Divisi
Blue mould
Peronospora hyoscyami f.sp. tabacina · Fungal
Photo: R.J. Reynolds Tobacco Company Slide Set, R.J. Reynolds Tobacco Company, Bugwood.org · CC BY 3.0 us
Blue mould is a fast-moving downy mildew — a water-mould — that attacks tobacco, and only tobacco, in the seedbed and in freshly transplanted young plants. In cool, wet, overcast weather it fruits at night on the undersides of the leaves and blows in on the wind, so a green, healthy seedbed can yellow and collapse within a few days. Its unmistakable mark is a bluish-grey to violet downy fuzz on the underside of yellow leaf spots.
Tashkoskip · CC BY-SA 4.0 
Tashkoskip · CC BY-SA 4.0
Why it matters
Blue mould is the tobacco seedbed's worst enemy: in cool, damp weather it can race through a nursery in a matter of days and take up to 90% of your seedlings — very nearly the whole bed. Once the mould works its way inside a plant, into the stem and growing tip, no spray can reach it, and every infected seedling you carry to the field plants the disease with it. That is why this battle is won or lost before you see much of it — the first yellow spot, or the first cold, wet, overcast spell, is already your signal to move.
When to act
Begin protective sprays in the seedbed when cool, wet, overcast weather sets in, or immediately if the first yellow leaf spots with grey-blue mould are found.
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 blue mould field guide
- Identify it
- Scout for it
- Control it
14 field-guide sections in the app
The full guide to blue mould — 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 blue mould?
What shows first: pale, yellow, roundish spots on the top of young leaves — small faded patches on otherwise green seedlings. In a crowded bed they often appear as yellowing patches creeping across the seedbed.
When should I take action against blue mould?
Begin protective sprays in the seedbed when cool, wet, overcast weather sets in, or immediately if the first yellow leaf spots with grey-blue mould are found.
How do I control blue mould?
Prevent it — dry the bed and give it air. Blue mould needs long leaf-wetness and still, humid air, so ventilate the seedbed (open the covers by day to dry the foliage), don't over-water — water early so leaves dry before the cool night — and don't crowd your seedlings; space them so air moves and leaves dry fast. Site the bed in an open, sunny spot away from older tobacco.
Which crops does blue mould affect?
Among the crops covered here: Tobacco.
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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