Pathogen
Black spot of elm, also known as elm anthracnose, is caused by the native fungal pathogen Stegophora ulmea (syn. Gnomonia ulmea and Dothidella ulmea) (Sinclair and Lyon 2005).
Hosts
All native North American elm species are susceptible, which includes American elm (Ulmus americana), slippery elm (U. rubra), rock elm (U. thomasii), winged elm (U. alata), cedar elm (U. crassifolia), and September elm (U. serotina). In addition, many European and Asian elm species are also susceptible, such as Scotch elm (U. glabra), European white elm (U. laevis), field elm (U. minor), Dutch elm (U. × hollandica), Siberian elm (U. pumila), Chinese elm (U. parvifolia), and even Japanese zelkova (Zelkova serrata) (Stipes and Campana 1981; Sinclair and Lyon 2005).
Symptoms & Signs
Stegophora ulmea is a common pathogen of elms in New England. The fungus overwinters in the buds and also survives within discarded leaves. In the spring, when temperatures reach at least 45° F for several days, spores are discharged from fruiting bodies and are spread by wind and rain to establish new infections. Spores germinate under moist conditions, penetrate the newly developing leaf tissue and produce foliar lesions within 10–20 days of infection (Sinclair and Lyon 2005).
Initial symptoms appear as small, circular, yellow-colored leaf spots that form as leaves unfold from the buds. Over time, the spots become black and raised on the surface. This makes black spot of elm highly conspicuous and relatively easy to diagnose when the leaves can be closely examined. When numerous leaf spots are present, they can coalesce to form large, irregularly shaped, foliar blotches that are surrounded by a yellow to white border of necrotic tissue. Stegophora ulmea can cause serious defoliation of susceptible trees during years with persistent wet and cool weather in the spring and early-summer (Sinclair and Lyon 2005). Under extreme conditions, Stegophora can also infect petioles and succulent stems, resulting in shoot blight (Stipes and Campana 1981). Symptoms tend to appear first in the lower canopy, as spores discharged from nearby leaf litter provide a large source of primary inoculum. When severe, the disease can cause premature defoliation early in the growing season.
Like most anthracnose fungi, the pathogen goes dormant during warmer and drier conditions in mid-summer. The disease is usually not a primary stress for elms in the landscape. More often, individual branches in the canopy are symptomatic, and the damage can appear similar to flagging from Dutch elm disease (Ophoistoma novo-ulmi). Secondary infection through the repeated production of asexual spores (conidia) continues throughout the growing season, as long as conditions remain cool and wet.
Management
In areas where elm anthracnose is a chronic problem (e.g. locations with cool weather and fog early in the growing season) pruning to promote air flow and sunlight penetration into the lower canopy may help to reduce disease incidence. Air circulation should be considered when planting trees in settings where elm anthracnose is a perennial problem. Elm species vary in their susceptibility to disease, with American elm (U. americana), Scotch elm (U. glabra) and European white elm (U. laevis) considered as the most susceptible to infection (Stipes and Campana 1981; Sinclair and Lyon 2005).
When black spot is confirmed, remove discarded leaves and prune any branches that may have died over the season to reduce inoculum. Active management is generally not required as elms tend to tolerate infections without lasting injury. Fungicide treatments can be utilized in the spring as new leaves are developing to prevent or reduce disease development. Young and recently transplanted elms that may be suffering from shock or other injuries may benefit from treatment. Fungicide applications should start prior to bud break and on labeled intervals until leaves are fully developed, especially if wet conditions persist in the spring. Chemicals labeled for use include copper hydroxide, mancozeb, myclobutanil, phosphites, propiconazole, tebuconazole, thiophanate-methyl, triadimefon, and ziram.
References
Sinclair WA and Lyon HH. 2005. Diseases of Trees and Shrubs, 2nd edn. Cornell University Press, Ithaca, NY.
Stipes RJ and Campana RJ. 1981. Foliar diseases pp. 42–45 in Stipes RJ and Campana RJ, editors. Compendium of Elm Diseases. APS Press, St. Paul, Minnesota, USA.