The April 24 to May 1, 2017 period has been a critical week for the Kansas wheat crop. Multiple freeze events and snowfall in much of western region of the state dominate the concerns. What about the impact of the freezing temperatures on the development of disease? Unfortunately, the freezing temperatures are unlikely to kill the stripe rust and leaf rust that were already established in our fields. The frequent rains may stimulate additional disease development in areas of the state that escaped the freeze events.
A look at the weather during the past two weeks indicates that conditions were conducive for continued disease development in central and southeastern regions of the state (Figure 1). Many areas of central Kansas had between 12 to 44 hours of favorable temperature and humidity conditions. The southeast region experienced more than 50 hours of temperature and humidity that were favorable for stripe rust.
Observations from the field indicate that stripe rust has moved to the upper leaves of wheat in some parts of central Kansas (Figure 2). Stripe rust is now severe in some fields that were not protected with fungicides in southeast Kansas. The disease had moved to the upper leaves in this region of the state a few weeks ago. Stripe rust was also detected at low levels in the middle or low canopy in new parts of north central and northwest Kansas. Leaf rust was also reported in some additional counties this week with more activity farther west than we had seen to date (Figure 3). Leaf rust was still restricted to the lower and middle canopy in most fields but there are a few locations where the disease has moved to the upper leaves.
Figure 1. Distribution of weather conditions that were favorable for the development of stripe rust in Kansas. Map shows the number of hours that temperature was between 45-55F and humidity was greater than 87%. Source: Kansas Mesonet and the KSU Weather Data Library.
The reports of disease moving to the upper leaves in central Kansas and additional reports of disease further west are cause for concern. However, the concerns about freeze injury and damage done by recent snow complicate the decision to apply a fungicide to manage rust or other diseases. Growers in areas that escaped the freeze should be considering fungicide applications to prevent further development of stripe rust and leaf rust in susceptible varieties. Growers that are uncertain about yield potential because of damage from recent freeze events and snowfall should be more cautious. Growers with wheat that is at the flag leaf emergence or boot stage of development may have 10-14 days to make their final decision about fungicide applications for this year. Where possible, growers should wait to see how the crop recovers before investing in a fungicide application.
Erick DeWolf, Wheat Extension Pathologist
Romulo Lollato, Wheat Production Specialist
Mary Knapp, Weather Data Library
Chip Redmond, Kansas Mesonet