June 1998
June weather provides the
fuel for scab, July the fire
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Prairie Grains is the
official publication of
the Minnesota
Association of
Wheat Growers,
North Dakota Grain
Growers Association,
South Dakota Wheat,
Inc., and the
Minnesota Barley
Growers Association.
NWES agronomist John Wiersma explains that in years of scab epidemics, we have had 10 days to two weeks of weather in June conducive for the development of the spores that cause scab. These spores have then developed into full-blown scab under sustained hot, humid weather in July.
"The two weeks before anthesis is just as critical, if not more, than the two weeks after anthesis. Planting date trials in 1993 and 1994 indicate that the environmental conditions two weeks before heading are critical for scab development," says Wiersma.
Recent research data indicates that the fungus causing scab has a relatively wide range of environmental conditions under which it can develop and cause severe infection, according to Marcia McMullen, NDSU extension plant pathologist.
Past research has indicated that optimum temperatures for the production and release of the spores that cause scab ranged from 55 to 91 degrees, with the optimum being around 77 to 80 degrees. Further, past research has indicated that scab infections can occur at temperatures as low as 59 degrees under 48 hours or more of continual moisture, but that only 18 hours of continual moisture were required at the warmer temperature of 68 degrees.
In 1993, Langdon, ND and Crookston received 320% and 176% of normal precipitation, respectively, in July of 1993. Measurable rainfall was recorded at Crookston on 26 of the 31 days of July in 1993. This July period coincided with the susceptible growth stage of the crop - flowering and early grain development. Although moisture was abundant, temperatures were quite cool in both locations, 2 to 6 degrees below the 30-year average of 66 degrees for July in these locations, and considerably below the optimum of 77 degrees for infection.
Records indicate that rainfall and high-humid conditions need not be continuous for scab to develop. For example, examination of the hourly records of rainfall and relative humidities at Crookston, in 1994, another year of severe scab, indicates that during the susceptible growth stages, relative humidities ranged from 85-100% for 12 to 15 hours during the evening and early morning periods, but relative humidities dropped to 60-85% during the afternoon hours. In growth chamber studies at Fargo, infection occurred with only 12 hours or less of moisture on the wheat spikelet at temperatures from 68 to 86 degrees.
Recent studies in Canada indicated that there is a daily pattern of release of scab spores into the air, with maximum release during the late evening hours and early morning hours, when relative humidities generally increase. Further, that the highest number of scab spores were captured from inoculated wheat plots 2 to 4 days after a rainfall event. In Argentina, researchers stated that the variable that showed the strongest association with scab incidence was relative humidity: two consecutive days of humidities at 80% or greater were needed during the susceptible growth period.
Research in Germany defined conditions critical for the development of scab and vomitoxin: the wheat crop must be at 50% heading to the end of flowering; a mean temperature for at least 24 hours greater than 64 degrees is required; and rainfall must be greater than or equal to 0.2 inches on this day with the mean temperature of 64 degrees, or on the day just preceding it.
Support Cyanamid scab research matching grant
Northern Plains growers are encouraged to support a scab research matching grant offered by American Cyanamid. Through the Scab Research Matching Grant, Cyanamid will match, dollar for dollar, every "Harvest Point" award credit contributed up to $25,000.
Growers earn Harvest Points award credits through the "Harvest Partners" preferred customer program with qualified purchases of American Cyanamid crop protection products, including Assert, Avenge, and other Cyanamid products purchased from participating dealers.
These points can then be redeemed for products and services offered exclusively through the Harvest Partners program. Contributions of Harvest Points award credits to scab research will be matched through December 31, 1998 or until the contribution equals $25,000.
The money will be donated by Cyanamid on a quarterly basis to the Minnesota Association of Wheat Growers and the North Dakota Grain Growers Association, who will be responsible for allocating the scab research funds.
Growers can simply indicate the amount to be contributed to scab research on their Redemption Request Forms, which are enclosed in Harvest Partners preferred customer program statements.
For more information about the Scab Research Matching Grant or to request a Harvest Partners enrollment form, growers can call the Harvest Partners program service center at 1-800-258-2345 or visit their local Cyanamid dealer for details.