Downy mildew on cucumber — symptoms and treatment

Severity: high

What is Downy mildew

Downy mildew is caused by Pseudoperonospora cubensis. It is one of the most destructive diseases of cucurbits worldwide. The pathogen is not a true fungus but an oomycete, a water mould, and it attacks the leaves only — it does not rot the fruit directly. That is small comfort: once the canopy is destroyed the plant has nothing left to feed developing fruit with, and yield collapses. On a susceptible cucumber crop in favourable weather the disease can move from a few spots to a dead canopy within a matter of days.

Symptoms

Look at the top of the leaf first. The spots start as pale yellow patches that are noticeably angular — squared off and blocky rather than round, because the veins act as walls the pathogen cannot cross. This vein-bounded, almost mosaic-tile pattern is the signature of the disease. Turn the leaf over and, in humid conditions, the underside of each spot carries a purplish-grey fuzzy growth. As spots multiply they run together, the tissue browns and dies, and leaves curl upward and crisp. Rapid leaf necrosis follows, usually working from the older leaves in the crown outwards.

• Early: angular yellow spots on upper leaf surfaces, bounded by the veins
• Early: purplish-grey fuzzy growth on the undersides of the spots
• Advanced: spots merge, leaves brown and die, rapid leaf necrosis strips the canopy

The angular shape distinguishes it from most other cucumber leaf spots. Angular leaf spot is the usual confusion — but powdery mildew sits as white powder on the surface, while downy mildew's growth is beneath the leaf and its spots are yellow on top.

Causes and conditions

The pathogen needs free water on the leaf to infect, so it thrives in humid weather, in heavy dew, after rain, and under overhead irrigation. Spores are produced on the leaf underside and carried on wind and air currents, which is why the disease can arrive in a garden with no infected plant anywhere nearby. Dense, poorly ventilated canopies hold moisture on the leaves for longer and give the spores the wet window they need. In greenhouses, still humid air and condensation dripping from the structure create ideal conditions.

Treatment

Ventilation and resistant varieties — cultural

Improve air circulation in greenhouses. Use resistant varieties. Avoid overhead irrigation. Anything that dries leaves faster works directly against the pathogen's requirement for leaf wetness — venting, spacing, trellising and watering at the base all count.

Metalaxyl + Mancozeb — chemical

Apply systemic fungicide preventively, at first symptoms or as a preventive treatment. Alternate with contact fungicides to prevent resistance. Pre-harvest interval: 14 days. Pesticide registrations vary by country — check local approval before use.

Prevention

• Grow resistant varieties — the single most effective measure available
• Water at the base and keep the foliage dry; never irrigate overhead late in the day
• Trellis and space plants so air moves through the canopy and leaves dry quickly
• Ventilate greenhouses actively to prevent condensation settling on leaves
• Scout the undersides of older leaves regularly so you catch the first spots

Frequently asked questions

Is it contagious to other plants? Yes — to other cucurbits. The spores travel on the wind and can reach plants some distance away, so nearby squash, melon and pumpkin are all at risk.

Can I still eat the cucumbers? Yes. The disease attacks leaves, not fruit. Observe the pre-harvest interval for any product you have sprayed.

When should I treat? Preventively, or at the very first angular spots. Once the canopy is browning, fungicides will not bring it back.