Landscape - Spring Leaf Spots and Blotches

Fungal leaf spots and blotches can occur in susceptible landscape plants in the spring. The following is an article on the subject from the Rutgers Plant and Pest advisory.

What would springtime be without leaf spots? Leaf diseases are caused by a wide variety of fungal and (some) bacterial pathogens. Most ornamental plants are susceptible to one or more of these diseases. For example, one leaf spot that we commonly see in Delaware landscapes is Phyllosticta leaf spot of maple.

The fungi that cause leaf spots and blotch reproduce and spread by means of spores. These spores are produced in fruiting structures in leaf litter on the ground. Once splashed to developing tissue after budbreak, the spores germinate and penetrate the leaf surface. The fungus then grows throughout the leaf, killing the tissue as it goes and leaving spots or blotches that are often accompanied by a yellow halo. Look closely at the lower leaf surface of leaf lesions later in the summer, and you may see the small, dark fruiting structures that serve to produce inoculum the following spring.

Horsechestnut leaf blotch is truly spectacular when it develops following environmental conditions optimal for disease development. Lesions of this fungal disease, caused by Guignardia aesculi, first appear on horsechestnut and buckeye in the spring as watery blotches that turn brown within a few days and are bordered by a yellow band. The blotches coalesce, causing leaflets to distort and curl, and fall prematurely. The disease also affects petioles and fruit. From a distance, severely affected trees appear scorched, so get close and look at the leaves before making a diagnosis.

Since the development of leaf spots and blotch is favored by abundant moisture and cooler temperatures, management benefits from a few basic strategies: reduce leaf wetness and humidity in plantings (e.g., improve air-flow through proper spacing and weed management, irrigate during early morning hours, and avoid overhead watering); remove leaf litter to reduce fungal inoculum; and improve plant vigor to help reduce disease severity. Remember, however, that the environment drives theleaf-spotting process, so expect to see more following wet springs.

Fungicides are effective only if they are present on leaf surfaces at the time the fungi are producing spores. Fungicides applied after leaf lesions are visible are not effective. Most damage caused by the fungi that cause leaf spot and blotch is merely cosmetic, thus the use of fungicides for disease control in the landscape is not usually recommended unless plants are severely stressed. If desired, however, leaf spots may be prevented with foliar applications of azoxystrobin, Benefit, chlorothalonil, ConSyst, copper, iprodione, Junction, mancozeb, maneb, propiconazole, Spectro, Stature, thiophanate-methyl, or Zyban. In general, these preventive fungicides are applied at budbreak and the spray repeated twice at 7- to 14-day intervals. Fungicides labeled for horsechestnut leaf blotch include chlorothalonil, Junction, mancozeb, maneb, Manhandle, Spectro, Stature, or thiophanate-methyl, applied at budbreak and repeated every 7 to 21 days. As always, check individual labels for specific host and application rate recommendations.

Reprinted with minor changes from "Diseases of Landscape Ornamentals in the Springtime (Part I)" by Ann B. Gould, Ph.D., Specialist in Plant Pathology, Rutgers University in the April 3, 2008 edition of the Plant and Pest Advisory; Landscape, Nursery, and Turf Edition.