Divisi
Asian soybean rust
Phakopsora pachyrhizi · Fungal
Photo: USDA · Public domain
Asian soybean rust is an aggressive, wind-borne fungal disease of soybean and other legumes such as common bean and cowpea. Its spores drift long distances on the wind and settle on the underside of the leaves, where warm, humid, dewy weather lets them infect and erupt into tan pustules. It builds up fastest once the crop is flowering and filling pods, stripping the leaves early and starving the seeds.
USDA · Public domain 
Reid Frederick, USDA Agricultural Research Service, Bugwood.org · Public domain
Why it matters
Asian soybean rust can strip the leaves off your crop just when the plant is filling its pods — and once the leaves are gone, the pods stay small and light. In a bad, wet year it can take up to 80% (four-fifths) of your harvest. It blows in on the wind and explodes fast in warm, humid weather, so by the time you see whole leaves yellowing and dropping, it is already very hard to stop. That is why the trick is to catch it early and spray protectively, before it climbs into the upper leaves that feed your beans.
When to act
Spray protectively from early flowering to pod fill when rust is reported in the area or the first pustules are found on lower leaves.
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 asian soybean rust field guide
- Identify it
- Scout for it
- Control it
15 field-guide sections in the app
The full guide to asian soybean rust — 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 asian soybean rust?
The damage you notice first: the lower leaves start going yellow and dropping early — while the plant is still flowering or filling pods, long before the crop should be drying down. It looks like the plant is 'ripening' from the bottom up at the wrong time.
When should I take action against asian soybean rust?
Spray protectively from early flowering to pod fill when rust is reported in the area or the first pustules are found on lower leaves.
How do I control asian soybean rust?
Prevent it — start with the variety: where an adapted one is sold, plant a rust-tolerant / rust-resistant variety. It carries a good part of the load for you; on its own it won't fully stop rust in a wet year, but it slows the disease and buys you time.
Which crops does asian soybean rust affect?
Among the crops covered here: Common bean, Cowpea, Soybean.
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