Powdery mildews have got to be just about the most widespread fungal problem in gardens. There are many types but the name of powdery mildew is given to them all and the symptoms and treatments are often very similar, regardless of the plant that’s being attacked. The fungus causes a white or pale greyish powdery coating, most often seen on the foliage but sometimes on stems, petals, thorns and buds too. On grapes the individual grapes may be attacked, each developing a tough greyish coating before splitting. When very young leaves are attacked, or if the infection is severe, the upper and lower leaf surface may be covered. On soft or young leaves, there may also be distinct distortion; the leaf may soon show browning and die. Exactly where it attacks depends partly on the species and the host plant involved, and also on how extensive the infection has become
Each species of powdery mildew generally only attacks a single genus or closely related host plants. It is easy to assume that one single powdery mildew fungus is spreading around the various plants in your garden, but in reality it is more likely that the weather conditions, soil moisture content etc. are simply ideal for the various different fungi involved. Just about every tree, shrub, climber, perennial and annual I know gets a powdery mildew from time to time – it can even attack grass. Some houseplants and greenhouse plants may succumb, in particular begonias, grapes and cinnerarias.
How can you prevent powdery mildew?
- Good hygiene will help to limit the spread of the powdery mildew fungal infections from spreading, for example by pruning out or picking off severely infected leaves. On plants such as gooseberry, rose and apple where stem infections are pretty commonplace, look out for signs of infection in these areas on a regular basis and then prune them out at the earliest opportunity, even if it is not the text-book time for pruning.
- There is a definite correlation between environmental and soil conditions and the development and spread of the powdery mildew fungi – a humid or rather damp or moist environment around the foliage or stems encourages the disease. In contrast if the soil or compost is a little too dry, this can make powdery mildew fungi more likely to be a serious problem.
- Improving the soil’s texture and ability to retain moisture during drier weather will help to prevent the powdery mildews getting a hold, as will soil improvement, regular watering and the use of plenty of bulky organic mulch on the soil surface.
- Wall shrubs and climbers are often subjected to very dry soil conditions because their roots are fairly close to the house foundations so the rain-shadow effect combined with rain-sheltering from any roof overhang adds to the problem.
- Planting at a minimum of 45cm (18) from the wall, and making these plants top priority for watering and mulching really helps.
- Plants growing in pots are often very prone to mildew, especially if the pot is quite small, so try to use larger pots if possible. Mulching the compost surface should also help, as will the use of moisture-retaining granules in the compost itself.
- Damp or muggy air problems can be reduced by avoiding watering plants from above and perhaps by some clever pruning to improve air circulation around them.
And if you need a control?
Wherever possible I try to avoid problems rather than treating them, and all of the above will make a massive difference but you could use a proprietary fungicide if you wish, making sure you follow the instructions with care of course. There are many recommendations for the use of bicarbonate of soda solutions or for spraying with cow’s milk as a control for mildew. Strictly speaking these materials are not listed ‘pesticides’ so using them as such is not legal but I do hear some pretty impressive reports from bicarb-wielding gardeners up and down the country.