The best way to avoid strawberry plant leaf diseases is to plant resistant varieties. This is especially important since control is difficult once symptoms appear in summer. Fortunately, if the plants are vigorous and the variety is one that resists local diseases the damage is usually minimal. Local garden centres are nurseries will know which varieties are best for the areas they serve. Cultural controls include removing dead or diseased leaves after harvest, weed control, and planting in an area where plants will have full sun and good air circulation. Fungicides may help to get leaf diseases under control.
Leaf scorch, which is caused by the fungus Diplocarpon earliana, is the most common leaf disease. Small, irregular dark spots less than 1/4 inch wide appear on the upper side of the leaf. In severe cases the spots run together and parts of the leaf will turn reddish-purple or brown and the entire leaf may die. The fungus overwinters on infected leaves then produces spore-forming structures on dead leaves in spring. In midsummer the spore-forming structures produce an abundance of spores that can germinate and infect the plant within 24 hours.
Leaf spot is caused by the fungus Mycosphaerella fragariae. The spots that appear on the upper surface of the leaves are purple with a white centre 1/4 inch across or less. As the disease progresses the spots enlarge and turn brown on older leaves and white to grey on younger leaves. The spots are surrounded by a reddish purple border. Under moist weather conditions superficial black spots about 1/4 inch across may appear on the fruit. The fungus overwinters in the leaf spots as spores and is spread in summer by splashing rain.
Leaf blight, which is caused by the fungus Phomopsis obscurans, causes round spots up to 1/2 inch in diameters. The spots enlarge to form a wedge shape with a light brown inner area surrounded by a dark brown margin. Runners may develop dark spots and severely affected runners may die. The disease overwinters on older leaves that are still attached to the plant and splashing rain spreads the disease in early spring.
Powdery mildew is caused by the fungus Sphaerotheca macularis. The white powdery coating is not as obvious on strawberries as it is on other plants. Powdery mildew causes young leaves to curl upward and the undersides of leaves may turn reddish in severe infections. Fungicides containing sulphur and neem oil are effective in treating powdery mildew.