Divisi
Late blight
Phytophthora infestans · Fungal
Photo: Downtowngal · CC BY-SA 3.0
Late blight is a fast-moving water mould (an oomycete, a fungus-like organism) that attacks potato and tomato. Its spores ride on wind and splashing rain in cool, wet weather and can turn a green field black in a matter of days. It rots potato tubers in the ground and in the store as well, so the loss follows you home after harvest.
Howard F. Schwartz, Colorado State University · CC BY 3.0 
Björn Andersson (SLU) · CC BY-SA 4.0
Why it matters
Late blight is the disease that once caused famine, and it can still take up to 8 in every 10 plants (80% of your crop) once it takes hold. It moves shockingly fast — a healthy-looking field can collapse into black, rotting stems within a week of cool, wet weather. It is so hard to stop because the spores blow in on the wind and wash down in the rain from a single sick plant, and by the time you see the blotches the mould is already spreading unseen. Worse, it follows the potatoes into the ground and rots the tubers in store, so a crop that looked saved can still be lost.
When to act
first water-soaked lesions seen, or cool wet weather forecast to persist; spray protectant before symptoms in high-risk conditions
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 late blight field guide
- Identify it
- Scout for it
- Control it
15 field-guide sections in the app
The full guide to late 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 late blight?
The first thing you see: dark green to brown water-soaked blotches on the leaves — they look wet, oily or greasy, as if the leaf was scalded, and they spread outward fast. They often start at the leaf tip or edge and are ringed by a pale, sickly halo where the healthy green fades.
When should I take action against late blight?
first water-soaked lesions seen, or cool wet weather forecast to persist; spray protectant before symptoms in high-risk conditions
How do I control late blight?
Prevent it first — choose the right seed and variety (your strongest non-spray tool): plant a late-blight tolerant/resistant potato or tomato variety where you can find one, and always use certified disease-free seed potatoes. Starting clean and tough is the single most effective thing you can do without a sprayer.
Which crops does late blight affect?
Among the crops covered here: African nightshade, Irish 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.
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