Reports on Plant Diseases

A wide variety of leaf spots and blotches on forage and ornamental grasses are caused by a number of species of the fungus Stagonospora. Orchardgrass, smooth bromegrass, reed canarygrass and timothy are attacked during periods of cool, damp weather from the early spring until late fall. Other forage, ornamental, and weed grasses affected by species of Stagonospora are listed in Table 1.

When Stagonospora leaf spot or blotch is prevalent, the infected leaves turn brown, wither, and die. The lower 30 to 40 percent of a severely damaged plant is often completely defoliated. Uncut plants are usually affected to a greater extent.

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Figure 1. Stagonospora leaf spot on orchardgrass.

Figure 2. Extreme closeup of Stagonospora leaf spot on smooth bromegrass.

Symptoms

Small, slightly elongate-to-elliptical spots or lesions form on the leaves, leaf sheaths, and culms early in the growing season. The lesions may be dark purple, purplish black, purplish brown, dark brown, or gray to tan, often with tan to straw-colored centers (Figure 1). Several lesions may later merge, causing the leaves to wither and die prematurely from the tip downward. In other cases, long brown streaks develop along the leaf margins. Pycnidia (fungal fruiting bodies), speck-sized and golden-brown to black, form in the dead or older affected leaves, culms, and glumes. However, the pycnidia do not always occur within the visible lesions (Figure 2).

Stagonospora leaf spots are often confused with those produced by species of the fungus Septoria. The fungal spores (conidia) must be examined microscopically to distinguish between the two pathogens. Septoria conidia are needle-shaped and, as a rule, are at least 10 times as long as they are wide. By contrast, Stagonospora conidia are broader or more cylindrical and are usually less than 10 times as long as they are wide (Figure 3).

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Figure 3. Left, typical spores (conidia) of Stagonospora species; right, typical conidia of Septoria species.

Disease Cycle

The Stagonospora fungi over-winter as pycnidia embedded in dead leaf or culm tissue. The fungi resume growth during cool, spring weather when the moisture conditions are damp to wet. The conidia exude from the pycnidia in yellow or hyaline tendrils and act as primary inoculum. The conidia are carried by air currents or splashing raindrops to nearby plants or adjacent fields. Under optimum conditions of continuous or abundant moisture and temperatures of 68° to 77°F (20° to 25°C), the conidia germinate and infect new host tissue, producing another "crop" of conidia within 72 hours. In cool, damp weather these cycles continue throughout the summer until late fall, reaching a peak just before or during heading time. Periods of dry weather or high temperatures check conidial germination and infection, thus halting disease development.

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