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Issue No. 9, Article 8/May 22, 2009

Wheat Disease Update

Scab. Fusarium head blight (scab) is starting to rear its ugly head in wheat fields in portions of southern Illinois. This comes as no surprise, as some fields in southern Illinois were beginning to flower at a time when the risk for scab was high there based on the wet weather at the time. (See "Wheat Scab Alert" in issue 7 of the Bulletin, May 8.) For fields in central and northern Illinois that have not yet begun flowering or that are beginning to flower this week, scab risk is currently low (check the Fusarium head blight risk assessment tool for current risk of scab at www.wheatscab.psu.edu/). Remember that the window for fungicide application to protect against scab is very small. The recommended timing is Feekes 10.5.1 (early flowering). Once the crop is beyond this stage, fungicides should not be applied to protect against scab. Once symptoms begin to appear (as is now true in southern Illinois), no treatments can be applied. The only management practice that can be done in symptomatic fields is to set the combine to blow as much light and shriveled grain as possible out of the back of the combine at harvest to reduce the amount of deoxynivalenol (DON) contamination.

Wheat scab is beginning to rear its ugly head in some fields in southern Illinois. (Photo by Dennis Epplin, crop systems extension educator, Mt. Vernon, IL).

Fungal leaf diseases. Currently, no wheat rust diseases have been reported in Illinois. Symptoms of leaf blights caused by the Stagonospora/Septoria complex and powdery mildew have been observed in lower leaves, but flag leaves have been relatively free of disease thus far.

Virus symptoms on leaves and bacterial mosaic. Some reports of flecking, mosaic patterns, and discoloration of wheat leaves have been observed and reported in portions of the state. In some cases, leaf discoloration (purple to reddish) has been attributed to barley yellow dwarf virus (BYDV), which is transmitted to wheat leaves by aphids that can feed on them in the fall and/or spring. Generally, fall infection of BYDV is more severe and can cause stunting and yield losses. At this point in time, no treatment is recommended for control of BYDV. For control next season, choose a variety with resistance to BYDV. Insecticide seed treatments may also help prevent aphid feeding in the fall, which can reduce the risk of fall BYDV infection.

Purplish-colored leaf tip symptoms of barley yellow dwarf virus (BYDV) on wheat leaves.

Symptoms of bacterial mosaic from a Wayne County wheat field. Photo by Nancy Pataky, University of Illinois Plant Clinic.

In addition to the BYDV symptoms, flecking and mosaic patterns on leaves have been reported. In some cases these may be due to wheat spindle streak mosaic virus (WSSMV) or wheat soilborne mosaic virus (WSBMV). Both viruses are transmitted to wheat by a fungal-like organism, Polymyxa graminis, which is a soilborne parasite of wheat roots. Symptoms of these viral diseases are usually best observed earlier in the spring, because plants tend to recover from the symptoms as temperatures begin to rise. Viruses may not always be the causal agent, however. Recently, bacterial mosaic (caused by the bacterium Clavibacter michiganense subsp. tessellarius) was confirmed in Illinois in samples sent to the University of Illinois Plant Clinic. Symptoms caused by bacterial mosaic can be very similar to those caused by WSSMV and WSMV; diagnosis using symptoms alone is thus difficult. So far, bacterial mosaic has been confirmed in wheat samples from Wayne and Madison counties that were sent to the Plant Clinic and in a sample from Mason County (Mason County confirmation came from Kelli Basset with Pioneer Hi-Bred). The only other known reports of this disease in Illinois came in 1990, where it was reported in Gallatin and Clay counties. No control recommendations are available for bacterial mosaic; however, because the pathogen could possibly contaminate seeds, it is recommended that you not save any seed from affected fields for planting.--Carl A. Bradley

Carl A. Bradley

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