Cool wet springs with inter-dispersed warmer and colder periods can increase fungal infections. Small, white afflicted heads in flowering orchardgrass may be related to the infection and/or may be a result of some hard frosts.
As discussed in a 1991 "Reports on Plant Diseases No. 312 - Stagonospora Leaf Spot or Blotch of Forage Grasses" is the likely culprit causing the tan to red leaves from the tip down. 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 spores (conidia) are carried by air currents or splashing rain drops to nearby plants or adjacent fields. Under optimum conditions of continuous or abundant moisture and temperatures of 68 to 74 °F, the spores germinate and infect new host tissue, producing another "crop" of spores within 72 hours.
In cool damp weather these cycles can continue throughout the summer until late fall, and may reach a peak just before or during heading time. Periods of dry weather or high temperatures check spore germination and infection, thus halting the disease development. Stagonospora leaf spots are often confused with those produced by the fungus Septoria. However, the fungal spores must be examined microscopically to distinguish the two pathogens.
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.
Many grasses, including most of our main cool season forage grasses, are affected by species of Stagonospora and are listed in the cited publication.
Suggestions for management
Where possible, sow only certified, disease-free seed of resistant varieties.
- Excessive rates of fertilizers high in quickly available nitrogen.
- Pure dense stands of a single variety. Where possible, seed a mixture of forages.
- Leaving a heavy mat of hay on the grass during damp weather.
Rotate with a non-grass crop where practical (to help prevent disease buildups.
Follow recommended mowing and grazing practices (Don't cut/mow too close.)
Keep down weed grasses by cultural or chemical means.
Figure 2. Stagonospora on Orchardgrass flag leaf and arrested head development (top) relative to unaffected leaves and normal head development (bottom.)