in storage if >37°F.
Symptoms: fleshy onion scales become spongy, water-soaked, pale yellow to light brown/gray. Interior of bulb breaks down to watery, foul-smelling, sticky mass inside dry outer scales. Liquid oozes from neck if bulb is squeezed. Foliage wilts, turns white.
Management: mature tops adequately before harvest; avoid bruising during harvest/handling; reduce mechanical injury during cultivation/weeding; cure onions thoroughly before storing; store onions at 32°F and <70% RH with good ventilation to prevent moisture collecting on bulbs.
Host plants: onion
Favorable conditions: high rainfall, strong winds, hail, heavy irrigation, persistent dews. Susceptibility increases with bulb maturity. Wounds provide points of entry. High temperatures, slow drying of bulbs.
Symptoms: no external symptoms during early stages, except soft neck when pressed. Few inner fleshy scales soft and watersoaked when bulb is cut longitudinally. Rot moves from top to bottom of infected scales, then to adjacent scales. Eventually all internal scales rot, dry, and bulb shrivels. Field symptoms may appear as 1 or 2 wilted leaves in center of leaf cluster. Wilted leaves pale yellow, die back from tips, older and younger leaves stay green. Lifted bulbs soft and watery – squeezing base of infected plants causes rotten inner portion to slide out through the neck (= “slippery skin”).
Management: harvest onions at proper maturity as soon as tops lodge; prevent injury during harvest; dry bulbs promptly after topping; store bulbs at 32-36°F.
Yeast soft rot
Host plants: onion
Favorable conditions: wounds and openings in neck; fruit flies and other insects. 20-30°C optimum.
Symptoms: similar to bacterial soft rots (isolation, microscopic examination, and pathogenicity tests needed to differentiate from bacterial pathogens). Soft, watery rot of bulbs evident after harvest, confined to either inner or outer fleshy scales and not spread readily from scale to scale. When squeezed, fluid exudes from neck. Large portion of bulb may be affected. Rot around bruises on outer scales.
Management: implement sanitation practices to reduce survival of the yeast; avoid bruising/injury of bulbs during harvest/handling; use cultural practices that promote production of tight, dry necks; store and transport onions at reduced temperatures.
Host plants: wild and cultivated Allium spp.
Favorable conditions: extended cool, humid/wet weather.
Symptoms: older leaves with pale, elongate lesions (3-30 cm). Velvet, gray-violet sporulation of fungus on surface of leaf or seed stem during moist periods. Violet to purple lesions may be confused with early purple blotch. Lesions become pale yellow, necrotic, and collapse. Lesions on scapes circular to elongate; weakened scape may break, seed shrivel. Flowers may be infected, leading to seed-borne infection. Infection may be systemic; bulbs become soft and shriveled or remain firm and sprout prematurely, forming pale green foliage. Lesions often invaded byStemphylium or Alternaria spp.
Management: rotate out of Allium crops for 3 years; avoid poorly-drained soils; plant in the direction of prevailing winds to increase air circulation; avoid overhead irrigation; destroy debris and cull piles; eradicate volunteers and wildAllium spp.; foliar fungicide sprays provide protection – consult PICOL for products registered for use in Washington at:http://picol.cahe.wsu.edu/labels/Labels.php
Iris yellow spot
Host plants: onion, garlic, leek, iris, lisianthus; IYSV has NOT been found in Washington, but has been reported in eastern Oregon, Idaho and Utah.
Favorable conditions: transmitted by onion thrips, not western flower thrips. Not seed-borne nor in bulbs.
Symptoms: straw-colored ringspots (eyespots) on leaves and flower stalks (scapes). Lesions may turn necrotic, spindle-shaped on scapes, with distinct green island effect. Some lesions have concentric rings of alternating green and necrotic/chlorotic tissue. Lesions may girdle scape, drying umbel and reducing seed yield/quality. Infection may be symptomless. “Straw bleaching”, general yellowing, dieback (may be rapid). No infection of bulb, basal plate, or roots.
Management: select onion cultivars less susceptible to thrips; use virus-free transplants; control volunteers and culls.
Host plants: onion, shallot, leek, garlic, chives
Favorable conditions: prolonged rain, moderate temperatures; necks not cured well at harvest; uncured onions stored at warm temperatures/humidity.
Symptoms: primarily on stored onions. Decay in neck moves down through bulb. Neck soft and spongy, scales soft, water-soaked, translucent. White-gray mycelium between scales. Sclerotia may form on outer scales or shoulder of bulb. Decayed tissue sunken. Soil-line rot from penetration of outer bulb scales.
Management: ensure onion tops mature well; lift and undercut onions; cure onions adequately; minimize injury during topping, harvest, and handling; store onions in ventilated facilities at 32°F; for seed onions – bury culls and debris from nearby onion fields; avoid planting new crops within several miles or downwind of current seed crops; use fungicide seed treatments or hot water treatments – consult PICOL for products registered for use in Washington at:http://picol.cahe.wsu.edu/labels/Labels.php
Stubby root nematodes
Host plants: wide host range
Favorable conditions: 68-35°F soil temperatures; sandy, sandy-loam, organic soils favorable.
Symptoms: conspicuous 1st few weeks; seedlings stunted, yellow; plants may be killed. Root systems develop stubby branches, in clusters, as a result of loss of meristematic activity and no root cap from nematodes feeding on root tips. Injury to side of root close to tip retards growth and elongation on that side, causing curled root growth. Affected roots darker; no distinct lesions; secondary organisms may cause discoloration and necrosis of roots. 16-17 day life cycle at 86°F.
Management: avoid planting onions after mint; use preplant fumigation – consult PICOL for products registered for use in Washington at:http://picol.cahe.wsu.edu/labels/Labels.php
Host plants: onion
Favorable conditions: stressed/wounded plants; warm (75-82°F), little disease <60°F.
Symptoms: infected roots light pink, becoming deeper pink to red, then dark purple, semi-transparent, water-soaked, dry, disintegrated. New roots become infected. Symptoms of nutrient deficiency and drought stress; fewer and smaller leaves, which turn white, yellow, or brown from tips back, and die; plants uprooted easily; premature bulbing, smaller bulbs (shriveled). Pink to purple red blemishes on outer scales of transplants or bulbs of white cultivars; water-soaked areas on outer scales of yellow or red cultivars.
Management: practice 3-6 year rotations out of onions; promote rapid root growth with adequate fertilizer and irrigation; plant resistant or partially resistant cultivars; fumigation (particularly fall fumigation) controls the pathogen on mineral soils – consult PICOL for products registered for use in Washington at:http://picol.cahe.wsu.edu/labels/Labels.php.
Host plants: onion, garlic, leek, probably other Allium spp.
Favorable conditions: hot, humid; leaf wetness >12 hours; uncommon disease in Washington
Symptoms: older leaves more susceptible. Small (2-3 mm), water-soaked lesions with white centers, enlarge, become zonate and brown-purple with red/purple margin surrounded by yellow zone. Leaf yellows above and below lesions. Dark brown to black concentric rings in lesion = sporulation of fungus. Lesions may girdle leaves, which collapse and die. Similar symptoms on seed stalks, which may collapse, seed shrivel or do not develop. Lesions may be invaded by Stemphylium vesicarium and turn black from spores. Bulbs may be infected at harvest through the neck or wounds on fleshy scales. Bulb decay semi-watery, conspicuous yellow then red wine color. Scales desiccate to papery texture.
Management: use long rotations out of Allium crops; reduce hours of leaf wetness; destroy cull piles and bury debris; avoid injury to onions; cure bulbs adequately; plant resistant or tolerant varieties; fungicide sprays provide control when conditions are conducive – consult PICOL for products registered for use in Washington at:http://picol.cahe.wsu.edu/labels/Labels.php.
= P. porri
Puccinia asparagi(asparagus rust)
Host plants: Allium spp. (differentially infected)
Favorable conditions: high humidity, low rainfall, moderate temperatures (50–75°F), plant stress.
Symptoms: small white flecks on leaves and stems, orange-red uredia (1-3 mm) between veins. Heavily infected leaves yellow and die. Dark brown teliospores form in pustules later in season. Bulb size/quality reduced if infection severe.
Management: plant healthy seed in well-drained soils; crop rotation, planting away from other Allium crops, and control of Allium weeds/volunteers reduce disease pressure; foliar fungicide sprays provide protection under conducive conditions – consult PICOL for products registered for use in Washington at:http://picol.cahe.wsu.edu/labels/Labels.php.
Host plants: onion, shallot, chive, asparagus
Favorable conditions: high humidity, low rainfall, dew (>3 hr).
Symptoms: uredial stage infects leaves and seed stalks; light yellow-orange to red, powdery pustules. As pustules enlarge, leaves or seed stalks may be girdled; tissues above girdling yellow and die prematurely; seed yields reduced.
Management: destroy wild asparagus near onion fields.
Host plants: onion, garlic
Favorable conditions: cool soils with adequate moisture for root growth.
Symptoms: premature yellowing, wilting, and dieback of older leaves after fungus invades stem plate or bulb. Foliar symptoms confused with onion maggot damage. Seedling death uncommon. Stunting of plants, rapid death of all foliage. White, fluffy mycelium on stem plate extends around base of bulb, moves inward across fleshy scales. Soft rot around base of bulb, roots destroyed; plants readily pulled from ground. Masses of tiny (poppy seed-size) black sclerotia on mycelium and infected bulbs.
Management: use disease-free transplants; avoid moving soil infested with sclerotia into new fields by cleaning equipment with a pressure sprayer; fungicides can provide some control if soil temperatures (top few inches) remain >75°F for most of the growing season – consult PICOL for products registered for use in Washington at:http://picol.cahe.wsu.edu/labels/Labels.php