HANDOUTS!!! (on memory stick) What lurks beneath? Soil borne Diseases of Vegetables CLUBROOT Clubroot Plasmodiophora brassicae Host Crops Cruciferous (Brassica) crops Canola, Cole crops (cabbage, broccoli, cauliflower, rutabaga, etc) Cruciferous weed species (stinkweed, wild mustard, flixweed, etc) Resting spores are viable for up to 18 years Disseminated by: drainage water with soil that clings to seed, equipment, shoes, tools, and transplants in contaminated manure in contaminated irrigation water Photo by R. Howard Infected Chinese Cabbage Root galls on Chinese Cabbage Resting spores germinate in the presence of susceptible roots and water Often more severe on wet, acidic soils Soil temperatures of at least 16 21 C are required for resting spore germination Photo by R. Howard 1
Ensure rigorous sanitation is practiced Ensure that all equipment or personnel leaving an infected field are free from potentially contaminated soil Do not remove straw or plant material from an infected field and spread it elsewhere Minimize soil erosion Ensure good soil drainage Maintain a higher soil ph (above 7), with ph adjustments made by liming the soil Follow a 5 to 7 year rotation between cruciferous crops Control cruciferous weeds Avoid applying contaminated manure (animals fed contaminated cull plants or pastured in infected crops) Avoid using contaminated irrigation water Use only healthy transplants for spring planting A transplant fungicide application may help to prevent infection follow label instructions Sclerotinia Rot (White Mold) SCLEROTINIA ROT / WHITE MOLD Sclerotinia sclerotiorum Hosts Carrots, lettuce, celery, beans, cole crops, potatoes, peas, cucurbits, solanaceous crops, various weed species (e.g. cruciferous) A.K.A Lettuce Drop, Cottony Soft Rot Pathogen causes a range of diseases in various crops Disease of field and storage Most serious disease of stored carrots Head rot of cabbage Photo by R. Howard White mold of beans Photo by P. Northover 2
White mold of carrots Sclerotia Soil borne sclerotia (hard mycelial bodies) produce mycelium, which infect plant tissues Sclerotia can also produce spores, which can infect weak, damaged or dying tissues in high and prolonged humidity conditions Spread occurs between carrots in storage Storage infections develop from field infected tissues Dirty pallet boxes can also cause infection Temperatures 13 18 C are optimum (but can develop above 0 C) Free moisture & RH greater than 92% favour disease Photo by P. Northover Maintain a 3 5 year rotation If possible, remove infected debris from growing areas Trimming canopy / Control weeds Improves air flow & reduces leaf wetness & RH Weeds = alternate hosts Ensure good air circulation and good soil drainage Rapidly cool harvested crops & store crops such as carrots at constant 0 C Ensure storage areas and containers are properly sanitized, to remove any sclerotia that may adhere to surfaces Preventative chemical sprays are available to protect various crops Damping Off DAMPING OFF Causal agents: Fusarium spp. Pythium Rhizoctonia Soil borne pathogens Often occur in complex Very wide range of host crops Young plants most susceptible Associated with poor growing conditions (cool, wet, etc.) 3
Lesions develop on seedling at soil level or below Plants may fail to emerge or may be stunted, chlorotic or slow growing Stems become girdled & collapse Plants wilt and fall over Appears to happen overnight Symptoms Tissues become soft, spongy and water soaked Fusarium spp. Pinkish masses of spores Internal discolouration of tissues Pythium spp. Water soaked lesions or grey white mould Brown to black root rot Rhizoctonia No spores, but otherwise resembles Fusarium Tends to manifest at cooler conditions May have brownish sunken lesions Ensure plants are growing in good conditions for rapid, healthy growth Avoid cool and wet soil conditions (if possible) Avoid overwatering or waterlogging/poor drainage Use raised beds (encourages improved drainage) Water to ensure plants dry off early in the day Maintain adequate crop rotations Use treated seed or post planting bed treatments RHIZOCTONIA 4
Rhizoctonia Rhizoctonia solani Hosts Wide range of host crops Potatoes Cruciferous vegetables Other Causes a range of diseases Damping off Wirestem (cruciferous veggies) Black Scurf (potatoes) Crown/Crater Rot (root crops) Bottom Rot (cabbage) Rhizoctonia canker (potatoes) Wirestem in Cole crop Photo by R. Howard Black Scurf Potatoes may fail to emerge Aerial tubers may be visible on infected stems Reddish brown & sunken root / stem / stolon cankers Cankers can kill sprouts & girdle stems Photo by R. Howard Superficial dark brown to black, irregularly shaped sclerotia on tuber surfaces Photo by R. Howard The dirt that doesn t wash off Crown / Crater Rot in carrots Photo by OMAFRA Sclerotia DO NOT penetrate the tuber 5
Disease Cycle / Conditions Favouring Disease Overwinters as sclerotia on tubers and in the soil Sclerotia germinate and mycelium infect tissues No spores are produced Cold and wet soils Short rotations Does not develop/spread in storage Ensure minimum 3 year rotations Rotate with grasses and cereals Plant clean seed tubers Seed treatments can control seed borne inoculum, but will not protect shoots from soilborne inoculum Top killing and early harvesting can reduce scurf development on tubers Stem/stolon cankers Plant into soil and in conditions which favour rapid emergence and good plant growth Avoid cool soils Warm seed Seed shallow and hill after emergence Delay irrigation until after sprouts emerge Avoid overwatering COMMON SCAB Common Scab Streptomyces scabies Fungus like bacterium Soil borne disease Hosts Potatoes Various root vegetables Carrots, beets, parsnips, rutabagas, turnips, radishes Disease Cycle Persists in the soil for 8 10 years Typically infects tubers during the 1st 5 weeks of tuber development Via the lenticels Colonizes several layers of cells Does not spread or increase in storage 6
Lesions vary in form Shallow and superficial Underground stems & stolons may also be affected Photo by inspection.gc.ca Round, irregular, brown lesions on the tuber surface Typically less than 1cm across Initially form around lenticels Deep & pitted Photo by inspection.gc.ca Raised & erupting Almost impossible to distinguish from Powdery Scab Raised, corky lesions Pitted scab Various other looks for scab (caused by the same organism) Netted scab Severity of symptoms determined by: Strain of pathogen Potato cultivar Soil organic matter Crop rotation Environmental conditions Moisture availability Photos by inspection.gc.ca Warm, dry conditions at or after tuber formation can increase incidence Reduces levels of antagonistic bacteria Soil ph can influence scab formation Prefers soil ph of 5.5 7.5 Use less susceptible cultivars There is a wide range of scab resistance between varieties Generally russet types = more resistant than smooth skinned types See Handouts for Variety Resistance Tables 7
Minimum 4 year crop rotation between host crops Maintain adequate & uniform levels of moisture, especially around tuber development Alter soil ph Difficult to accomplish, as soil buffers and pathogen and host both reduced at lower or higher ph levels Avoid introducing pathogen Use clean seed Avoid applying manure from animals fed scabby produce Seed treatments can provide some control of tuber borne scab Does not provide protection for daughter tubers ONION BULB ROTS BOTRYTIS NECK ROT Botrytis Neck Rots Botrytis species (B. aclada, B. byssoidea, B. squamosa) One of most common storage diseases of onions Hosts Onion Shallots Leek Garlic Chives Definite margin Cooked appearance Photo by P. Northover 8
Fungal growth evident in neck area of stored onions White mycelium Sclerotia Photo by P. Northover Photo by P. Northover Overwinter as sclerotia on cull piles, onion debris, unharvested bulbs and soil Germinate under moist conditions Spread is rapid during wet seasons and ripening and harvest periods Injury (bruising / damage) during harvest can increase incidence Does not spread bulb to bulb Remove sources of inoculum such as cull piles, unharvested crops Maintain minimum 2 year rotations Keep fields separate from each other Ensure proper curing of mature bulbs Grade out damaged, bruised or thick necked onions before storage Store onions at 0 C and 65 70% RH FUSARIUM BASAL PLATE ROT 9
Fusarium Basal Plate Rot Symptoms Fusarium oxysporum f.sp. cepae Onions and everything in the Allium genus Can cause losses in field and storage Lasts in the soil as chlamydospores (sturdy resting spores) Spread by soil, water or air Infection through roots or wounded / weakened tissues Initially Tip dieback and yellowing of the leaves Foliage withers and dies Infected plants pull easily out of the ground Plants often appear lopsided All or parts of the basal plate may be affected Symptoms Roots rot Rot may have a pinkish brown colour White mycelium may develop on rotted areas in moist conditions Cut bulbs will have a discoloured basal plate, starting outer layers, moving upward Wounded tissues Temperatures between 15 C & 30 C High soil temperatures Adequate moisture In storage, temperatures between 20 C & 30 C Allows for rapid disease development High relative humidity Maintain a minimum 3 year rotation Start with disease free plant material Some cultivars are considered to be resistant Ensure soils are well drained Keep crops stress free Cure bulbs before storage Tighten necks Cull out infected bulbs Store onions below 15 C to slow disease development 0 C and relatively dry (65 70% RH 10
Fusarium Dry Rot FUSARIUM DRY ROT Fusarium sambucinum; F. solani var. coeruleum; F. avenaceum Affects potatoes Doesn t spread between tubers in storage Carried over in contaminated soil or in infected tubers Requires a wound or entry point for infection Initial = darkened, shallow lesions on surface Tissues become sunken and wrinkled, with concentric rings of discoloured tissues Early (wet) stage of FDR Early (wet) stage of FDR gradual cavity rot Gradual development of internal cavity rot Gradual development of internal cavity rot tissue collapse 11
Photo by OMAFRA Avoid wounding during planting, harvest and post harvest Ensure good skin set prior to harvest Encourage wound healing through curing Store tubers are low temperatures to slow disease development Tissues collapse and dry up, with fungal development on surfaces The surface of the tuber may appear sunken Bacterial Soft Rot BACTERIAL SOFT ROT Pectobacterium carotovora subsp. carotovora; P c subsp. atroseptica; other species (Bacillus, Clostridium, Flavobacterium, Pseudomonas) Wide range of root, fruit and leafy vegetables Often occurs as secondary rot Spread in water, etc. Requires a wound Symptoms picture and description Typically occurs on tubers, fruit or roots Small water soaked lesions or brown sunken lesions around a wound or entry point Quickly enlarge and coalesce Tissues become watery and mushy Tissues may become brown, slimy and stinky Sharp delineation between healthy and diseased tissues Some oozing and leaking out onto other healthy product Wounded / weakened tissues Overripe or immature tissues = more susceptible High temperatures Moist conditions Soil Humidity Poor ventilation & high humidity / wet tubers in packaging Frozen produce or harvesting under wet conditions 12
Grow in well drained soils Maintain adequate crop rotation Control diseases/insects that create wounds or weak points Harvest only mature products Minimize bruising and wounding during harvest and post harvest activities Store crops as cool as possible Properly disinfect wash water (change regularly) Disinfect equipment and storages periodically HANDOUTS!!! (on memory stick) Rob Spencer, BSA, MSc, P.Ag. Commercial Horticulture Specialist QUESTIONS??? robert.spencer@gov.ab.ca Alberta Ag Info Centre 310 FARM 13