Scab: Planting Date,
Crop History Matters

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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.

Last summer a team of plant pathologists headed by Marcia McMullen with the NDSU Extension Service surveyed six barley fields and 40 wheat fields in eight ND counties for scab incidence and severity.

Overall results from wheat fields sampled following grain flowering and after harvest indicate that previous crop history does have an effect on scab severity. The lowest level of scab was found in wheat where dry beans or potatoes were the previous crops, followed by sugarbeets and sunflower. The highest level of scab observed was where wheat or barley had been the previous crop. Comparing samples from grain flowering and after harvest, results indicate that scab levels often increased as the growing season progressed.

Wheat yields from the fields sampled last year were highest with sunflower as the previous crop and lowest with sugarbeets as the previous crop. Yields of the fields on sugarbeet ground may have been more impacted by heat stress in June, however, than by scab, as these fields were some of the earliest planted and were in a critical stage of spikelet formation last June.

In the several fields of barley surveyed, scab severity was half the level where sunflower had been the previous crop, as opposed to when wheat had been the previous crop. However, yields were highest in the two fields where wheat had been the previous crop, most likely because these fields had been planted later.

Planting date may have had the biggest impact on scab severity and yields in 1995. The highest scab severities were observed with the May 17-31 planting dates, regardless of previous crop, and the lowest yields were observed with the very earliest planting dates. Planting prior to May 6 or after June 1 last year allowed fields to escape the severest damage because they either flowered before or after major periods of rainfall.

Previous crop history, especially in the case of sugarbeets, had an effect on planting date, as these fields were the driest first in those areas and were the first planted. McMullen says that in a year such as 1993 or 1994, when the June temperature was lower and rainfall occurred over a longer period during flowering and grain fill, the cropping history associated with row crops in the rotation may have played a bigger role in subsequent yields.

The NDSU survey results lend support to strategies which help reduce disease risks:

• Plant early so grain flowering occurs before mid July, when disease pressure and high temperatures are usually greatest.
• Stagger planting dates so grain flowering doesn't occur at the same time.
• Grow at least three different varieties with different heading times.
• Plant small grains on last year's broadleaf ground.

Copyright Prairie
Grains Magazine
May 1996