As temperatures and relative humidity values are creeping up so are the hours of leaf wetness and with that the risk for fungal infections. Since last Saturday about two-thirds to three-quarters of the days were favorable for tan spot to develop and about half the days were favorable for leaf rust and Septoria spot blotch. The latter two diseases like it a little warmer than tan spot does. Surprisingly both the National Fusarium Risk Tool as well as the NDAWN Small Grains Disease Forecast model for FHB continue to indicate a low risk of initial infections of Fusarium head blight. There are a few locations in the NDAWN system, however, where the model indicates moderate risk for Fusarium Head Blight. Greenbush, MN, is one of those locations.
Both models share the same origins and rely in a sense on the same framework of sufficiently high enough relative humidities for at least seven days preceding the current day. The higher relative humidity triggers the development of ascospores that subsequently can infect the crop during the most critical period of the wheat development - anthesis and the first week of grainfill.
While rain events are not directly in the model they are in many ways a prerequisite to creating these higher relative humidities. There in lie two possible sources of error in the models' accuracy. Rain events in Minnesota and North Dakota are often very localized with lots of 'have' and 'have nots' over very short distances. That means that the relative humidity in your fields might be much higher than the models indicate. The second weakness is that our soils have lots of organic matter and tremendous water holding capacity, even near the surface. Add to this the fact that the wheat will use water to cool itself by evaporating it during periods of the day that are too warm for the plant to function optimally. With daytime highs in the mid-eighties, the crop uses about a quarter of an inch of water per day. Most of that will end up as water vapor in the canopy. This in turn means that the microclimate in the canopy can be more favorable for ascospore development than is suggested by the recorded relative humidity of a nearby weather station.
Bottom line - I'm more nervous about the risk of FHB infections than the NDAWN FHB risk model and the National Risk Tool currently indicate. The immediate weather forecast continues to allow for extended leaf wetness periods in a temperature range that favors fungal diseases, including FHB, to develop.
Source : umn.edu