Onion: purple blotch and thrips control

Purple blotch and thrips together can cut onion yields by half, and they feed on each other: thrips wounds let the purple blotch fungus in. Control both from the start of the crop.

Doses in this guide are taken from the product labels. Always read the label and leaflet before use, respect the pre-harvest interval, and check with your nearest Biostadt retailer or agronomist for advice specific to your field.

Before and at transplanting

• Dip seedling roots in a fungicide solution before transplanting to reduce early infection.
• Pre-emergence weed control: Hitbac (pendimethalin 38.7% CS) 1500–1750 ml per hectare, sprayed on moist soil within 2–3 days of transplanting.
• Use raised beds with drip or furrow irrigation. Standing water on leaves favours purple blotch.

Purple blotch

• Symptoms: small white sunken spots that enlarge into purple patches with yellow edges; leaves bend and dry from the tip.
• Control: Alliance 750 ml/ha or Tagma 625–750 ml/ha. Start at the first spots or 30–35 days after transplanting in humid weather, and repeat every 12–15 days.
• Onion leaves are waxy, so add a sticker such as 4 IN 1 (500–700 ml) to every spray.

Thrips

• Symptoms: silvery white streaks on leaves, twisted tips; thrips hide deep in the leaf folds.
• Scout from 20 days after transplanting; spray when you see 30 or more thrips per plant.
• Use a high water volume and a sticker so the spray reaches the leaf base. Rotate between products labelled for thrips, such as Ultimo Super and Wapkil.
• Blue sticky traps (10–12 per acre) help you see when numbers start rising.

Bulb size and storage quality

• BioGreen Potash 2–3 ml per litre at bulb initiation improves bulb size and firmness.
• Stop irrigation 10–15 days before harvest and cure bulbs in shade for about a week to reduce storage rots.

Spray safely

• Wear gloves, a mask, full sleeves and eye protection while mixing and spraying.
• Spray in the early morning or late evening, never against the wind or before expected rain.
• Use clean water and the full recommended water volume. Poor coverage is the most common reason a spray "does not work".
• Do not mix products unless the label allows it. If unsure, do a jar test first.
• Keep children and animals away, and never reuse empty containers. Rinse them three times, puncture them and dispose of them safely.