Since the Fourth of July holiday temperatures and dew points have been unpleasantly high for small grains. Needless to say, the risk models for FHB have been high for all of Minnesota as the Northern Red River Valley has also received some much-needed rain since my last report two weeks ago.
The first symptoms of FHB take about 7 to 10 days to develop. This week, therefore, is a good time to start accessing how much damage FHB may inflict this year as a lot the spring wheat across the state headed between the 4th and 10th of July.
These early FHB infections have the most impact on grain yield. First, the affected florets and spikelet will produce the tell-tale, chalky-white tombstone kernels. Secondly, these early infections have the best chance to grow into the rachis is cut off the nutrient flow to the developing kernels in the florets and spikelets above the initially infected floret, thereby halting grain fill.
One key difference between varieties that are rated very susceptible to FHB and those that are rated moderately resistant, is their ability to slow the disease development into to rachis and cutting off the development of the grain above the initial infection.
And although FHB is really a monocyclic disease, i.e. the current infections will not produce spore that will infect the current crop, the crop remains susceptible until you have the crop in the bin. The spores will come from the same sources of inoculum that provided the ascospores for these first infections. The yield losses of these later infections, however, are much less dramatic and the concern is more the presence of DON in the grain.
Finally, these high temperatures and dew points are already taking their toll on the yield potential of this crop. Especially the high nighttime temperatures shorten the grain fill period, resulting in fewer kernels and lower kernel weight.
--Jochum Wiersma, Extension Agronomist, University of Minnesota Crookston
For more details, go to the FHB Risk assessment tool at http://www.wheatscab.psu.edu
For the latest news and updates from the U.S. Wheat & Barley Scab Initiative, go to https://www.scabusa.org