Plant Disease Management Handbook

Cause Puccinia striiformis, a fungus that overwinters on infected plants. In the Willamette Valley, stripe rust is a serious disease in bluegrass. It also attacks orchardgrass. An alternate host for stripe rust is unknown. This population of stripe rust does not attack wheat, or vice versa. In spring, primary infection is from windborne urediniospores produced on infected plants. Secondary spread continues from subsequent infections.

Cause Puccinia graminis subsp. graminicola, a fungus that overwinters as slow-growing mycelium in overwintering plants. Susceptible hosts include perennial ryegrass, tall fescue, Chewing's fescue, orchardgrass, bluegrass. The principal alternate host is barberry (Berberis spp.). In spring, the fungus resumes more vigorous growth and produces primary inoculum (urediniospores) that are windborne.

Cause Unknown, but it is suspected that physical damage caused by thrips as the females insert eggs individually through the epidermis of the stem opens up the stem area to invasion by pathogenic bacteria. If bacteria are responsible, they likely survive in infested residues. This problem has been observed since before the mid-1990s in orchardgrass but is not considered to be yield limiting.

Cause Several insects, including thrips, stem borers and plant bugs, cause physical damage and/or inject plant toxins during feeding. Poa species and, to a lesser extent, orchardgrass are affected. The fungus Fusarium poae is associated with silvertop but only after damage has been done.

Cause Myxomycetes (Physarum spp., Fuligo spp., and Mucilago crustacea) that live in the soil or thatch and are favored by wet weather and an abundance of leaf litter. The appearance of slime mold (sporangia) can cause great concern but is not considered a disease and will recede after 1 to 2 weeks. Sporangia persist in the soil or litter where they have fallen, and they germinate during or after wet conditions.

Cause Septoria spp. (teleomorph: Mycosphaerella) or Stagonospora spp. (teleomorph: Mycosphaerella), fungi that survive winter and summer as mycelia or pycnidia (minute, black, flask-shaped fungal structures) in infected debris. These fungi attack a broad range of hosts, including bentgrass, bluegrass, fescue, and ryegrass. Bluegrass is more susceptible than other grasses.

Cause The fungi, Fusarium sambucinum, F. culmorum, and F. avenaceum, have been found on orchardgrass plants in Oregon seed production fields. Other Fusarium species are known to pathogenic on turf grasses: F. acuminatum, F. crookwellense, F.

Cause At least two species of Rhynchosporium scald fungi. Rhynchosporium orthosporum infects bentgrass, fescue, orchardgrass, ryegrass, and bluegrass; R. secalis is best known as the cause of scald in barley, but it can also infect bentgrass and ryegrass. Rhynchosporium scald is one of the more common causes of leaf spot on orchardgrass, perennial ryegrass, and tall fescue in winter and spring.

Cause A bacterium, Rathayibacter rathayi (formerly Corynebacterium rathayi), that is seedborne in orchardgrass and survives in affected debris and soil. It does not spread rapidly through a field or appear to affect seed yield except in years with heavy rain during orchardgrass heading. Rathayibacter depends on the Anguina sp.

Cause Mastigosporium rubricosum, a fungus that overwinters as mycelium in infected leaves and affects orchardgrass, bentgrass, and timothy. Conidia develop from overwintered mycelium and are disseminated in water and wind. The fungus requires cool, wet weather to infect hosts. The disease is most severe in the cold, rainy, or foggy weather of winter and early spring.