The 2015 growing season was one of contrasts. On the positive side, losses to diseases in soybeans were the lowest in at least 20 years. On the negative side, yield loss to disease in corn was higher than in any recent year. Overall, grain sorghum losses were average to somewhat above average.
Corn diseases
Frequent rains in May and June allowed gray leaf spot to get off to a quick start, especially in northeast, north central, and central Kansas. Because of low corn prices, many growers chose not to make an investment in fungicide applications and this turned out to be a big mistake. Statewide, yield losses to gray leaf spot were estimated to be 3%. This calculates to a loss of 17 million bushels or about 4.5 bushels for every acre of corn grown in the state. Of course many corn acres in western Kansas had no loss while fungicide trials in northeast Kansas produced an 18% gain in yield over unsprayed plots. Losses in some individual fields likely reached the 25 to 35% range.
Southern rust was a second major disease in corn in 2015. Tropical storm Bill, which struck the Texas and Louisiana Gulf coasts on June 16, pushed southern rust spores all of the way up to Kansas. The first reported case in Kansas came from Parsons on July 11, but based on the level of disease, initial infection likely occurred in the third or fourth week of June. Nearly all counties surveyed had some level of rust present by the end of the growing season. Some corn planted in late April in southeast Kansas had an 8% yield loss from rust. While not estimated, losses in June-planted corn were likely much higher.
Fusarium stalk rot levels were also above average in 2015. Loss of photosynthetic leaf area due to gray leaf spot and southern rust directly contributed to the increase in stalk rot.
Soybean diseases
Soybean disease losses were estimated at 3.6% across the state, which is well below the long-term average of 12%. The most significant reductions in disease incidence were in sudden death syndrome (SDS) and charcoal rot. Delayed planting, use of more tolerant varieties, changes in the timing and amount of late-season rains, and the use of seed treatment all contributed to the decline in SDS from the record 2014 levels. Timely rains, especially in southeast Kansas, contributed to the lowest levels of charcoal rot in many years.
The most significant soybean disease in 2015 was soybean cyst nematode. The nematode continues to gain tolerance to the currently deployed resistance genetics in varieties. Growers are encouraged to rotate varieties and never plant the same variety in the same field twice.
Sorghum diseases
Sorghum was mostly healthy throughout the season until late when significant levels of Fusarium stalk rot developed. Above-average rains in many areas likely resulted in the development of root rot that then moved up into the stalk and the loss of nitrogen from leaching or denitrification. Low nitrogen levels are frequently associated with higher stalk rot levels. The highest levels of stalk rot and lodging occurred in north central and northwest Kansas.
Doug Jardine, Extension Plant Pathology
jardine@ksu.edu
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