Bob Mulrooney, Extension Plant Pathologist; bobmul@udel.edu
The wet weather and the warmer temperatures will accelerate the development of scab if it is present and the symptoms will become more evident as the temperatures get warmer this weekend. Some scab is present on Delmarva. The first symptoms of Fusarium head blight include a tan or brown discoloration at the base of a floret within the spikelets of the head. As the infection progresses, the diseased spikelets become light tan or bleached in appearance. The infection may be limited to one spikelet, but if the fungus invades the rachis the entire head may develop symptoms of the disease. The base of the infected spikelets and portions of the rachis often develop a dark brown color. When weather conditions have been favorable for pathogen reproduction, the fungus may produce small orange clusters of spores or black reproductive structures called perithecia on the surface of the glumes. Infected kernels are often shriveled, white, and chalky in appearance. In some cases, the diseased kernels may develop a red or pink discoloration.
Fusarium head blight or scab on wheat.
Fusarium head blight or scab on wheat head.
Grain produced in heads damaged by Fusarium head blight is often shriveled, white, and chalky in appearance.
Fusarium graminearum is known to produce two important mycotoxins, deoxynivalenol (DON) and zearalenone, which can contaminate the diseased grain. The mycotoxin DON can cause reduced feed intake and lower weight gain in animals at levels as low as 1-3 ppm, especially in swine. Vomiting and feed refusal can occur when levels of DON exceed 10 ppm. Humans are also sensitive to DON, and the FDA has recommended that DON levels not exceed 1 ppm in human food. Ruminant animals, including dairy cows and beef cattle, are less sensitive to the toxin. The fungal toxin zearalenone has estrogenic properties and produces many reproductive disorders in animals. Swine are the most sensitive to the toxin, but cattle and sheep may also be affected. Zearalenone concentrations of 1-5 ppm can result in negative effects in animals and humans. Producers concerned about these mycotoxins should have grain tested prior to feeding to animals. Contact the state department of agriculture or local extension office for more information about testing for mycotoxins.
When high levels of Fusarium head blight are present in fields, precautions can be taken to reduce mycotoxin contaminations of the grain. The mycotoxin contamination is often highest in the severely diseased kernels. Adjusting the combine to blow out the small, shriveled kernels can help reduce mycotoxin levels. Harvested grain should be dried to 13.5% moisture as soon as possible to limit continued fungal growth. Grain suspected to have been damaged by Fusarium head blight should be tested for DON and zearalenone. Do not mix contaminated grain with good grain prior to a mycotoxin analysis. The mixing will result in more contaminated grain, which may be difficult to sell. Edited from Penn State fact sheet on Head Blight authored by Eric DeWolf. http://www.wheatscab.psu.edu/PDF/Fusarium_Head_Blight_.pdf
Other wheat diseases that we are seeing are take-all and, just recently, tan spot. Take-all is characterized by patches in the field that can vary in size but the wheat is generally stunted and the heads bleach out prematurely. Infected plants can be easily pulled out of the ground due to the extensive root rot that occurs. The other symptom is the dark streaking at the base of the stem (lowest node under the leaf sheaths). See picture. Take-all can be controlled by rotating out of wheat for a year. However planting wheat followed by double crop soybeans followed by wheat is not an effective rotation for take-all control. Manganese levels also interact with take-all. Be sure that soil levels of manganese are adequate for the crop and check pH so that the managanese is available. High pH makes manganese unavailable.
Take-all symptoms on the lower nodes. Note lack of roots as well.
Tan spot was identified in my wheat fungicide trials near Middletown. This foliar disease can look like Septoria (Stagnospora) leaf and glume blotch. It is caused by the fungus Pyrenophora tritici-repentis. It is too late for any control but this disease will be favored by the wet and warm weather. Most of the spots were in the lower canopy and may reach the flag leaf before the plants begin to dry down. Applications of foliar fungicides at heading or earlier should provide pretty good control of this disease. At present most of the infection is in the lower canopy and the effect on yield should be minimal if the disease does not move up to the flag leaf or the leaf below the flag leaf.
Tan spot symptoms on wheat.