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Sunflower An Ally in The Battle with Scab

Monday, April 1, 1996
filed under: Planting Systems

Can sunflower’s presence in a rotation help reduce the threat of Fusarium head blight (scab) in Upper Midwest wheat and barley fields? Yes it can, confirms North Dakota State University plant pathologist Marcia McMullen. But so can that of other row crops such as soybeans and sugarbeets.

Fusarium attained epidemic proportions in eastern North Dakota and western Minnesota in 1993 and 1994. Wheat and barley fields in the region suffered yield losses ranging from five percent up to 90 percent or higher. In North Dakota alone, an estimated $500 million in lost yield was blamed on Fusarium in 1993, with that figure placed at $250 million in 1994.

Though the Upper Midwest scab problem was substantially lower and confined to a smaller geographic area in 1995 compared to the previous two years, it remains a serious threat — particularly in years when precipitation is abnormally high during wheat and barley flowering stages.

While common sense would suggest that rotating away from host crops would reduce the amount of Fusarium inoculum in succeeding grain fields, numerous producers around the Red River Valley reported severe head scab infection in 1993 and 1994 grain fields regardless of the previous crop. Other producers, however, said that yield loss due to scab was less in small grain fields where the prior year’s crop consisted of a nonhost row crop.

In an effort to document the effect of cropping systems on the incidence of Fusarium head blight, NDSU’s McMullen coordinated a twin-faceted study of the relationship between scab incidence and crop rotations in wheat and barley fields.

In the first phase, McMullen and her associates surveyed 46 eastern North Dakota scab-infected fields (40 wheat, six barley) for which the producer had provided planting date, variety and previous crop information. The plant pathologists found that the lowest severity of scab at the time of the survey (latter half of July) was in those grain fields where the previous crop had been dry beans or potatoes; next lowest were in fields that had been in sunflower the prior year; and the highest degree of scab severity was observed where sugarbeets comprised the previous crop.

However, producer observations on scab severity at harvest indicated that severity levels generally had increased by that time; also, the highest level of scab was observed in fields where wheat or barley had been the previous crop. The severity index then declined in the following order: sunflower, sugarbeets and dry beans or potatoes. Early (before May 6) or late-planted (after June 1) wheat fields had lower scab levels than did those planted during the latter half of May in 1995. Wheat yields were highest where sunflower was the previous crop and lowest with sugarbeets as the preceding crop. Those lower yields of wheat planted on sugarbeet ground probably could be attributed to hot, dry weather in June (while wheat was in the critical spikelet formation stage) rather than to impact from scab, McMullen notes. The fields were among the earlier-planted ones in the area and were most impacted by the June heat stress.

In the second phase of the 1995 producer survey, a one-page questionnaire was inserted into the annual NDSU survey of North Dakota and Minnesota sugarbeet growers — many of whom incurred severe scab problems in grain fields during 1993 and 1994. More than 200 growers responded to the insert, and they reported on 720 fields (595 wheat, 125 barley).

The table below summarizes the grower responses regarding scab problems and preceding crops in those 720 small grain fields. Since the insert was going specifi-cally to sugarbeet growers, it’s not surpris-ing that one-third of the 1995 wheat/barley fields had been in beets the previous year.

Perhaps most revealing are the relative percentages of the sugarbeet growers’ wheat/barley fields with “severe” scab problems. As might be expected, those percentages were highest where the prior year’s crop was wheat, corn or barley (nine, eight and nine percent, respectively). The four nonhost crops — sunflower, soybean, sugarbeets and dry beans — resulted in a “severe” rating well below that level (three, four, two and three percent, respectively).

“Both surveys showed that cropping history did make a difference in the severity of the scab,” McMullen summarizes. “There was less scab where you had wheat growing on ground that had been planted the previous year to sunflower, sugarbeets or soybeans.”

However, previous crop was not the only factor influencing head scab severity. Planting dates also played a role, with the highest severities associated with wheat planting dates between May 21-30.

The 1995 cropping system/scab severity survey results will be detailed in an NDSU agronomy report, to be published this summer. As of early April, McMullen was unsure whether follow-up surveys would be conducted in the region during 1996.
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