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Black Spot in Elm Trees

Ned Tisserat   Extension Specialist, Plant Pathology

 

Black spot (and insect feeding damage) on American Elm. Note the black, shiny spots on the leaf

Early spring leaf spotting and shoot dieback on Siberian elm

Figure 2. Early spring leaf spotting and shoot dieback on Siberian elm.

Figure 1. Black spot (and insect feeding damage) on American Elm. Note the black, shiny spots on the leaf.

 

Introduction

 

Elm black spot, sometimes referred to as anthracnose, is the most widespread foliar disease of elms. It is caused by the fungus Stegophora ulmea. Black spot is generally considered to be a minor problem on mature elms, but it may cause significant defoliation and twig dieback on susceptible elm cultivars during nursery production.

Symptoms

Small yellow spots initially develop on the upper leaf surface as leaves unfold in spring. On susceptible elms, the spots may rapidly expand and coalesce to blight the entire leaf. Lesions may also girdle petioles and succulent shoot growth. Twig blighting in successive seasons may cause the formation of witches- brooms. On more resistant elms, leaf spots remain smaller in size (0.5 to 3.0 mm in diameter) and eventually develop a slightly raised, black crust or stroma containing fruiting structures (acervuli) of the fungus. The spots may be surrounded by a white to light yellow halo. Repeated infections in early summer result in multiple spotting and premature defoliation.

Epidemiology

 

The fungus overwinters in dead leaves on the ground and in dormant buds. In spring, spores (ascospores) are released from fruiting bodies (perithecia). These spores infect newly developing leaves and stems. Secondary spores (conidia) are released from acervuli embedded in the stroma by rain splash impaction. Leaves are most susceptible to infection in spring and early summer. A third, noninfectious spore stage (sporidia) is formed later in summer.

Host range

 

Most elm species are susceptible to black spot, although there is considerable variation in disease severity among cultivars of the same species. Certain cultivars of Chinese elm appear to be highly susceptible.

Management

Remove leaf debris and dead shoots from trees during winter to reduce overwintering inoculum of the fungus. Avoid close spacing and overhead irrigation of cuttings and young trees. Preventive fungicide applications may be required for susceptible elm cultivars. Make the first application at bud break in spring and continue at regular intervals until leaves are fully developed.

 

Table 1. Fungicides labeled for black spot control
Fungicide Typical Application Interval Examples of Products
mancozeb 14 days Dithane T/O, Dithane WF, Flowable Mancozeb, Fore

Protect T/O
myclobutanil 14 to 21 days Systhane

Homeowner:  Immunox

 

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Web updated 9/01/06
Web comments to jpierz@ksu.edu