Weather Favorable for Wheat Scab in 2001

Weather this summer has been overwhelmingly favorable for development of fusarium headblight, or scab, in the northern half of North Dakota, according to Marcia McMullen, plant pathologist with the North Dakota State University Extension Service.

"North Dakota wheat and barley fields are certainly no stranger to this disease, but until recent years, head scab has been found most frequently in the eastern part of the state, areas with traditionally higher moisture levels," McMullen says. "Last year, some areas in north central and northwestern North Dakota had weather favorable for infection during flowering, and head scab was a problem in some durum fields."

Scab, caused by the fungus Fusarium graminearum, may invade developing small grain heads from flowering stage through soft dough stage of kernel development. The primary source of the fungus is previously infected grain residue. Weather conditions that favor scab infection include frequent rain showers, high humidities, and heavy dews for extended periods of time during these critical stages of growth. Optimal temperatures for infection are 75 to 85 degrees, but infection can occur at cooler temperatures when moisture or dews persist for 72 hours or longer, conditions which did happen in much of the northern tier of counties this year.

McMullen says preliminary evaluations of cultivar response indicate that cultivars known to have more tolerance to the disease, such as Alsen and Gunner, have some infections in the severely impacted fields but much less than very susceptible cultivars such as McNeal or Amidon hard red spring wheat or Monroe or Dressler durum. Preliminary evaluations of fungicide applications to flowering wheat also indicate that available fungicides reduced scab field severities from 60-80 percent.

"Growers currently need to consider harvest options for scab infected fields. Options include harvesting diseased fields as soon as possible, harvesting the severest damaged areas of the field separately, using appropriate combine settings to remove the lightest diseased kernels, cleaning of the grain, binning fields separately, storing grain at appropriate moisture contents, and holding grain until the extent of yield and quality losses are known and markets stabilize, says McMullen

The weather has been overwhelmingly favorable for infection from July 11 onward in much of the northern half of the state, she says. For example, N.D. Ag Weather Network (NDAWN) data from sites in the north central and northwest crop reporting districts indicate that a dry spell from June 28-July 10 was followed by many days of frequent rain showers. At Berthold rainfall was recorded on 11 of 20 days from July 11 to July 31, and 17 of those days had relative humidity levels of 90 percent or greater for nine to 20 hours of the day. This meant environmental conditions were very favorable for scab infection at the same time much of the hard red spring wheat and durum crops were flowering or in early kernel development in this area.

Yield losses may range from 5 to 25 percent, or more, depending on timing of infection and cultivar grown, says McMullen. Similar weather conditions also occurred in northeast counties, with late planted grain in these areas also at risk.

"Producers in affected areas will suffer yield and quality losses at varying degrees. Any crops that flowered early in July during the dry weather should have less damage from the disease than those that flowered during the wet spells," she says.

Effects of scab infection are greater the earlier the disease strikes during kernel development. The kernel is vulnerable to infection as long as kernel moisture is 28 percent or above. A flowering infection results in either a completely aborted kernel or one that is very shriveled and chalky. An infection shortly after flowering results in a kernel that is still shriveled and chalky, but not as small as ones infected at flowering. Kernels infected at watery ripe stages may be large enough not to be discarded out the back of the combine, but still be discolored and non-vitreous. Late infections result in some discoloration, but kernels may be almost normal size and are more difficult to remove with harvest or grain cleaning equipment. Grain cleaning will improve test weight and reduce levels of the mycotoxin byproduct, DON or vomitoxin, in the grain samples.

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