Worldwide, many diseases attack cabbages. Some, like black rot, are seed-borne diseases, and some are soil-borne, such as clubroot. Powdery mildew is an example of a fungal disease. Gardeners should contact a professional or an agricultural agent to learn about the latest prevention methods and treatments.
Cabbage is a member of the cole, or Brassicaceae, family, which also includes broccoli, Brussels sprouts, and cauliflower. Most of the diseases that attack cabbage will affect other cole crops. Other cole crop examples are members of the mustard family, kale, and turnips. Many gardeners rotate their cole crops to areas where they did not plant these types of plants in the previous growing season because crop rotation is one of the control methods to avoid soil-borne cabbage diseases.
Pests generally damage cabbage plants in two ways. First, they attack the plant, causing stress to the plant, and secondly, the wounds inflicted by the pests may allow a plant disease to enter the plant. Other wounds include hoe or spade cuts, scratches or damage from animals traveling through the cabbage patch, and weather-related wounds.
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Alternaria brassicae, known as alternaria leaf spot, is a seed-borne disease that often is not noticed until the plants are older. Infected plants get spots resembling bulls' eyes on the oldest leaves. The brown or gray-brown spots are typically more prevalent during spells of warm weather and wet conditions. Generally, wind-dispersed spores, poor handling of infected plants, and using infected seeds distribute the disease.
Black rot, or Xanthomonas campestris, is another seed-borne bacterium that is more prevalent in humid conditions. V-shaped patches of light brown or yellowish lesions grow at the edges of the leaves and may cause blackened leaf veins. Often the disease survives in infected plant debris for up to a year, but it does not live long in soil without plant debris as a host.
Black spot and dark leaf spot are two species of Alternaria. Like black rot, infected debris abandoned in the growing area can spread the diseases. Generally, the fungi spores can live for about 12 weeks after the harvest. Plant rotation and careful field clean-up may help control these cabbage diseases.
Although an obligate parasite causes clubroot, or Plasmodiophora brassicae, most people classify it as one the fungal cabbage diseases. The name comes from the club-like roots that develop in infected plants. Above ground, symptoms are yellowed leaves, and plants wilt during hot weather. Soil-borne clubroot spores can survive up to 10 years. Agricultural experts suggest plant rotation and treating alkaline soils with lime.
The fatal disease known as damping off, or Rhizoctonia solani, prevents young seedlings from sprouting or causes them to die shortly after sprouting. Symptoms include light brown stems and wilting in young plants. Typically, planting treated seeds and letting the soil dry out completely between waterings may lessen the chance of contracting the disease. Wet weather and high humidity are two conditions that encourage damping off.
Like damping off, downy mildew thrives in high humidity and wet conditions, such as persistent fog, heavy dew, and overwatering. High winds and splashing rain frequently spread the spores. Usually, symptoms include whitish-gray, fluffy spots on the underside of the leaves, black or purple spots on the cabbage heads or florets, and green-yellow to brownish leaf spots that may spread to other plant parts. The Latin name for cabbage-related downy mildew is Peronospora parasitica.
Fusarium oxysporum conglutinans, called fusarium wilt or yellows, is one of the fungal cabbage diseases, and because it exhibits many of the characteristics of black rot, it is often misdiagnosed. Typically, the most obvious symptom is yellow to greenish-yellow leaves, which may drop off. Frequently, the yellowing occurs on only one side of the plant. This fungus may live for years in the soil and rain, or improper handling may spread it. Unlike some other diseases, an infected plant may shed the affected leaves and recover enough to produce a healthy head of cabbage.
Sclerotinia rot, or white mold, scientifically known as Sclerotinia sclerotiorum, affects several plants, but in cabbage this fungal disease attacks any part of the plant that touches infected soil. The symptoms include tan, watery spots that usually develop into a cottony, white appearance. Some people control Sclerotinia cabbages diseases by letting the soil dry completely between waterings, removing disease-carrying weeds, and by applying the appropriate fungicides. Growers should get a professional diagnosis before applying fungicides.
One of the soil-borne cabbage diseases is wirestem of the Rhizoctonia solani anastomosis group of fungi. Wirestem kills seedlings by girdling the stem. Infected older plants may fail to produce a full-sized head of cabbage or die before producing any head.