Divisi
Alternaria leaf blight
Alternaria spp. · Fungal
Photo: Dwight Sipler · CC BY 2.0
Alternaria leaf blight (also called early blight on tomato and potato) is a common fungal leaf disease — caused by Alternaria moulds — that attacks tomato, potato, cabbage, carrot, onion, sunflower and sweet potato. It lives on old crop debris, volunteer plants and seed, and its spores ride wind, rain-splash and irrigation onto wet leaves, where warm, muggy weather lets them infect. It shows as dark target-ring spots that start on the oldest, lowest leaves and climb up the plant as the crop ages.
Clemson University - USDA Cooperative Extension Slide Series · CC BY 3.0 
Agronom (Wikimedia Commons) · CC BY-SA 3.0
Why it matters
Alternaria leaf blight is a fungus — a tiny mould, not an insect — that eats away the leaves of tomato, potato, cabbage, carrot, onion, sunflower and sweet potato. In a warm, wet season a bad attack can strip a plant almost bare and take up to half your crop (50%). It is hard to stop once it takes hold: every spot makes new spores that rain and wind carry up the plant, and a spray can only protect the healthy leaves that are left — it cannot heal a leaf that is already spotted. Fewer leaves means smaller, sun-scalded fruit and tubers, so catching it early, on the bottom leaves, is everything.
When to act
Begin control when the first target-like spots appear on lower leaves, especially in warm humid weather.
This guidance is general, compiled from published agronomic references including FAO and CABI. Thresholds and timing shift with your crop, your variety and the season.
In much of the region a spray is simply called mushonga — medicine. But a chemical group you cannot name is one you cannot rotate, and a product you did not write down has no pre-harvest interval you can observe. Mushonga, and other farming words →
The full alternaria leaf blight field guide
- Identify it
- Scout for it
- Control it
15 field-guide sections in the app
The full guide to alternaria leaf blight — every symptom to check, how and when to walk your field looking for it, and the controls that actually work, ranked. Built into your crop plan, so you are told to look before it costs you a harvest.
Crops it affects
Frequently asked questions
How do I identify alternaria leaf blight?
Where it starts: look on the oldest, lowest leaves first — the ones nearest the ground. The first sign is scattered dark brown to black specks, no bigger than a pinhead, often showing up after a spell of warm, wet, muggy weather.
When should I take action against alternaria leaf blight?
Begin control when the first target-like spots appear on lower leaves, especially in warm humid weather.
How do I control alternaria leaf blight?
Start with a tougher plant (your best non-spray defence): where you can get them, grow varieties bred to resist or tolerate blight, and avoid the ones you know break down badly in a wet season. A resistant plant slows the disease right from the start.
Which crops does alternaria leaf blight affect?
Among the crops covered here: Cabbage, Carrot, Onion, Sunflower, Sweet potato, Tomato.
Catch it before it costs you a harvest
Divisi builds scouting into your crop plan — it tells you when to walk your field, what to look for at that stage, and what to do when you find it. Free for farmers, works offline.
Check what grows on your farm