Purple Blotch of Onion Control

Symptoms

Small water soaked whitish spots appear first on older leaves and flower stalks, which enlarge into spindle shaped lesions with a sunken purple centre and yellow margin. The spots often show concentric rings. Lesions girdle the leaf or stalk causing it to break and dry. In seed crops the flower stalk lodges, reducing seed yield.

Damage

Purple blotch is a major fungal disease of onion and garlic, especially in warm humid weather, causing premature drying of leaves, poor bulb development and bulb rot in storage. Losses can reach 30 to 50 percent in seed crops and reduce both bulb yield and seed yield. It often occurs together with thrips and Stemphylium blight.

Organic & cultural control

Use disease free seed and treat seed with Trichoderma viride before sowing. Practise crop rotation of two to three years away from onion and garlic and destroy infected crop debris. Apply a foliar spray of Trichoderma harzianum as a biocontrol where available. Ensure good field drainage and wider spacing to lower leaf wetness, and control thrips which worsen the disease.

Chemical control

Protective fungicide sprays of mancozeb or a copper based fungicide are recommended at first appearance, repeated at ten to fifteen day intervals as per label, and a sticker should be added because onion leaves are waxy. For severe pressure a recommended combination or systemic fungicide may be advised by your KVK. Use only labelled products at the correct dose, follow the pre-harvest interval and consult your KVK or agronomist before spraying.

Always follow the product label, the recommended pre-harvest interval, and your local KVK or agronomist. Pesticide approvals change, so verify before use.

Prevention

Start with healthy treated seed and resistant or tolerant varieties where available. Follow crop rotation, avoid waterlogging and excess nitrogen, and provide adequate spacing and drainage. Remove and destroy infected leaves and debris, control thrips, and store only well cured healthy bulbs to avoid storage rot.