Nathan Kleczewski, Extension Specialist – Plant Pathology; nkleczew@udel.edu
Growers should be aware that Stripe Rust was confirmed in different parts of Virginia, including the Eastern Shore, over the weekend. As mentioned last week, it was confirmed in Kentucky, North Carolina, and Tennessee the previous week. Currently the level of infection is low in areas of VA where the pathogen is found, so the potential for any significant movement into our area is low. Regardless, growers in southern parts of Sussex County Delaware and southern counties in Maryland should plan on scouting for the disease over the next 2 weeks, as the forecast continues to look favorable for potential disease development.
Figure 1. Stripe rust infected wheat from Eastern Shore Virginia.
The stripe rust fungus produces yellow to orange pustules that will rub off on your hands, leaving a rusty residue. Often the pathogen will form pustules in strips following the leaf veins (Figure 1). Under cool wet conditions the pathogen can spread rapidly on susceptible varieties. Early stripe rust can look like any number of foliar diseases. Suspect foliage can be placed in a plastic sandwich bag with a moist (not wet) paper towel at room temperature. If rust is present you should see pustules develop on these lesions within 72 hours. If stripe rust is detected, applications of a premix fungicide at early flag leaf will provide excellent control, provided that significant infections have not already occurred. Fungicides containing a strobilurin – (QoI, Group 11) fungicides should not be applied later than Feekes 8/9. Make sure you scout your fields and assess it for diseases before pulling the trigger. Look for foliar disease symptoms on upper, green foliage. In addition to stripe rust, look for leaf blotch complex (tan spot, Stagonospora) and powdery mildew. I cannot stress the importance of looking at your fields now to see what diseases are present.
I have had questions about powdery mildew on susceptible wheat varieties and if the cold weather has had any impact. The answer is yes, it likely has had an impact. Powdery mildew is active when temperatures are above 59°F, and certainly, we have been under this temperature the past few weeks. However, when canopies are dense you have an increased number of insulated or protected areas of the canopy that will take longer to be impacted by cold temperatures. The result is that although the total number of active pustules will decrease, there will still be some active fungus within the canopy. When our daytime temperatures increase to over 59°F, the active fungus can continue to develop and potentially spread.
That leads me to another question I have been asked several times this week: How do I know if the powdery mildew is active? Aside from asking the fungus, you can look at the color of the pustules as well as the pustule location. Old pustules will often be grey in color, spent pustules may show up as necrotic flecks or small lesions on the foliage. Right now you should be looking at the new foliage- this will tell you if the pathogen has been on the move recently. Do you see white pustules or early lesions on the upper 2-3 leaves? This may indicate a recent infection.