Hartley Magazine

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Removing Powdery Mildew on Gooseberries

I have found symptoms of American Gooseberry mildew, a form of powdery mildew on one of my gooseberry plants, as there is a purple bronze tint to the leaves. It is one of many forms of this disease that can be a problem in summer, on a range of plants. The upper leaf surfaces are coated with a dry whitish powder that can spread to the undersides, shoot tips and flowers. Growth is stunted and distorted, particularly on young shoots and fruits split and crack. On rhododendron leaves, yellow patches appear on upper surfaces with corresponding brown felty blotches below. Look out for Powdery mildew on shrubs and herbaceous plants like roses, clematis, apple trees, sweet peas, plants in hanging baskets; the leaves of vegetables like spinach can be affected too. Keep plants well watered. It doesn’t matter if you make the leaves wet, powdery mildews like humid conditions but hate wetness on the leaf surface though the effect of knocking the foliage with water can disperse the spores and use mulches to keep the soil moist. Improve air circulation around plants by pruning and open vents or install fans in the greenhouse.

Don’t plant too densely and avoid high-nitrogen fertilizers that encourage vulnerable soft sappy growth. Grow plants in their preferred position so they are stress free and grow resistant varieties. Prune out infected parts immediately and ensure that white patches on stems, the resting stage is pruned out of roses during winter. Collect and burn or dispose of all infected debris and spray plants with soft soap or sulphur, a method which has been used since Roman times and if you wipe mildew from Gooseberry fruit, they are still edible. I have found the gooseberries ‘Keepsake’, ‘Leveller’ and ‘Hinnomaki Red’ and Hinnomaki Yellow’ to be resistant too.

Although it cannot be officially recommended, bicarbonate of soda at 5g per litre controls powdery mildew and spraying leaves of vulnerable plants with a 10% solution of skimmed milk and water is very effective too.. Researchers in Brazil discovered that milk solution was effective on courgettes and boosted the plant’s immune system. A weekly spray of one part milk to nine parts water reduced the severity of infection by 90 per cent, but if milk concentrations rose above 30 per cent fungus grew on the plants. In New Zealand, researchers found skimmed milk to be as effective as full fat and reduced the chance of odours. The whole plant needs to be sprayed thoroughly as it is a ‘contact’ control.

Finally, it is essential that early flowering plants like Camellia’s and Rhododendrons are not allowed to dry out at the roots from this month onwards until the end of September when the flower buds for next year are being formed. Keep the compost moist but not waterlogged using rainwater or soft water. This is particularly important for plants growing in containers which should be checked and watered even if it rains as very little water reaches the surface of the compost. Happy gardening!

  • Hartley Botanic

    Reply from Matthew Biggs – Dryness at the roots increases susceptibility to American Gooseberry Mildew, so keep plants well-watered (without splashing the foliage) and mulch around the plants with well rotted organic matter to keep the soil moist. Improve air circulation around plants by pruning and avoid high-nitrogen fertilizers which encourage vulnerable soft sappy growth and feed in spring with Sulphate of Potash instead. Gooseberries in their preferred position, in moist soil in preferably in sun though they will take part shade. Grow resistant varieties like ‘Hinnomkai’ Red, yellow and green, ‘Captivator’ ‘Rokula’.

    Wipe mildew from gooseberry fruits, for example, and they will still be edible.

    Unofficially, because neither product is legally approved for powdery mildew control, so I cannot recommend them! Bicarbonate of soda at 5g per litre controls mildew. Researchers in Brazil discovered that milk solution was effective on courgettes and boosted the plant’s immune system too. A weekly spray of one-part milk to nine parts water reduced the severity of infection by 90 per cent, however if milk concentrations rose above 30 per cent fungus grew on the plants. In New Zealand, researchers found skimmed milk to be equally effective and the reduced fat content decreased the chance of odours. The whole plant needed to be sprayed. The findings were published in the journal Crop Science (Vol. 18, 1999, pp. 489–92) and are available on the internet