Clubroot: inspect swollen roots and the wet parts of the field
Clubroot affects cultivated and wild crucifers, including cabbage, cauliflower, Brussels sprouts, swede, turnip and mustard. The organism, Plasmodiophora brassicae, causes characteristic root galls that disrupt nutrient and water uptake. Affected plants may perform poorly above ground even when the diagnostic symptoms are beneath the soil surface.
The pathogen can survive in soil for up to 15 years. A clean intervening crop therefore does not show that an affected area is free of inoculum.
Zoospores move through soil water. Compacted ground, poor drainage and flood-prone areas favour disease spread, especially in warm, wet conditions. Maintain drainage and reduce movement of contaminated soil on machinery, boots and other equipment.
Prevent clubroot introduction with transplants and soil hygiene
Use vegetable transplants grown in compost free of clubroot spores. Clean and disinfect reused trays thoroughly.
Control cruciferous weeds and volunteers within and between susceptible crops. Charlock and shepherd's purse are hosts, so leaving them in a rotation preserves a route for the disease.
Acidic soils carry a greater risk of severe clubroot symptoms. Check soil pH before establishing a susceptible crop and aim to keep it above 7: even a rise of 0.5 to 1 pH unit decreases severity.
Brassica downy mildew: compare both leaf surfaces
Look for yellow patches on the upper leaf surface, often bounded by veins, and whitish fuzzy growth underneath. On seedlings, infection can spread rapidly through the plant. In mature crops, affected tissue can die, shrivel and fall out. Cauliflower curds and radish flesh can also develop internal browning.
The causal organism is Hyaloperonospora brassicae, an oomycete. It penetrates plant tissues under wet conditions and produces spores carried by wind and rain. Young plants in propagation are vulnerable in spring; established outdoor crops can be affected in summer and early autumn.
Space brassicas to improve airflow, remove host weeds and promptly remove infected material.
Keep your spray records in one place. Free to start.
Onion white rot: inspect the bulb base
Onion white rot, caused by Stromatinia cepivora, also named Sclerotium cepivorum, affects onions, garlic, leeks and other alliums. Yellowing or wilting foliage is an early above-ground sign. In wetter conditions plants may remain upright but become loose in the soil as their roots decay. Inspect the base of the bulb for fluffy white fungal growth and small black sclerotia.
Contaminated soil carries the organism on tools or muddy footwear, and it persists for many years. Preventing introduction to a clean site is therefore central to control. A short rotation is not a reliable reset.
Seed health: germination is a different measurement
A germination result measures the ability to produce seedlings under the test conditions. It does not establish that the lot is free of disease. Important vegetable pathogens can be seed-borne, and a seed treatment active against damping-off organisms may have little effect on a particular seed-borne pathogen.
Pathogen-free seed is especially important for diseases with few effective chemical options, including several bacterial problems.
Decide between prevention and an authorised product
The HSE product register shows the crop use and notices of a product. The garden database covers products used on an allotment or home vegetable patch. A label for an ornamental crop does not give a vegetable use.
For tomato and strawberry diseases, see the protected-crop disease guide.
Ask which pathogen the seed test targets
The seed-health list distinguishes bacterial and viral disease from fungal infections. Examples include Xanthomonas hortorum pv. carotae associated with bacterial blight of carrot, Septoria apiicola associated with celery leaf spot, and pea seed-borne mosaic virus. A negative test for one organism is not a universal disease-free result.
Next step
Read about vegetable pests, seed treatments and strawberry and tomato diseases.