| Cabbage Clubroot | |
|---|---|
| Scientific classification | |
| Kingdom: | Rhizaria |
| Phylum: | Cercozoa |
| Class: | Phytomyxea |
| Order: | Plasmodiophorales |
| Family: | Plasmodiophoraceae |
| Genus: | Plasmodiophora |
| Species: | P. brassicae |
| Binomial name | |
| Plasmodiophora brassicae Woronin, 1877 |
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Cabbage Clubroot is a disease of Brassicaceae (mustard family or cabbage family) caused by the soil-borne slime mould Plasmodiophora brassicae.[1] The disease first appears scattered in fields, but in successive seasons it will infect the entire field, reducing the yield significantly and sometimes resulting in no yield at all. Symptoms appear as yellowing, wilting, stunting, and galls on the roots. It is transmitted by contaminated transplants, animals, surface water runoff, contaminated equipment, and irrigation water.[1] The pathogen can survive in a field for years as resting spores without a host present and will infect the next crop planted if it is a susceptible host. This pathogen prefers a wet climate and a pH around 5.7, so proper irrigation and the addition of compounds that raise the pH can be used to control this disease.[2] Other control methods include sanitation to prevent transmission, chemical control, and resistant varieties.
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Cabbage Clubroot affects cabbage, Chinese cabbage, and Brussels sprouts most severely, but it has a range of hosts that it affects less severely like kohlrabi, kale, cauliflower, collards, broccoli, rutabaga, sea kale, turnips, and radishes.[3]
Developing plants may not show any symptoms but as the plants get older they will start to show symptoms of chlorosis or yellowing, wilting during hot days, and exhibit stunted growth. Below ground, the roots experience cell proliferation due to increased auxin or growth hormone production from the plant as well as the pathogen.[2] This causes the formation of galls that can grow big enough to restrict the xylem tissue inhibiting efficient water uptake by the plant. Galls appear like clubs or spindles on the roots. Eventually the roots will rot and the plant will die.[3]
In the spring, resting spores in the soil germinate and produce zoospores. These zoospores swim through the moist soil and enter host plants through wounds or root hairs. A plasmodium is formed from the division of many amoeba-like cells. This plasmodium eventually divides and forms secondary zoospores that are once again released into the soil. The secondary infection by the zoospores can infect the first host or surrounding hosts. These secondary zoospores can be transmitted to other fields through farm machinery or water erosion. They form a secondary plasmodium that affects plant hormones to cause swelling in root cells. These cells eventually turn into galls or “clubs”. The secondary plasmodium forms the overwintering resting spores which get released into the soil as the “clubs” rot and disintegrate. These resting spores can live in the soil for up to 20 years while they wait for a root tip to come in close proximity for them to infect.[4]
Cabbage Clubroot is a disease that prefers warmer temperatures and moist conditions. Ideal conditions for the proliferation of this disease would be a soil temperature between 20-24°C and a pH less than 6.5; Therefore, this disease tends to be prominent in lower fields where water tends to collect.[4]
Cabbage clubroot is very hard to control. The primary step for management and long-term control is exclusion of the disease. Good sanitation practice is important with regard to the use of tools and machinery in order to prevent the introduction of the pathogen to a disease-free field. It is not uncommon for an inattentive farmer or gardener to unknowingly carry in the pathogen after being previously exposed to it at a different time. Additionatlly, one should avoid purchasing infected transplants of cabbage so as to prohibit the infestation of P. brassicae. Soil type is also an important factor in the development and spread of cabbage clubroot; the use of sand will allow for the plants to grow in well-drained soil, thereby eliminating the possibility of the pathogen to proliferate in a hospitable environment.[1]
Although it is difficult to eradicate the pathogen once it is introduced to a field, there are several methods for its control. Keeping the soil at a slightly basic pH of 7.1-7.2 by the addition of agricultural lime as well as the integration of crop rotation will reduce the occurrence of cabbage clubroot in already infected fields. Fumigation using metam sodium in a field containing diseased cabbages is yet another way to decrease the buildup of the pathogen.[5] Control and management practices on already infected fields help to reduce the overall impact that P. brassicae has on a field of cabbage and other cruciferous plants, but it is extremely difficult to rid a singular plant of the disease once it is already infected.
Cabbage clubroot can be a reoccurring problem for years because it is easily spread from plant to plant. P. brassicae is able to infect 300 species of cruciferous plants, making this disease a recurring problem even with crop rotation.[6] This wide host range allows the pathogen to continue its infection cycle in the absence of cabbages. Additionally, Cabbage clubroot may be a stubborn disease due to its ability to form a microbial cyst as an overwintering structure.[6] These cysts may last many years in the soil until it comes into contact with a suitable host, making it difficult to entirely avoid the introduction of the disease. Those growing cabbage need to be aware of the possibility of Plasmodiophora infestation by simply growing in particular fields that may have had Cabbage clubroot previously.