Grass for Seed-Rathay's Disease {Bacterial Head Blight}

Note the necrotic seed heads (orchard grass).
Note the necrotic seed heads (orchard grass).
Infected orchardgrass plant, note the yellow-orange material on seed head.
Infected orchardgrass plant, note the yellow-orange material on seed head.

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. nematode (see Grass for Seed-Nematode, Grass Seed Gall or Seed Gall) in orchardgrass to infect plants. Rathayibacter adheres to the Anguina and is carried into the plant as the nematode migrates among leaf sheaths to the base of the plant. The nematode infects the developing panicle, carrying Rathayibacter to the infection site. Anguina sp. infects the ovary and replaces the seed with a gall containing nematodes but in some cases Rathayibacter will overtake the Anguina and discrete, lemon yellow colored galls will be produced instead of the nematode galls. More commonly, as Rathayibacter proliferates, a yellowish, slimy mass exudes from infected florets. Anguina nematodes do not survive the excessive bacterial growth.

Symptoms A gooey, sticky mass of yellow-orange material is visible on seed heads and this later dries to a brittle, glass-like covering. The panicle is the only organ of the plant affected but the yellow exudate can stick to leaves or stems. Yellow colored galls that replace seed galls from Anguina can also occur.

Cultural control Control of Rathayibacter has not been established but control of Anguina is believed to control Rathay's disease (see Grass for Seed-Nematode, Grass Seed Gall or Seed Gall). Removal of infected residues after harvest by baling is believed to reduce the level of Rathayibacter. In addition, burning is assumed to destroy infested residue.