By Emmanuel Byamukama
Weather conditions drive the type and severity of plant diseases. The majority of fungal and bacterial pathogens thrive under wet, humid, and warm conditions, whereas virus and nematode problems tend to be exacerbated by moisture stress on infected plants. This wheat growing season started with less than optimal moisture conditions and this may influence what diseases develop. With the exception of cereal rusts (stripe rust, stem rust, and leaf rust), the majority of pathogens survive in our environment and thus there is enough inoculum for these diseases to develop should favorable weather conditions exist. Producers should always scout their fields before making management decisions. Some of the diseases to look out for include: Stripe Rust Although this type of rust has not yet been found in South Dakota this season, it has been reported in Nebraska. Several reports indicated stripe rust developing in Kansas, Oklahoma and Texas.
Stripe rust
travels north on wind blowing from the southern states. This would mean that stripe rust spores are likely to reach our area soon. The optimal temperature for stripe rust development is 50-65° F, and current temperatures are conducive for this. The only limiting factor would be leaf wetness duration.
Figure 1. Stripe rust pustules are orange-yellow in color and appear in a linear manner between veins on a wheat leaf.
As winter wheat is just starting to joint in some areas and spring wheat is just emerging, an early development of stripe rust could result in yield losses. Producers should scout for this rust and plan a fungicide application depending on the level of the disease and weather conditions. For fungicides that are effective against stripe rust and other fungal pathogens see the recommended fungicides by the NCERA 184 working group.
Leaf Spot Diseases
Several fungal diseases can infect wheat early, especially for wheat planted into wheat stubble. Common fungal leaf spot diseases include tan spot and Stagonospora/Septoria leaf blotch. The pathogens for these diseases survive on wheat residue and are favored by warm humid conditions. Tan spot can develop early in the season whereas Stagnospora/Septoria leaf blotch requires warmer temperatures. For no-till wheat on wheat fields, scouting and applying an early fungicide at herbicide timing may be beneficial.
These fungal leaf spot diseases develop under certain weather conditions, and a weather tracking system that records when favorable conditions occur was developed. This prediction tool forecasts development of leaf spots and leaf rust based on weather conditions (Figure 2). The prediction tool can aid producers in making the decision about fungicide application. Fusarium head blight (FHB, scab) prediction starts at when wheat is heading.
Figure 1. An example of an output when a weather station is selected. The output prints “yes” or “no” for each day. Accumulating more than 6 yes’s for each disease indicates infection is likely.
Wheat Streak Mosaic Disease
Wheat streak mosaic disease is caused by Wheat streak mosaic virus (WSMV) and is transmitted by wheat curl mites (WCM). This disease can cause significant yield losses in winter wheat especially when infections take place in fall. Symptoms are most obvious when temperatures warm up late May/early June. Infected plants are stunted, and leaves have streaking (short yellow/chlorotic and alternate with green and general yellowing). Under heavy WSMV infection, symptomatic plants can be seen from edges of the field where the WCM would have deposited it first. WSMV can be managed by eliminating the ‘green bridge’ before planting wheat.
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