Leaf rust was identified on wheat in North Dakota earlier than usual this year. Reports of the disease during late May surprised even the most seasoned rust spotters. Fortunately, spore populations didn’t build as rapidly as was expected which allowed time for the crop to grow.

Ardell Knudsvig, plant health specialist with the Minnesota Dept. of Ag, has reported an increasing incidence of leaf rust on wheat in Pennington and Marshall Counties in northwest Minnesota. Most severely infested wheat fields in those areas are approaching uniform disease incidence. For those of you that don’t normally think in plant pathological terms, that means nearly every plant within a field has at least one rust spot, or pustule. No matter how it’s stated, that’s a lot of pustules. On average, wheat growth stages in those counties range from milk to dough.

Growth stage of plants is an important consideration before getting too excited about leaf rust. Generally, when plants are severely infected early, the possibility of yield loss is substantial. Since the spores must arrive in Minnesota on winds from southern states, early plant infection isn’t too common. According to Marcia McMullen and Jack Rasmussen, authors of the North Dakota Wheat Leaf Rust bulletin (http://www.ext.nodak.edu/extpubs/plantsci/smgrains/pp589w.htm), susceptible varieties may have yield losses as high as 30% if the plants are severely infected prior to flowering. Alternatively, if plants are severely diseased during grain maturation (soft to hard dough) there could be losses of 5-15%.

Fungicides labeled for use on wheat leaf rust are effective in controlling the disease if a timely application is made. A general rule of thumb for preventing yield loss is to apply fungicides before 5% of the flag leaf is covered by pustules. Strobilurin-based fungicides (e.g., Quadris, Stratego, and Headline)* are not recommended for disease control at heading. Research indicates that strobilurins can increase DON concentrations when Fusarium head blight (FHB or scab) is present. Triazole fungicides (e.g.: Tilt, PropiMax, or Bumper)* do an excellent job of controlling rust at heading without causing an increase in toxin levels. Do not apply triazoles within 40 days of harvest. Always read and follow pesticide label directions.

*Mention of product names does not constitute a guarantee or warranty of the product by the University of Minnesota Extension Service or the author.