Corn diseases

Significant rainfall amounts across most of the state alleviated drought conditions in 2016, but at the same time, the precipitation provided ideal conditions for many foliar diseases. Gray leaf spot, while not present at the record levels seen in 2015, was still higher than the long term average.

Unfortunately, because of low commodity prices, many producers chose not to apply a fungicide and this was a significant mistake where more susceptible hybrids were being grown. Yield losses could easily be more than 15 percent.

Figure 1. Gray leaf spot on V7 corn in Harvey County, mid-June 2016. Photo by Doug Jardine, K-State Research and Extension.

 

For the second consecutive year, southern corn rust made its first appearance in mid-June rather than the historical time period of late July to early August. The disease quickly spread across the entire state and certainly resulted in yield losses of 10 percent or more where corn was planted later and fungicides were not applied.

Goss’s bacterial blight was present at near normal levels, with most of the reports coming from the western half of the state. A new bacterial disease, commonly referred to as corn bacterial leaf streak, was identified for the first time in Kansas. This disease has been present in Nebraska since at least 2014, and may have been in Kansas since at least last year, but 2016 was the first year that the causal bacterium, Xanthomonas vasicola pv vasculorum (Xvv) was positively identified as the cause. By year end, Xvv was officially diagnosed in 16 Kansas counties, most of which are in the western third of the state. The disease is most common and severe in fields that are in a continuous corn, no-till production system with overhead irrigation. At this time, it is not clear if this disease is associated with any yield loss.

Rainy weather at silking time also resulted in a record epidemic of Diplodia ear rot. This disease can cause entire ears to become moldy, shrinking and discoloring kernels. The disease can also penetrate the cob, causing “cob rot,” which ultimately leads to large amounts of foreign material in the grain from infected fields and results in significant dockages at the point of sale.

Aspergillus ear rot, the cause of aflatoxin problems was present on a very localized basis. The most severe problem area was in southern Harper and Barber counties and into Oklahoma. While a few samples tested more than 1,000 ppm, most samples were well below the 20 ppm safe level established by the Food and Drug Administration.

Lastly, Fusarium, anthracnose and Diplodia stalk rots were present to varying degrees across a large part of the state. Stalk rots led to premature death of infected plants with the result being reduced yields from smaller ears and the additional threat of losses from lodging.

Grain sorghum diseases

Disease levels in grain sorghum varied with the location in the state. The highest levels of disease were reported from south central Kansas, where frequent rainfall events throughout the summer caused significant levels of the foliar disease sooty stripe. Sooty stripe is a splash-dispersed disease. Levels on susceptible hybrids were the highest seen since the late 1990’s from the Wichita area southward. This disease is capable of producing yield losses up to 35 percent on susceptible hybrids. The other significant problem in sorghum in 2016 was the development of sooty mold in fields where high sugarcane aphid populations were present. Sooty mold, while not a true pathogen, forms on the upper surface of leaves that have large amounts of aphid honeydew on them. The black mold does not penetrate the leaf, but effectively blocks sunlight from reaching the leaf and thereby reduces yields due to reduced photosynthesis.

Sorghum rust, an occasional problem in Kansas, was present at higher-than-normal levels in 2016, again due to frequent rains. Most fields were mature enough for it not to be an issue, but some later-planted fields may have suffered some yield loss.

Fusarium stalk rot was also present, but appeared to be at normal levels compared to 2015, when levels were much above normal in some parts of the state.

Lastly, for the first time in several years, sorghum ergot was reported in the state in both Jewell and Republic counties. In both instances, late-planted forage sorghum was the host. While the disease causes some yield reduction on its own, the biggest threat is harvest delays associated with having to clean off the sticky sap produced by the fungus from harvesting and other crop handling equipment. This disease does not overwinter in Kansas, and hopefully its presence will continue to be just isolated incidences.

Soybean diseases

Heavy rains early in early July resulted in numerous cases of Rhizoctonia root rot and Phytophthora root rot, especially in east central and south central Kansas. Even with all the rain, there were numerous cases of charcoal rot reported later in the season. Sudden death syndrome was common in late August and September in parts of east central and southeast Kansas. Levels of frogeye leafspot were well above normal across large portions of northeast and north central Kansas. Numerous fields reached levels where a fungicide application would have been practical.

Good growing conditions hid damage from soybean cyst nematode. Numerous samples with nematode counts in the moderate to high range were received in the Plant Disease Diagnostic Clinic. Research indicates that many of the current “SCN resistant” varieties are losing their effectiveness and yield loss is much greater than what growers realize.

Late in the season, many mature fields had plants that were still green. Samples of these plants usually tested positive for tobacco streak virus, a reemerging problem in the western soybean belt. Fortunately, there are usually only a handful of affected plants in any given field and yield loss is negligible.