CRUCIFEROUS VEGETABLES - FUNGI

Causal organisms: various species of Pythium, Rhizoctonia, Fusarium, Alternaria and other fungi.

Host range:
All these species are rather polyphagous - with some exceptions all vegetable and ornamental species, sugar beet and some weeds can be host plants.

Occurrence and importance:

The disease occurs wherever vegetable or sugar beet are grown, but serious damage occurs only under unfavourable conditions. For Cruciferous vegetables it is mainly important during growth of transplants for early cultures.

Symptoms and confusable diseases: Typical symptoms are softening and darkening of hypocotyle and root tissues of newly-emerged plants. The affected plant either suddenly collapses or gradually yellows - in this case plant and root system development is very poor. Preemergence attacks result in darkening of tissues and failure of the plant to emerge. Tissue can be covered by mycelial growth of various colours.
The symptoms caused by all these pathogens are very similar and it is rather difficult to differentiate them.

Causal organism: Alternaria brassicae, A. brassicicola

Host range: All cultivated Cruciferous species and some weeds.

Occurrence and importance: The occurrence of this disease is rather frequent but direct losses are usually not important with the exception of Chinese cabbage. Alternaria species can produce mycotoxins. The disease is widespread throughout Europe.

Causal organism: Verticillium dahliae, Verticillium longisporum (and Verticillium dahliae var. longisporum) Verticillium albo-atrum.

Host range: All plants except cereals and rice are hosts of Verticillium wilt. All Crucifers are hosts of Verticillium pathogens and many weeds too.

Occurrence and importance: Verticillium wilt is one of the most destructive and economically - significant plant diseases worldwide. The economic impact can be severe. Verticillium wilt is a typically fatal disease that affects various vegetable plants, including cabbages. It remains dormant in the soil for several years and, when it enters the canopy due to extreme changes in temperature and precipitation, it causes mass mortality. The disease is less contagious in cool, wet conditions, so it is not significant in Poland and mountainous areas. The appearance of the pathogen may be observed in extreme drought periods, after rainfall or irrigation. 

Symptoms and confusable diseases:  Initial symptoms of Verticillium infection include a change in colour at the edge of foliage from green to yellow, that initially appears on the one side of the plants. Infected leaves wilt, dry out and die. Soft rot at the base of the stem and root tissues causes the infected leaves to dry. The vascular tissue appears as a dark ring in cross section.

Disease cycle: Verticillium spp. are soilborne pathogens. Susceptible plants and plant debris can serve as an overwintering site for these fungi. Verticillium pathogens can survive long periods in soil by producing microsclerotia in senescent tissue. Infection begins in the root area where the resting hyphae of Verticillium germinate and penetrate feeder roots. The pathogen also can enter wounds in the root. The disease spreads within the plants by mycelium or microconidia that travel through the xylem vessels to other parts of the plants. The affected tissue becomes necrotic, dark brown (dead) because the vascular tissue is clogged with mycelium, conidia and by products of fungal metabolism. As a result, water flow is restricted and the plants wilt. The necrotic tissue causes the dark streaks that are symptomatic of this disease.

Control:

• Fungicides are not effective in controlling Verticillium;
• Appropriate fertilization based on soil analysis;
• Watering during periods of drought;
• Weed control is helpful since the fungus can infect a variety of weeds;
• Soil treatments such as fumigation;
• Soil solarization, to reduce or eliminate of soilborne pathogen inoculum, including Verticillium. This method could be an option if soil can be heated at significant depths;
• Soil biosolarization with different organic amendments, including dry plant residues;
• Biocontrol by antagonistic fungi, bacteria, actinomycetes;
• Biocontrol by endophytes such as Trichoderma

Causal organism:  Leptosphaeria maculans, (syn. Plenodomus lingam, anamorph: Phoma lingam) and Leptosphaeria biglobosa (syn. Plenodomus biglobosus)

Host range:  These pathogens can infect many Crucifers. Host range includes Brassica species (cabbage, cauliflower, broccoli, Brussels sprouts, collards, canola, kale, turnip, etc.), Sinapis (yellow mustard and white), and Raphanus (daikon and radish) and Cruciferous weeds.

Occurrence and importance: Black Leg is a destructive disease of Brassica crops in most parts of the world and can reduce seed and vegetable yields. Typical present in countries with cool, wet weather.

Symptoms and confusable diseases: The earliest symptoms of the black leg are slight spots on stems at cotyledon scars, which elongate in larger areas, turn brown with a black to purplish halo and become sunken. Formed lesions extend up and down the stems. The stems become blackened and girdled. Many fruiting bodies (pycnidia) are formed there. Lesions can develop below ground on roots. Badly affected seedlings may wither and die. In field crops the plants may be attacked at all stages of growth. Symptoms are irregular pale spots developing on leaves and stems. Later initial spots become oval to circular, grey-brown coloured with scattered fruiting bodies. Typically stem cancer develops at the soil line after the pathogen grows systemically down from leaf spots. There are usually cancer lesions elongated with purple-dark borders. Later, the roots die from below and turn black - the origin of the name “Black leg”.

Disease cycle: The pathogen survives with plant debris as pycnidia or/ and pseudothecia, in living plants as pycnidia and in infected seeds.  Ascospores may release from pseudothecia at wet or humid conditions with mean temperatures between 8° - 15° C. Ascospores may be windborne long distances and infect through stomata or via wounding of plants by herbicides, insects and mechanical means, which can increase the severity of disease. Pycnidia produce asexual spores, named pycnidiospores, or conidia, and can be found on infected live plants. Conidia are a secondary infective form of the pathogen, that are dispersed by irrigation water or splashing rain. Their movement is limited to a short distances. The process of infection by conidia is similar to that of the ascospores, at temperatures between 5°- 25° C and high relative humidity.

Control:

• Planting seeds certified free of Leptosphaeria;
• Hot water seed treatment for 25 min at 50° C for cabbage and 20 min for cauliflower.
• Prevent pest damage;
• Rotate out of crucifers for at least three years.
• For organic production, removal of affected leaves shortly after leaf spots develop can reduce secondary spread within a planting, as well as the incidence of stem cankers;
• Fungicide seed treatment;
• Fungicide with a different mode of action applied during vegetative growth .

Causal organism:  Plasmodiophora brassicae

Host range: All cultivated species and most of weeds from the family Cruciferaceae can be infected.

Occurrence and importance: Clubroot occurs very frequently in areas where cole crops are cultivated. In heavy infections 100 % loss may occur. The importance of the disease is also the longevity of soil contamination. Based on the EPPO Database map (Fig. 3), Plasmodiophora brassicae is present in all INPACT partner countries (Polland, Czech Republic, Greece, Bulgaria, Romania and Hungary). Due to the emerging extreme rainfall patterns, on different soils (but mainly on heavy soils) we should expect an epidemic of Clubroot, and farmers should take preventive action.

Symptoms: Typical symptoms develop on the roots, which are irregularly galled and the root system is stunted. Galls are white initially at the beginning but later become brown to black and decompose relatively rapidly. Galled and clubbed roots are often invaded by secondary organisms such as soft rot bacteria; this results in the rapid decay of roots, the further decline of infected plants, and the release of additional inoculum into the soil. Later symptoms are stunting of the above-ground part of plants which becomes blue-greyish colour and wilts. The edible parts, if developed at all, are of poor quality.
Similar symptoms on above-ground parts can be also caused by larvae of the turnip gall weevil Ceutorhynchus assimilis (=pleurostigma), but in this case galls are relatively smooth and round with yellowish larva inside.

Disease cycle: The pathogen survives for a long time (several years) in the form of thick-walled resting spores in soil. In the presence of host plant roots, these resting spores germinate by releasing swimming zoospores, which infect and colonize roots. Under favourable conditions it multiplies rapidly and spreads in the form of secondary zoospores. Acid pH and high soil humidity favour the pathogen but infection also occurs above pH 7.0. Within a field, the pathogen spreads slowly by means of zoospore movement but rapidly during soil cultivation. Spread to other fields can occur by any transport of soil containing resting spores: water and wind erosion, machinery, transplants, etc.

Control: This pathogen is difficult to control. A complex system of measures should be used:

• resistant hybrids
• crop rotation (include cereals, if possible, avoid Brassicaceous fodder and green manure crops)
• neutral reaction of soil (lime application if pH is under 7.2)
• liquidation of infected plants (not in the compost!!!)
• avoid transport of contaminated soil outside of the field on machinery, other plants etc. do not use tailwater from contaminated fields to irrigate non-infested fields because the pathogen can be transported in water
• avoid direct sowing of seeds into contaminated soil but use containerized transplants, healthy transplants
• use of "enemy" plants in hearts of contamination (e.g. mint),
• fungicide soil treatment.