Fungal Leaf Spots of Trees and Ornamentals
By Dr. Sharon M. Douglas
Department of Plant Pathology and Ecology
The Connecticut Agricultural Experiment Station
123 Huntington Street
P. O. Box 1106
New Haven, CT 06504-1106
Telephone: (203) 974-8601 Fax: (203) 974-8502
SYMPTOMS AND DISEASE DEVELOPMENT:
Fungal leaf spots are the most prevalent of plant diseases in the Northeast. Most, if not all, commonly grown trees and ornamentals are subject to attack by one or more leaf-infecting fungi. Leaf spot symptoms vary depending on the plant host and the causal fungus. A typical leaf spot is a rather definitely delimited necrotic lesion, often with a brown, black, tan, or reddish center and a darker margin. These spots vary in size from pinhead to those that encompass the entire leaf. Partial to complete defoliation of the tree or ornamental may occur under certain circumstances.
Although many different fungi are known to cause leaf spots, their disease cycles are often similar. In most cases, the causal fungus overwinters on fallen leaves. In spring, often in conjunction with rain and wet weather, spores are produced by the fungus and subsequently discharged and carried by wind or splashing rain to newly emerging leaves. Once on the leaf surface, and with appropriate environmental conditions, the fungal spores germinate, penetrate the leaf, and cause infection. Leaf spots are generally favored by cool, wet weather early in the growing season. By the time symptoms are apparent, it is usually too late to apply chemicals for control.
Control of fungal leaf spots can be achieved using a multifaceted approach. They are often effectively controlled by following good sanitary and cultural practices and are rarely serious enough to warrant chemical control. Since many of the leaf-spotting fungi overwinter on fallen leaves, it is important to rake and remove fallen leaves from the vicinity of the tree in autumn. This reduces the number of spores available to infect emerging leaves in spring. Good plant or tree vigor should be maintained by proper watering and fertilizing, appropriately timed pruning, and insect control. Leaf spots can become serious, causing permanent injury or even plant death, on weakened or stressed plants. In such cases, chemical control is often necessary. There are several fungicides registered for use in control including thiophanate methyl, chlorothalonil, ferbam, and mancozeb. The pesticide label will contain information for use with specific plant hosts and fungi, dosage rates, and safety precautions. Since most leaf-spotting fungi infect in spring as leaves are unfolding, the first fungicide spray is applied at budbreak. Two or three additional sprays are subsequently applied at 7- to 14-day intervals. Additional applications may also be necessary in unusually wet springs. Once again, when symptoms are visible on the leaves, it is too late for chemical control.
Fungal leaf spots are the most commonly encountered of all plant diseases in the landscape. Symptoms can vary depending on the plant host and the causal agent although typical leaf spots usually appear as dead areas scattered over the surface of a leaf. Significant defoliation can occur, but it is usually considered to be more aesthetic than life-threatening to the plant. Strategies to minimize the impact of these types of diseases are discussed.