Figure 1. Stripe rust	Figure 2. Tan spot	Figure 3. Septoria tritici blotch.	Figure 4. Powdery mildew

May 2, 2013

Cool weather in April and continuing into early May delayed wheat growth in Nebraska by two to three weeks. The cool weather similarly delayed wheat disease development. The predominant disease in the southeast and south central parts of the state is wheat soilborne mosaic (see April 26 CropWatch article).

As of May 3, stripe rust (Figure 1) had been reported as far north as southeast Kansas. The chances are good that we may see stripe rust in Nebraska. However, if it arrives after temperatures have risen above 65°F, it may cause less damage than if it had arrived in early to mid-April as it has in previous years. If cool, wet weather continues, growers should be prepared to take preventive measures against stripe rust.

Fungicides are effective in preventing or controlling stripe rust. A list of wheat fungicides and their efficacy ratings is provided in Table 1, compiled by North Central Regional Committee on Management of Small Grain Diseases (NCERA-184).

Regular scouting of wheat fields for early detection of diseases is critical in making timely decisions about fungicide application. In addition to stripe rust, scout for tan spot (Figure 2), Septoria tritici blotch (Figure 3), and powdery mildew (Figure 4). Septoria tritici blotch can be distinguished by the tiny black structures visible in the lesion. These are asexual fruiting structures of Septoria tritici, the causal fungus of the disease. Also scout for leaf rust (Figure 5) which usually appears in Nebraska in mid- to late May, but this year is expected in early to mid-June.

Fungicide application should be timed to protect the flag leaf. However, if stripe rust is detected, cool, wet weather is forecast, and a susceptible cultivar was planted, an earlier fungicide application may be warranted. This strategy also applies to tan spot, especially if a susceptible wheat cultivar was drilled into wheat stubble.

Root and crown rot diseases are likely to be more widespread this year due to the dry soil conditions that prevailed during planting last fall. Bare patches in wheat fields (Figure 6) and stunted and yellow plants are indicative of root and crown rot diseases, but also may be due to failed germination due to dry soil conditions at planting. Little can be done to control root and crown rot diseases during the growing season. Future damage can be avoided by using fungicide seed treatments.

Stephen Wegulo
Extension Plant Pathologist