Downy mildew on cabbage — symptoms and treatment

Severity: medium

What is Downy mildew

Downy mildew of brassicas is caused by Hyaloperonospora parasitica. It is common in cool, wet weather — the disease of a long damp spring or a cool, drizzly autumn rather than of high summer. It attacks the leaves, and it hits hardest at the seedling and young transplant stage, where a heavy infection can set plants back badly or kill them outright. On older cabbage it is more often a quality and vigour problem: leaves are lost and growth slows.

Symptoms

Downy mildew is a two-sided disease and you must look at both sides of the leaf to identify it. From above you see yellow patches on the upper leaf surface — often angular, because the growth is bounded by the leaf veins rather than spreading in a neat circle. Turn the leaf over and directly beneath each yellow patch there is a white to gray fuzzy growth, the downy layer that gives the disease its name and the feature that clinches the diagnosis. As patches age, the tissue browns and dies, and affected leaves drop from the plant.

• Early: yellow patches on the upper leaf surface, often angular between the veins.
• Diagnostic: white to gray fuzzy growth on the underside, directly under the yellow patch.
• Advanced: leaf necrosis and leaf drop.

Do not confuse it with powdery mildew, which is a white powdery coating on the upper surface. Downy mildew's fuzz is on the underside and pairs with a yellow patch above. The yellow patches alone can also be mistaken for nutrient deficiency — check the underside before concluding.

Causes and conditions

The pathogen produces spores on the fuzzy underside growth and those spores are carried by wind and splashing water to healthy leaves. Infection needs free water or persistently high humidity on the leaf surface, and the pathogen prefers cool conditions — hence its reputation as a cool, wet weather disease. Extended leaf wetness from rain, heavy dew, fog or overhead irrigation is the trigger. Crowded seedbeds and dense plantings that hold humidity and dry slowly are especially favourable, which is why seedlings raised close together under cover are the classic casualties. The pathogen survives on infected brassica debris and volunteer brassicas between crops.

Treatment

Metalaxyl + Mancozeb — chemical

Timing: seedling and transplant stage. Apply the fungicide to seedlings and young transplants preventively in cool, wet weather — preventively is the operative word, because this is a protectant strategy aimed at the stage when the crop is most vulnerable, and it works far better applied before an outbreak than in response to one. Pre-harvest interval: 14 days.

Pesticide registrations vary by country — check local approval before use.

Prevention

• Sow thinly and give seedlings room; crowded trays and beds hold the humidity that downy mildew needs.
• Space and thin transplants for airflow so foliage dries quickly after rain and dew.
• Water at the base rather than overhead, and water early in the day so leaves are not wet overnight.
• Remove and destroy infected leaves, crop debris and volunteer brassicas, which carry the pathogen between crops.
• Rotate away from brassicas and use resistant varieties where they are available.

Frequently asked questions

When should I treat? At the seedling and young transplant stage, preventively, when the weather is cool and wet. Waiting until the yellow patches and fuzz appear on a young crop means the pathogen already has a foothold — protection ahead of a damp spell is what pays.

Is it the same as the white powder I see on other plants? No. That is powdery mildew, which sits as a powdery coating on the upper leaf surface. Downy mildew shows yellow patches on top with white to gray fuzz underneath, needs wet conditions rather than merely humid ones, and is treated differently.

Can I eat cabbage from an affected plant? Yes — trim off the affected outer leaves and use the sound head. If you have sprayed, respect the pre-harvest interval given above before harvesting.