White, powdery patches on leaves, as if someone had dusted them with flour. That is powdery mildew, one of the most widespread plant diseases in the world. It attacks cucumbers, melons, squash and pumpkins, wheat and barley, grapes, peas, tomatoes and peppers, apples, mangoes, roses and many more. Unlike most leaf diseases, it does not need rain. It thrives in warm, dry days with humid nights, in shaded, crowded canopies, and in greenhouses.
In this video: how to recognise powdery mildew and tell it from downy mildew, how it spreads and survives, the conditions that favour it, how to prevent it with resistant varieties and good crop management,
and how to use fungicides by FRAC group, because powdery mildews are among the fastest fungi to become resistant.
The advice is general and global. Your local extension service and product labels always come first.
Powdery mildew starts as small, round, white spots, usually on the upper surface of older or shaded leaves,
and sometimes on stems, flowers and fruit. The spots grow and merge until the whole leaf looks dusted with white or grey powder.
Rub it with your finger, and the powder comes off. That powder is made of the fungus and millions of its spores.
Badly infected leaves turn yellow, then brown, and die early, so fruit is smaller, sweetness and quality fall, and in grapes the berries can split.
One group behaves differently.
Leveillula taurica, which attacks tomato, pepper, onion and other crops, grows inside the leaf. You first see yellow patches on the upper surface,
and only a sparse white growth underneath.
It is easy to mistake for a nutrient problem.
Powdery mildew is often confused with downy mildew, but they are different organisms that need different control.
Powdery mildew is a true fungus.
It forms white powder, mostly on the upper side of the leaf, that you can rub off, and it is favoured by dry days and humid nights.
Downy mildew is a water mould, an oomycete. It causes yellow patches on the upper surface,
often angular and limited by the veins, with a grey to purple fuzz on the underside, and it needs wet leaves. A fungicide for one may do nothing against the other, so get the diagnosis right first.
Each crop group has its own powdery mildew species.
The one on wheat does not attack cucumbers,
and the one on grapes does not attack apples.
So your neighbour's roses are not the source of your squash mildew, but your neighbour's squash may be.
The spores are carried by wind, often over long distances.
They germinate on a dry leaf surface in high humidity.
In fact, free water on the leaf can burst the spores, which is why the disease is common in dry seasons and under plastic.
A new generation can form in about a week in favourable weather, so the disease builds up very fast.
Between seasons, powdery mildews survive as small, dark, round fruiting bodies, called chasmothecia, on leaves and bark, as mycelium hidden in buds of grapes and apples, on volunteer cereals that bridge the gap between crops, or on crops and weeds growing year-round in warm regions.
Resistant varieties are the most effective and cheapest control.
Many modern cucumber, melon, squash, pea and cereal varieties carry good resistance.
In barley, for example, a resistance gene known as mlo has held for decades.
Check the variety description, and the local trial results, before you buy seed.
Then manage the crop. Avoid excess nitrogen, because soft, lush growth is most susceptible.
Give plants room and sun, with good airflow: wider spacing, training and pruning, and in vineyards, removing some leaves around the bunches.
In greenhouses, control humidity and avoid big swings between warm days and cool, humid nights.
Remove volunteer cereals and heavily infected old crops promptly,
and rotate crops where the species can survive on residues.
And scout the shaded, lower and older leaves at least once a week, because the first colonies are easy to miss.
Fungicides work best when started early, at the first colonies or before them in very susceptible crops, with good coverage of both sides of the leaves and of new growth.
Several protectants act on contact.
Sulphur, in group M two, is one of the oldest and still one of the most useful, and it is allowed in organic systems. But it can scorch some cucurbit varieties, and many crops in hot weather, roughly above thirty degrees, and it should not be used close to an oil spray. Potassium bicarbonate, horticultural oils and some biological products, such as Bacillus strains and the mycoparasite Ampelomyces quisqualis, which attacks the mildew itself, are other options.
Single-site groups used against powdery mildews include group three, the DMIs, group seven, the SDHIs, group eleven, the strobilurins, group thirteen, group fifty, and group U six. Powdery mildews are high-risk for resistance.
Resistance to group eleven, and to the older group one, is widespread in several mildews, and reduced sensitivity to group three is common. So use single-site products in mixtures or alternation with a different group or with sulphur, limit the number of applications of each group per season,
and follow the label and local resistance advice.
Registrations differ from country to country.
To recap.
Powdery mildew is white powder you can rub off, mostly on the upper side of leaves, and it thrives in dry days with humid nights.
Downy mildew is a different disease, with angular yellow patches and fuzz underneath, and needs different fungicides.
Each crop has its own powdery mildew species.
Plant resistant varieties first. Avoid excess nitrogen, give the crop light and air,
control greenhouse humidity, remove volunteers and old crops, and scout shaded leaves weekly.
Spray early, cover new growth, use sulphur and other protectants, and rotate and mix single-site FRAC groups.
For more on crop diseases, visit laisha.ai. Thank you for watching.
