The warmest April on record has provided ideal conditions for one garden fungus – powdery mildew, to thrive so be on your guard for early infections on vulnerable plants. It’s a dry whitish powder that coats the tops of the leaves but soon spreads to the undersides, shoot tips and flowers stunting and distorting foliage and weakening plants. Infected fruits split and crack, though if you catch them early, gooseberries can be wiped clean and eaten. Powdery mildew affects woody and herbaceous plants, like roses, apples, sweet peas, some clematis if they become too dry and plants in hanging baskets, but beetroot, parsnips and spinach leaves can be affected too. Yellow patches appear on the upper surfaces of Rhododendron leaves with corresponding brown felty blotches below. Since rhododendrons are evergreen, there’s plenty of opportunity for the disease to spread so always buy healthy plants and treat those that are infected immediately. Look out for resistant varieties like the swede ‘Marian‘, courgette ‘Sebring‘, Brussels sprout ‘Wellington‘, gooseberries like ‘Hinnomaki Red‘ and species clematis which are less susceptible to attack. Mildews can spread to cultivated plants from closely related weeds, so weed control in the garden is important too.
As dryness at the roots increases susceptibility, keep plants well watered, don’t splash the foliage and mulch when the soil is moist. Improve air circulation around plants by pruning, open vents or turn on the fan in the greenhouse and don’t plant too densely, as the fungus loves stagnant air, plant vegetables at the correct spacing and avoid too much high-nitrogen fertilizer which encourages vulnerable soft sappy growth. Grow plants in their preferred position so they are not under stress and at risk of infection. Prune out infected parts immediately and ensure that white patches on stems (the resting stage) are pruned out of roses during winter pruning. Collect and burn or dispose of all infected debris. Many local authorities now collect for composting so dispose of any disease infected material in there, the temperatures created on their giant heaps is sufficient to kill spores.
Spray with the relevant chemicals, sulphur or fatty acids as soon as you notice any infection. There are no fungicides recommended for use on most vegetables, so you need to find an alternative method. Bicarbonate of soda at 5g per litre controls powdery mildew and 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 needs to be sprayed as it is a contact control, however neither product is legally approved for powdery mildew control, I am not allowed to recommend them! It has started to appear on my gooseberry cordons so immediate action is needed! Make sure that all spraying equipment is clearly labelled and wash it out thoroughly after use.