Downy mildew on cauliflower — symptoms and treatment

Severity: medium

What is Downy mildew

Downy mildew of cauliflower is caused by Hyaloperonospora parasitica. Seedlings and young plants are most susceptible, which makes this a nursery and early-transplant problem above all. The pathogen colonises leaf tissue and produces its spores on the underside of the leaf, and in young plants it can move through the plant systemically rather than staying as isolated spots — the reason a seedbed can go from healthy-looking to badly stunted in one damp spell.

Symptoms

Look at the top of the leaf and you see purplish-brown spots; turn the same leaf over and, under the spots, you find a white sporulation layer. That pairing — discoloured patches above, white fuzz below — is the signature of downy mildew and the fastest way to separate it from other leaf spots. On seedlings the picture is different and more serious: systemic infection in seedlings causes stunting, so the plant is held back overall rather than simply carrying a few marked leaves.

• Early: purplish-brown spots on the upper leaf surface
• Early: white sporulation on the undersides, directly beneath those spots
• Advanced: spots enlarge and leaf tissue dies back
• Advanced (seedlings): systemic infection, stunted plants that never catch up

Do not confuse it with powdery mildew, which produces a white powdery coating on the upper surface and is not tied to cool, wet conditions in the same way. The white growth of downy mildew is on the underside, and it appears under the discoloured spots.

Causes and conditions

Downy mildew is favoured by cool, wet conditions. The spores produced on the leaf undersides are carried by wind and by splashing water, and they need leaf wetness to infect — extended dew, fog, overhead irrigation or a run of wet weather all provide it. This is why the disease concentrates in the crowded, humid microclimate of a seedbed or a propagation tray, where leaves stay wet and air movement is poor. Infected transplants carry the disease into the field, and the pathogen also survives on brassica crop debris and related weeds between crops.

Treatment

Metalaxyl + Mancozeb — chemical

Timing: seedling stage. Apply to seedlings and young transplants during cool, wet conditions. Pre-harvest interval: 14 days.

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

Prevention

• Space seedlings and transplants so air moves between the plants and leaves dry quickly.
• Water at the base rather than overhead, and water early enough in the day that foliage is not wet overnight.
• Ventilate propagation structures and avoid the dense, humid seedbed that gets this disease started.
• Inspect transplants before planting and reject any showing spotted leaves or white growth underneath.
• Clear brassica debris and related weeds, which carry the pathogen between crops.

Frequently asked questions

Is it contagious to my other plants? It spreads readily between brassicas — cauliflower, cabbage, broccoli and their relatives. Unrelated vegetables in the same bed are not the concern; other brassicas, especially young ones in the same tray or seedbed, are.

When should I treat? Treatment is aimed at the vulnerable stage: apply to seedlings and young transplants during cool, wet conditions, which is when infection actually happens. Waiting until an established crop is visibly spotted is late — the seedbed is where this disease is won or lost.

Can I eat cauliflower after spraying? Respect the pre-harvest interval of 14 days for Metalaxyl + Mancozeb. In practice this treatment is applied at the seedling stage, long before there is a curd to harvest, so the interval is rarely the limiting factor.