Tree Health in Focus

Foliar Diseases

Why do we have to treat for leaf diseases in the spring before we see any sign or symptoms of disease?

The answer gets to the heart of plant medicine. For most foliar diseases, the pathogen over winters on bark and even old leaf litter, and as soon as the conditions are right they infect new leaf tissue as it begins to expand in the spring. By the time people notice the symptoms, such as leaf curling and dead tissue, the infection has already occurred and is inside the leaf.

To prevent infection we need to catch the pathogen before it has a chance to infect the leaf. We apply fungicides to prevent the pathogen from getting into the leaf. It is just like washing your hands to prevent yourself from getting the cold. Once you are infected you have to go through the cycle, but if you wash your hands and prevent it from infecting you, you can avoid all the symptoms and remain healthy.

Apple scab and anthracnose are two such diseases and now is the time when people should be contacting Rainbow to schedule these preventive treatments.

The following information has been compiled by Steve Schumacher, Consulting Arborist for Rainbow Treecare.

Apple Scab
Infected leaves initially appear with velvety brown to olive colored spots. Later, these spots turn black.  Heavily infected leaves, with many spots, may turn yellow and drop from the tree. Fruit may also be infected, initially displaying lesions similar to those appearing on leaves. Later these lesions become brown and corky. Severe infections cause fruit to mature unevenly and crack.

In late fall and early spring, fungal reproductive structures, appearing as round black pimples, develop primarily on infected leaves from the previous year. Spores are released from these structures in the spring and are blown by wind or splashed by rain to new growth on nearby trees. There they start new infections. The fungus may infect and colonize the leaves, fruit, petioles, and blossoms of the tree, with symptoms commonly observed on the leaves and fruit. Later in the spring, these primary infections produce secondary spores which infect other leaves and fruit. These secondary infections may continue throughout the growing season during wet periods.
The length of time required for infection to occur depends on the number of hours of continuous wetness and the temperature during the wet period. Young leaves remain susceptible for five to eight days, but their lower surfaces may become infected in late summer. For fruit, the duration of the wet period required for infection increases with the age of the fruit, which remains susceptible until harvest.

 

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