Elm – Black Spot

by Sherrie Smith and Jason Pavel

Elm Black Spot – Stegophora (syn. Gnomonia) ulmea

Photo by Sherrie Smith, University of Arkansas Cooperative Extension

We are seeing some defoliation of elm, in some cases severe. A fungal disease called Elm Black Spot is an important foliar disease in North American elms (Ulmus spp.). This disease is also called elm leaf scab, elm leaf spot, or anthracnose. The causal agent of this disease is the fungus Stegophora (syn. Gnomonia) ulmea. Most elm species are susceptible to this disease, but Ulmus americana is the most susceptible. This disease is considered a minor disease on mature trees but can cause significant defoliation and twig dieback in nursery production on susceptible cultivars. 

What are the signs of elm black spot?

Symptoms start as small, whitish, or yellowish flecks or blotches on the upper surface of the leaf that later enlarge with black specks developing inside. These black specks can merge to form a single, irregular-shaped stomata that is 0.5 to 3.0 mm in diameter. In general, individual stomata remain separate surrounded by a yellow-white band of dead tissue, but sometimes these stomata can grow so close together to appear to cover the entire discolored area. When this happens, the symptoms can look like Rhytisma tar spot. Besides infecting the leaves, this fungus can infect and girdle petioles and shoots. Successive seasons of twig blighting can cause the formation of witches’ broom. Disease infection often begins early in the growing season and can be very damaging during wet summers. 

How do you manage elm black spot?

Management of this disease includes removing infected leaf debris and dead shoots. Avoid overhead irrigation and close spacing of cuttings and young trees. Chemical control of this disease is generally not warranted for large trees. For small nursery trees, preventive fungicide applications of chemicals such as mancozeb, copper, or chemicals with the active ingredient chlorothalonil may be made starting at budbreak in the spring and continuing at regular intervals until leaves are fully developed.

Take Aways

• Clean up fallen leaves and dead shoots.
• Avoid overhead irrigation on young trees.
• Start fungicides at budbreak in the spring on young trees.
• Large trees do not need fungicides as a rule.

This work is supported by the Crop Protection and Pest Management Program [grant no. 2017-70006- 27279/project accession no. 1013890] from the USDA National Institute of Food and Agriculture.