Figure 1. Stripe rust lesions on a wheat leaf.
Stripe rust was found in one winter wheat field in Hand county last week. A few plants at the edge of the field had stripe rust pustules (Figure 1). This is the first observation of stripe rust in 2017 in South Dakota. Stripe rust forms yellow/orange lesions (pustules) full of millions of spores. These spores are blown by wind into other fields.
The presence of stripe rust at this time indicates that there is potential inoculum for this rust in our area. Stripe rust has already been reported in Nebraska and other southern states. It is possible that the stripe rust observed in this field overwintered there or could be from spores blown in from southern states.
Scouting is encouraged to assess presence of this rust and to plan a fungicide application, depending on the field incidence level and weather conditions. Stripe rust is caused by Puccinia striiformis f. sp. tritici and infection and progress is favored by cooler temperatures (optimum range 45-59 °F) and wet conditions (at least 8 hours of leaf wetness). Warmer conditions (>75 °F) curtail progress of this rust but infection can still take place under cool night temperatures.
Powdery mildew was seen in a few fields in central South Dakota. Powdery mildew is common in fields with quick, thick growth. It starts to develop in lower leaves where there is poor air movement and long periods of leaf wetness. Small cottony mycelia on the stems and the upper leaf surface is the first visual symptom of a powdery mildew infection (Figure 2). If environmental conditions are conducive to powdery mildew growth, infection points expand and coalesce and can cover the entire leaf leading to a reduction in photosynthesis.
Figure 2. Powdery mildew developing on lower leaves.
Powdery mildew is caused by a fungal pathogen Blumeria graminis f. sp. tritici. This pathogen survives on infected crop residue. Infection of wheat by this pathogen can take place any time from when seedlings emerge in the fall. However, wheat is most susceptible during the rapid growth period in the spring. Powdery mildew develops under humid conditions and at temperatures between 60 and 72 °F.
Tan spot was found in every winter wheat field scouted last week. However, the level of tan spot was very low and mainly on lower leaves. Tan spot is caused by a fungal pathogen, Pyrenophora tritici-repentis, which survives in wheat stubble. Infected plants in spring and summer provide the source of inoculum for the following season. The fungus requires a wet period of at least 6 hours for spores to be produced. The spores are then dispersed by wind and rain splash onto lower leaves. Maturing lesions also serve as a source of secondary inoculum. Tan spot lesions start as dark brown spots on the lower leaves and have a yellow halo around them (Figure 3). The lesions may expand and coalesce to form large necrotic (dead) areas on the leaf.
Figure 3. Tan spot symptoms. Notice the dark brown center and a yellow halo around the lesion.
Early Disease Management
Scouting is crucial to determine the level of disease and to decide on the need for a fungicide. Scout each field by walking in a “W” pattern making a stop at each turn. Observe the lower leaves of at least 10 plants at each turn. Begin scouting about 15 ft from the field edges. If stripe rust if found at 2 out of 5 stops, a fungicide may be necessary. If stripe rust is present, it can spread very quickly in fields under the right environmental conditions. If tan spot and powdery mildew are just starting, a fungicide may be delayed as these diseases do not typically spread as quickly. It should be noted that the most beneficial fungicide timing for managing foliar fungal pathogens is at flag leaf. This ensures that the top three leaves are protected from fungal disease development. However, for fields planted into wheat stubble and with moderate levels of fungal leaf spots developing, a fungicide applied at herbicide timing may be beneficial. The weather outlook will affect the spread of all the above mentioned diseases and should be considered when making fungicide application decisions.
Several fungicides registered for wheat in South Dakota are effective against stripe rust and other fungal leaf spot diseases. The North Central Small Grain Diseases Working group provides rating of various fungicides against these diseases. Consult the table on this link for various fungicides and their efficacy rating.