Mold and mildew are common names given to some kinds of fungus. In general, prevention and treatment are handled as they are for other fungi.
Grey Mold
This fungal disease is caused by Botrytis cinerea. It is very common and affects many kinds of plants. The symptoms vary. In very humid conditions, a rot develops that is covered by a fuzzy gray layer that gives the disease its name. If humidity is low, the gray mold layer may not develop even through the plant still rots and shrivels. On some plants, like tomatoes or cyclamen flowers, small discolored spots may appear.
Spores typically infect healthy plants through wounds. They can also attack delicate tissue directly, such as ripening fruit or flowers. The fungus can remain dormant in developing plants, only breaking out and causing rot later. In primulas, the fungus can actually be carried in the seed. Botrytis is also able to form small structures called sclerotia, which can survive in the soil for many months.
There is no specific fungicide for treating gray mold. Good garden care is the best prevention, and prompt removal of damaged material is the best control method.
Sooty Mold
Saprophytic fungi cause the black growth sometimes found on trees and shrubs. Many gardeners find it unsightly, but it is not a disease and will not directly damage the plant. The presence of sooty mold often indicates insect activity, which can cause damage to plants.
Powdery Mildews
A range of closely related fungal species cause powdery mildew. Each fungus has a limited host range, so although powdery mildew may appear on several kinds of plants in the garden, it is unlikely to be transferred between species. The most typical sign is a dry whitish power that coats the leaves, shoot tips, and sometimes flowers. The plant may have stunted or distorted growth, and flowering may be reduced. On some plants, the foliage is discolored.
Good air movement is the most important way to prevent powdery mildew. Avoid overcrowding, and prune plants as necessary. Water the garden in the morning, so plants can dry completely. Grow resistant varieties when possible, and plant them under optimal growing conditions. Remove infected material promptly and destroy it.
Fungicides available to the home gardener for powdery mildew control are myclobutanil, penconazole, flutriafol, and sulphur mixes.
Downy Mildew
Downy Mildew is caused by several different kinds of fungi. It first appears in patches or spots on the upper surface of leaves, with a furry growth on the underside of the leaf directly below the spot. The growth often has a slightly violet color. Downy mildew needs warm weather and moist conditions to thrive. Under favorable conditions, it spreads rapidly.
Downy mildew is prevented by the same horticultural practices that prevent powdery mildew. Copper fungicides can be used for treatment.
Snow Variety
Grass is damaged by snow mold, which appears as brown patches followed by fluffy white fungal growth and by other fungi. They seldom kill grass, but can cause unsightly spots. Good lawn care is the best control.
