Nathan Kleczewski, Extension Specialist – Plant Pathology; nkleczew@udel.edu
As I mentioned last week, there have a few instances of growers with fields of wheat or barley with bleached heads. Generally the bleaching is found throughout the field and is not located in any specific parts of the field. Whole heads to individual spikelets have been observed. The incidence and severity varies from nearly nonexistent to roughly 40%. In a few cases this bleaching is a direct result of Fusarium head scab. This is not surprising as we did have favorable conditions for a short time and once spores are produced, infection can occur as long as humidity and temperature are favorable for the pathogen. In addition, we are seeing some damage that is a result of frost. Lastly, some samples we have received appear to have frost damage, but also contain Fusarium. This could be due to secondary colonization (the fungus colonizing frost-damaged tissue) or a direct result of scab. Scab can cause abortions above the point of infection if a highly susceptible variety is planted, but the fungus will be present at the base of the spikelets. As a result, this may look like frost damage. Regardless, fields with high levels of head bleaching and confirmed Fusarium, which is accomplished by sending samples to the University of Delaware Plant Diagnostic Clinic, need to be carefully handled. This is because Fusarium can increase mycotoxin (e.g. DON) levels in grain. DON is toxic to humans and some animals. The FDA regulations for mycotoxin levels can be found here: http://www.fda.gov/Food/GuidanceRegulation/GuidanceDocumentsRegulatoryInformation/ChemicalContaminantsMetalsNaturalToxinsPesticides/ucm120184.htm. The following are some precautions you should consider if you have severe bleaching of wheat heads and confirmed presence of Fusarium.
1) Harvest your grain as soon as you can to help prevent continued growth of the fungus into other, healthy grains in partially damaged heads.
2) Increase the fan speed on your combine to help blow out infected kernals, which are lighter than healthy kernels,
3) Dry your grain down to between 13 and 20%. Additional fungal growth and DON levels are reduced under these conditions.
4) Store grain harvested from symptomatic fields separate from grain harvested from asymptomatic fields.
5) Growers may also wish to sample fields or bins and send samples off for DON analysis. The presence of Fusarium on grain does not guarantee high test levels of DON. A number of commercial and government laboratories can do this for a fee. The UD Plant Diagnostic Lab is not equipped to provide this service. There are also various test kits that you may purchase and test for DON yourself (www.gipsa.usda.gov/fgis/tech-servsup/metheqp/testkits.pdf)
The University of Nebraska offers an excellent publication on diagnosing head disorders of wheat, which can be located here: http://www.ianrpubs.unl.edu/live/ec1872/build/ec1872.pdf. Information on freeze injury to wheat, including critical temperatures at given stages of development can be found in the following publication: http://ianrpubs.unl.edu/live/ec132/build/ec132.pdf.