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Club root of crucifers - Plasmodiophora brassicae
Club root rot caused by Plasmodiophora brassicae Woronin occurs on crucifers, generally in cooler conditions. The roots of infected plants develop large swellings that later decay, often leading to the death of the plant. The disease occurs all year in the Da Lat region of the Lam dong Province and the Sa Pa region of the Lao cai Province. The disease can also occur in the winter and spring crops grown in the Red River Delta.
The first symptom of the disease is that plants will wilt during the hotter parts of the day. Swellings develop on the underground parts of the plant, including the taproot, feeder roots and underground parts of the stem. The swelling on affected roots are of variable shapes, often spindle or fist-like. The swellings are larger on the tap root and smaller on the secondary roots, initially firm in texture. If they are cut in half, they are white internally. Later, they shrivel, decay and become a dark brown, soft, smelly mass. Infected roots often decay before the end of the season, with the decay occurring rapidly after heavy rain. Decay usually begins in those parts of the root system deepest in the ground. Further wilting and collapse of the plants occurs upon decay of the enlarged roots.
Root knot nematodes cause similar symptoms, but the swellings are usually smaller and more evenly distributed in the lateral feeding roots.
Description of the Pathogen
Plasmodiophora brassicae is a fungal-like organism in the Plasmodiophoramycetes in the Kingdom Protozoa. It is closely related to the slime moulds and is an obligate endoparasite. The main body of the pathogen is called a plasmodium, that produces zoospores in sporangia in free soil water.
Distribution within Vietnam
The disease occurs all year in the Da Lat region of the Lam dong Province and the Sa Pa region of the Lao cai Province. The disease can also occur in the winter and spring crops grown in the Red River Delta. The distribution of the disease is limited to rgions with cool climates.
Plants in the crucifer family are susceptible to this fungal disease, especially cabbage, but also cauliflower, broccoli, kohlrabi, Chinese cabbage, mustard and tobacco.
The pathogen is most active in cool, wet, acidic soils. It enters the roots through hairs or injured areas. Upon decomposition of the infected roots, spores are disseminated through the soil and the organism is spread when these soils are moved. This may occur with infected seedlings when transplanted, in soil with cultivation equipment and in water movement. Once introduced into the soil, it can remain there indefinitely. The pathogen is favoured by high soil moisture, and temperatures between 18 and 25°C.
In free soil water, resting spores germinate to produce a biflagellate zoospore. These motile spores are responsible for penetrating the root tissue and forming plasmodia in the host cytoplasm. Plasmodia produce zoosporangia which produce 6-8 zoospores. The secondary zoospores reinfect the host to give rise to secondary plasmodia in the cortical cells. It is at this stage that the characteristic club root symptoms are evident. The plasmodia then form individual resting spores that, upon decomposition of the host root tissue survive in the soil for 7-15 years.
Ensure that nursery soil is effectively pasteurised or sterilised prior to raising seedlings. Seedlings themselves should be examined for symptoms prior to transplanting, especially if bought externally.
A 3-4 year break from growing cruciferous crops will reduce losses. The removal of weeds belonging to the mustard family will make this rotation more effective in reducing disease severity.
Liming can also reduce the severity of the disease. The amount required depends on the soil pH, with more applied to acid soils. Applying 4t/ha is a common rate, but a trial in an affected area is best to determine if the lime will have any affect on the disease severity. The lime is best applied at least six weeks before planting in order for the pH to change significantly. In order for the lime to be mixed thoroughly into the soil, it is best applied in late summer when the soil is driest. Hydrated lime may be easier to mix than other forms. In any case, the soil pH needs to be raised to 7.3. Adding too much lime will interfere with the uptake of important nutrients into the plant, so it is best to check the pH before reapplication in the following year. The suitability of a higher pH must also be considered with the other crops included in the rotation.