Issue No. 14, Article 2/June 27, 2008
Wondering about Wheat Diseases?
Wheat in western Illinois has seen a fair share of disease on the flag leaf in the past month. Familiar diseases showing up include scab, leaf spots, rust, and viruses. Photos of some foliar diseases on wheat can be seen online in issue 13 of the Bulletin (June 20, 2008). However, this year's very long, cool, and wet spring may have predisposed wheat to a few less-characteristic diseases that the plant might typically outgrow in a normal season.
Pythium root rot was found in a wheat field in western Illinois just prior to anthesis. Symptoms of Pythium root rot on adult wheat plants include stunting, yellowing of leaf tissue, and the appearance of nitrogen deficiency. Plants infected by Pythium spp. often have delayed heading and maturity, so they develop heads that are small and poorly filled. Pythium damages the wheat root systemi--n particular, the fine roots are damaged and lacking, and the root systems are smaller, with very few root hairs. Lateral roots and root tissue are brown with yellow-brown root tips.
Brown discolored wheat roots from Pythium root rot (L. Ortiz-Ribbing).
Root systems will die back when Pythium infestations are severe. Infection by Pythium can be difficult to diagnose, and symptoms caused by these root rot fungi are often more uniformly distributed throughout the field compared to symptoms caused by other soilborne pathogens, so symptoms may go unnoticed because most plants may show symptoms to some degree.
Many of the wheat plants infected by Pythium spp. were also infected with very common bacterial organisms (Pseudomonas spp.) that can cause bacterial leaf blight. Under appropriate conditions, bacterial leaf blight develops on the upper wheat leaves after boot stage. Symptoms begin as small, water-soaked leaf spots that turn brown as they expand, eventually turning gray-green to tan-white in color. The spots often grow together to form streaks or blotches that may initially be mistaken for virus symptoms. The heads and glumes of wheat plants infected with bacterial leaf blight may remain symptomless, while eventually entire leaves appear brown and dead. Infection by this bacterial organism can result in loss of more than 50% of wheat leaf tissue.
The last pathogen infecting these wheat plants was wheat streak mosaic virus (WSMV), which is vectored by the wheat curl mite, most frequently in autumn. However, symptoms rarely show up until spring, when stunting and yellowing streaking become more obvious. This virus causes the wheat head to be partially or totally sterile, and individual leaves or the entire plant can become yellow and die.
Wheat plant showing symptoms from three pathogens (L. Ortiz-Ribbing).
Management. None of these pathogens can be controlled by foliar fungicides. The virus is transmitted by the wheat curl mite, so control includes cultural practices that minimize the mite in newly emerged wheat fields in fall, such as destroying volunteer wheat or any green-bridge crop like corn, millet, and susceptible grasses like buffalograss, and planting later after the Hessian fly-free date. Some tolerant wheat varieties may be commercially available--check with your seed dealer. The Pythium organisms remain in soil for a long time and have a wide host range, so crop rotation is not an effective control. Initial infection often begins in the germinating wheat seed, so wet conditions at planting encourage infections. Planting into relatively dry or well-drained seed beds, using high-quality seed and supplemental phosphorus, and using systemic fungicide seed treatments can help where this disease is a problem. Pseudomonas bacteria are everywhere, but cool, wet, humid weather favor disease development. The bacteria are also spread by wind-driven rain and enter the plant through wounds or natural openings in leaves like stomata. Check with your seed dealer for wheat cultivars that may be less susceptible to this organism.--Loretta Ortiz-Ribbing