CONTROL OF POWDERY MILDEW IN THE HOME GARDEN

Many home garden plants are affected by powdery mildew diseases, eg. cucumber, squash, begonia, calendula, dahlia, delphinium, phlox paniculata, zinnia, rose, catalpa, lilac, gooseberry, grape, ornamentals and turfgrass. The attractive colour of green foliage may be spoiled by the growth of the white powdery mildew fungus on the surface. Affected leaves do not manufacture as much food as they should. These weakened plants will not grow as well and not yield as many blossoms or fruits as healthy plants. Some plants may be so weakened that they do not survive the winter or in the case of tuberous begonias, they may be killed during the growing season.

A number of different powdery mildew fungi are responsible; some attack only one or a few kinds of plants while others attack many different plants. All of the powdery mildew fungi are spread as spores by the wind. They also may be spread when infected plants are transplanted into an area where the disease was not present previously.

LIFE CYCLE

Throughout the season powdery mildew fungi produce mycelium and spores on the surface of infected foliage. the spores are carried by air currents to other foliage where they can cause new infections. There may be several overlapping generations of spores produced throughout the season. Late in the growing season, some powdery mildew fungi produce cleistothecia, the sexual reproductive stage by which the fungus overwinters. Cleistothecia are black spherical structures about the size of a pin head. These remain in the infected foliage or drop to the soil and the following spring release ascospores to produce new infections. As a final overwintering mechanism, some powdery mildews may invade buds. Plant parts that grow from infected buds may be stunted, distorted and covered by mildew. Powdery mildews of apple, firethorn, hawthorn, plum and rose overwinter this way.

Powdery mildews seldom kill their hosts but utilize their nutrients, reduce photosynthesis, impair growth and reduce yields. Late season infections commonly do not have a serious impact on the overall health of plants but can render then unsightly.

Powdery mildew spores do not need free moisture on the leaf surface to germinate. Some species require high humidity but sufficient humidity can develop at the leaf surface when cold nights are followed by warm days or when plants are grown in crowded or shady conditions without sufficient air circulation. Once infection has begun, the mycelium continues to spread on the surface regardless of the moisture conditions.

CONTROLS

There may be less powdery mildew in gardens where there is good air circulation and full sun. In many gardens it is not possible to provide these conditions for the entire garden. Trees, shrubs, and buildings may severely limit air circulation and provide too much shade.

When planting the garden, select suitable varieties that are resistant to powdery mildew. Consult seed and nursery catalogues or experienced gardeners regarding susceptibility or resistance to powdery mildew. Be prepared to apply more mildew sprays and perhaps accept some powdery mildew, if susceptible varieties are used.

It may be possible to remove some trees or shrubs to reduce shade and improve air circulation without reducing privacy, protection from wind, and summer sun. Judicious pruning of trees to open the top a little, or removal of some low hanging branches, may help.

Destroying the tops of annual and perennial flowers at the end of the season will help to reduce the amount of powdery mildew fungus that survives the winter. This may only delay the first appearance of the disease, because spores of the fungus may be blown from other gardens. Cut off the tops and bury them in a vacant space in a flower bed or vegetable garden, or, bury them in a compost pile where the spores will be killed in the heat of the decaying compost.

CONTROLS WITH FUNGICIDES

For information on control with fungicides consult Ontario Ministry of Agriculture and Food (OMAF) Publication 64, "The Gardener's Handbook" available from your local OMAF office or by ordering at http://www.gov.on.ca/OMAFRA/english/products/product.html.

C.B. Kelly

PDCF-107 (Revised 2003/01/14)