Heading Small Grains, Scab Risk, and a Few Fungicide Questions (06/17/21)
The heat is speeding up small grain development as heat units are accumulating at a record pace. Water stress is accelerating this pace even further, so the decision to apply a fungicide is here. Currently, the risk for Fusarium head blight to develop is low across the entire state of ND and Western MN. (Figures 1 and 2). We expect the risk to remain low until at least the end the end of this week.
The Fusarium pathogen needs several days of high humidity to produce spores and infect small grain florets. Although, we had a day or two of rain, hot and dry weather will hinder Fusarium production and keep the risk for scab to remain low.
Last week’s thunderstorm events might increase scab risk for very susceptible varieties by the end this week in individual fields. However, this will be dependent on if additional precipitation is received this week, and if we have some dew in the mornings over the weekend. We are expecting scab risk to remain low for most. To help determine your risk in individual fields, visit NDSU Small Grains Disease Forecasting or http://www.wheatscab.psu.edu/.
Do fungicides help with drought stress?
Producers have also contacted us with the question if fungicides help reduce water stress. A very thorough and well-designed study by Nason et al. (2007) evaluated the effect of our commonly used strobilurin fungicides and a triazole fungicide on water-use efficiency in soybean, wheat and barley. Their study indicated that fungicides improved water-use efficiency in well-watered plants. However, in water-stressed environments, fungicides actually decreased water-use efficiency (negative effect). This study supports once more that fungicides should be used for control of fungal diseases.
Is there a yield benefit?
The greatest yield response from a fungicide occurs when fungal diseases are present. Fungicides do not alleviate drought stress, nor can they increase yield - simply put, fungicides protect yield. As the yield potential dwindles because of drought stress so will be the portion that can be saved with a fungicide should diseases have a chance to reach economically damaging levels. After all, 5% of 80 bushels is not the same as 5% of 30 bushels. To get an estimate on the yield response of a fungicide in a heat stressed, low scab risk crop and with no fungal leaf diseases present, we went back through some of the research data collected in Fargo. In trials conducted in 2012-2014, the research trials in Fargo had very low scab risk, but had either some leaf rust, stripe rust or fungal leaf spots present on the flag leaf. In these trials, the yield response from Prosaro, Caramba, or Folicur (generics) was approximately 3%. Those yield responses were likely the result of managing fungal leaf diseases in these trials. Given that 2021 has very low foliar disease pressure and low scab risk, we would expect an inconsistent and very low yield response if a fungicide is applied at early-flowering.
Extension Plant Pathology, Cereal Crops
University of Minnesota Crookston