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Fungal diseases in vegetable crops

Fungal diseases - an overview:

This page provides an overview of the fungal diseases in vegetable crops. The related tools listed at the end of the page provided detailed information about identification, symptoms, and management of fungal diseases. It is important to have a plant diagnostics laboratory confirm the pathogen causing any diseases in a crop so that the disease can be appropriately managed.

Fungi constitute the largest number of plant pathogens and are responsible for a range of serious plant diseases. Most vegetable diseases are caused by fungi. They damage plants by killing cells and/or causing plant stress. Sources of fungal infections are infected seed, soil, crop debris, nearby crops and weeds. Fungi are spread by wind and water splash, and through the movement of contaminated soil, animals, workers, machinery, tools, seedlings and other plant material. They enter plants through natural openings such as stomata and through wounds caused by pruning, harvesting, hail, insects, other diseases, and mechanical damage.

Some of the fungi are responsible for foliar diseases – Downy mildews; Powdery mildews; and White blister are some of the highly prevalent foliar diseases. Other fungi – Clubroot; Pythium species; Fusarium species; Rhizoctonia species; Sclerotinia and Sclerotium species – are soilborne diseases.

Some fungal diseases occur on a wide range of vegetables. These diseases include Anthracnose; Botrytis rots; Downy mildews; Fusarium rots; Powdery mildews; Rusts; Rhizoctonia rots; Sclerotinia rots; Sclerotium rots. Others are specific to a particular crop group, e.g. Clubroot (Plasmodiophora brassicae) in brassicas, Leaf blight (Alternaria dauci) in carrots, and Red root complex in beans.

Common fungal diseases and crops affected:

Some examples of common fungal diseases of vegetable crops are provided in the table below with some typical symptoms.

Fungal disease

Factors conducive to spread

Crops affected

 Symptoms
 White blister/White rust (Albugo candida) Optimum conditions for disease development are 3-4 hours in mild temperatures (6- 24?C). Brassicas (including Asian leafy brassicas). White blisters and swellings on leaves and heads of affected plants; blisters consist of masses of white dust-like spores; up to 100% losses have been reported.
Downy mildews (individual species damage particular crop families) High humidity, leaf wetness and cool to mild temperatures (10-16 °C). Wide host range including onions; peas; lettuce; celery; spinach; kale; herbs; cucurbits; brassicas; Asian leafy brassicas. Symptoms usually begin with yellowish leaf spots which then turn brown; downy growth appears on underside of leaves.
 Powdery mildews (some species are restricted to particular crops or crop families) Moderate temperatures (20-25?C); relatively dry conditions (unlike downy mildews). Wide host range and very common, especially in greenhouse crops: cucumber; melons; pumpkin; zucchini;parsnip; beetroot; potato; herbs; peas; bitter melon;tomato; capsicum; Brussels sprouts; cabbage; swedes. Small, white, powdery patches on most above-ground surfaces; usually observed first on undersides of leaves but eventually cover both surfaces; affected leaves become yellow, then brown and papery and die.
Clubroot (Plasmodiophora brassicae) Warm weather; acidic soil (pH less than 7); high soil moisture. Brassicas (including Asian leafy brassicas). Plants are yellow and stunted and may wilt in hotter parts of the day; large malformed ‘clubbed’ roots which prevent the uptake of water and nutrients, reducing the potential yield of the crop.
 Pythium species Cold, wet soil conditions; known as water moulds, they enter untreated water supplies; water supplies for irrigation and hydroponics should be tested regularly. Many vegetable crops in including cucurbits; brassicas; lettuce. May kill seedlings, which die before they emerge or soon after emergence; plant collapse.
Sclerotinia rots (S. sclerotiorum and S. minor) – a range of common names are used Windy, cool, humid weather; wet soil; survival structures known as sclerotia remain viable in soil for long periods (10-15 years). Most vegetable crops. Water-soaked rotting of stems, leaves, and sometimes fruit; followed by a fluffy, white and cottony fungal growth which contain hard black pebble-like sclerotia.
Sclerotium rots (Sclerotium rolfsii and S. cepivorum) S. rolfsii – Warm, moist conditions. S. cepivorum –Development is favoured by cool soil conditions (14-19?C) and low moisture. S. rolfsii – Wide host range including: beans; beets; carrot; potato; tomato; capsicum; cucurbits. S. cepivorum – only affects onions, garlic and related Alliums (shallots; spring onions; leeks). S. rolfsii – Lower stem and root rots; coarse threads of white fungal growth surround the diseased areas; small brown fungal resting bodies. S. cepivorum – Yellowing and wilting; fluffy fungal growth containing black sclerotia forms at the bases of bulbs.
 Fusarium wilts and rots (Various Fusarium species including F. solani and F. oxysporum) Warm to hot weather. Wide host range including: brassicas; carrots; cucurbits;onions; spring onions; potato; tomato; herbs; peas; beans. Causes severe root and crown rots or wilt diseases by attacking roots and basal stems; cucurbit fruit and potato tubers can be affected in storage.
 Botrytis rots – for example Grey mould (Botrytis cinerea) Cool, wet weather. Celery; lettuce; beans; brassicas; cucumber; capsicum; tomato. Softening of plant tissues in the presence of grey fungal growth.
 Anthracnose (Colletotrichum spp. except for in lettuce – Microdochium panattonianum) Cool, wet conditions. Wide range of crops including: lettuce; celery; beans; cucurbits; tomato, capsicum; potato; globe artichoke. Typical symptoms begin with sunken and water-soaked spots appearing on leaves, stems and/or fruit.
Rhizoctonia rots (Rhizoctonia solani) – range of common names, e.g. Bottom rot (lettuce) and Wire stem (Brassicas) Warm, humid weather; can survive for long periods in the soil in the absence of a host plant. Wide host range including: lettuce; potato; brassicas;beans; peas; beets; carrots; capsicum; tomato; cucurbits. Range of symptoms depending on the crop being grown but can affect roots, leaves, stems, tubers and fruit; plants wilt and may collapse and die.
Damping-off (Pythium, Rhizoctonia, Phytophthora, Fusarium or Aphanomyces) Occurs under cold, wet soil conditions; shore flies and fungus gnats can spread Pythium and Fusarium. Many vegetable crops including: leafy vegetables;brassicas; carrots; beetroot; cucurbits, eggplant; tomato;coriander; spring onions; beans Young seedlings have necrotic stems or roots; seedlings die or show a reduction in growth.
Cavity spot (Pythium sulcatum) Growing carrots after carrots; acidic soil; not harvesting carrots as soon as they reach marketable size. Carrots. Cavity spots are small elliptical lesions often surrounded by a yellow halo.
Tuber diseases (Various species) Potato and sweetpotato. Potato tubers may be infected with superficial skin diseases, such as common scabs, powdery scab, and Rhizoctonia. Sweetpotatoes may be infected by scurf.
 Rusts (several species, e.g. Puccinia sorghi – sweet corn; Uromyces appendiculatus – beans; Puccinia allii – spring onions). Wind can spread spores great distances; favoured by low rainfall, 100% relative humidity and cool to mild temperatures. Sweet corn; beans; onions; spring onions; beets; celery; silverbeet; endive. Small, red or reddish-brown pustules that form on the underside of the leaves and sometimes on the pods as well; dusty reddish-brown spores released from pustules (may be black in cold weather).
Black root rot (Different species on different vegetable crops) Cool soil temperatures; high soil moisture. Lettuce; beans; cucurbits. Blackening of roots; stunted plants; plants may die.

Other fungal diseases of vegetables include:

  • Target spot – Alternaria solani (tomatoes)
  • Aphanomyces root rot – Aphanomyces euteiches pv. phaseoli (beans)
  • Aschochyta collar rot (peas)
  • Gummy stem blight – Didymella bryoniae (cucurbits)
  • Alternaria leaf spot – Alternaria cucumerina and A. alternata (cucurbits)
  • Black leg – Leptosphaeria maculans (brassicas)
  • Ring spot – Mycosphaerella brassicicola (brassicas)
  • Late blight – Septoria apiicola (celery)
  • Cercospora leaf spot – Cercospora beticola (beets)
  • Leaf blight – Septoria petroelini (parsley)
  • Septoria spot – Septoria lactucae (lettuce)
  • Leaf blight – Stemphylium vesicarium (spring onions)
  • Leaf blight – Alternaria dauci (carrots).

Management:

Integrated Crop Protection (ICP) or the Integrated Pest Management (IPM) approach has achieved success in the management of the fungal diseases. ICP considers the production system as a whole, including all pests and the importance of soil health. It requires a good understanding of the fungi; the periods during which the crops are susceptible; and the influence of environmental conditions.

Tips for managing fungal diseases include:

  • Understand the lifecycles, survival mechanisms, and conducive environmental conditions for fungi
  • Be committed to farm sanitation – clean up your farm and remove all weeds, crop debris, and volunteer hosts
  • Use resistant or tolerant varieties
  • Use clean transplants and seed (and seed treatments)
  • Monitor weather conditions (particularly temperature, humidity, and leaf wetness)
  • Have knowledge of relevant disease prediction models
  • Understand the implications for irrigation timing and minimise free moisture and high humidity periods (e.g. irrigating at around 4 am rather than at dusk, not irrigating during peak periods of spore release)
  • Appropriate crop rotations (long rotations with non-host crops may be necessary)
  • Avoid heavily infested blocks by testing soil for soilborne diseases prior to planting
  • Monitor crops regularly and be able to detect early symptoms on your crop
  • Amend and manage soil to disadvantage the fungi (some fungal diseases can survive in the soil for 30 years or more)
  • Minimise ways in which the disease can spread on-farm – remove and destroy sick plants when symptoms first show
  • Understand the influence of planting time, plant spacing and overlapping crops
  • Apply preventative fungicides based on weather conditions
  • Understand fungicide resistance and rotation of chemical groups.

Source of information and related tools: