Botrytis leaf blight on onions — symptoms and treatment

Severity: medium

What is Botrytis leaf blight

Botrytis leaf blight is caused by Botrytis squamosa. It produces white lesions on the leaves and premature die-back of the foliage, and on an onion that matters more than it first appears: the leaves are what fill the bulb, so leaves lost early mean a smaller bulb at harvest. Severity is rated medium — a light peppering of spots costs little, but heavy infection in wet weather can strip the foliage while the bulb is still sizing.

Symptoms

The lesions are small, white and oval, scattered over the leaves, and each sits in a light green halo — a pale ring where the tissue is reacting. That combination is the diagnosis: small white ovals, light green halos, distributed over the leaf rather than concentrated at one end. On their own the spots look almost trivial. The problem is what happens next, because the lesions expand in wet weather, and as they merge the tissue dies and the leaf tips die back. A crop that looked lightly spotted can go to scorched, dying foliage in a wet spell.

Key signs:

• Early: small white oval lesions scattered on the leaves
• Early: a light green halo around each lesion
• Advanced: lesions expanding and merging in wet weather
• Advanced: leaf tips dying back, foliage lost prematurely

Downy mildew on onion is a different picture: pale oval spots carrying a grayish-purple fuzzy sporulation, with leaves collapsing from the tips. Botrytis leaf blight gives you discrete white spots with green halos and no fuzz on them.

Causes and conditions

Botrytis squamosa survives on onion debris and cull piles, and releases spores that travel on air currents and splashing water. Free moisture on the leaf is what it needs to infect, so the disease runs on wet weather, heavy dews and overhead irrigation. This is why it can sit almost static in dry conditions and then explode: the fungus needs the leaves to stay wet, and when they do, every existing lesion produces spores that start new ones. Dense stands, weedy rows and still air keep the foliage wet longer and drive it.

Treatment

Chlorothalonil — chemical

At first symptoms. Apply contact fungicide when lesions first appear. Repeat every 7-10 days during wet weather. Being a contact fungicide shapes how you use it: it forms a protective film on the leaf surface, so it guards healthy tissue but does nothing to lesions already formed. It also does not move into new growth — onion leaves keep extending, so tissue grown since your last spray is unprotected, which is why the 7-10 day repeat matters in wet weather. Coverage is difficult on onions because the leaves are upright and waxy; without a suitable adjuvant much of the spray runs off. Pre-harvest interval: 14 days. Pesticide registrations vary by country — check local approval before use.

Prevention

• Rotate away from onions and other alliums rather than growing them in the same bed each season.
• Space plants and keep rows weed-free so air moves through and leaves dry quickly.
• Irrigate at the base rather than overhead, and early enough that the foliage dries.
• Destroy crop debris and cull piles, which carry the fungus between crops.
• Scout in wet spells — this is when a mild infection turns serious.

Frequently asked questions

Can I still eat onions from an infected crop? Yes. The disease is on the leaves and does not make the bulb unsafe — the cost is a smaller bulb from a crop that lost its foliage early. After Chlorothalonil, respect the 14-day pre-harvest interval.

Is it contagious to my other onions? Yes. Spores move on air currents and splashing water through the planting, and in wet weather each lesion becomes the source of many more.

When should I spray? At the first lesions, and mainly if wet weather is driving them. In dry conditions the disease often stalls; in a wet spell you need the spray on the leaves before the lesions expand, and repeated as new leaf grows.