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CropTalk: A Triple Threat of Emerging Pests and Disease

June 2019

Published on Monday, June 10, 2019

DISEASE: TAR SPOT

Caused by the fungus Phyllachora maydis, Tar Spot was first identified in the U.S. in 2015 and has been found in each crop year since. The infection causes small, circular, raised black lesions called stromata on the corn leaf. Stromata can spread to stalks and husks in severe cases. The infection will result in rapid loss of green tissue on the corn plant causing reduced photosynthesis. This will cause the plant to scavenge carbohydrates from the stalks, ultimately leading to lodging issues in the fall. With Tar Spot being such a new disease in the U.S., there is still much to learn about the disease. Researchers have confirmed that Tar Spot has the ability to overwinter in the Midwest, meaning it is more likely that some areas will continue to encounter the disease as long as environmental conditions are favorable. In fields with severe Tar Spot infections, yield loss has been seen in the 30 to 40 Bu./A. range. Currently, the most effective way to manage against Tar Spot is to select tolerant hybrids, although no current hybrid is completely resistant to the disease. Fungicide applications have also been shown to reduce the severity of Tar Spot, although the perfect application timing is still unknown. — Jon Skinner, CCA - Field Agronomist

PEST: SOYBEAN GALL MIDGE

Soybean Gall Midge (Resseliella maxima) has been infecting soybean fields since it was first uncovered in Nebraska in 2011. In 2018, observations of the pest increased dramatically and it was recorded in four states in the Midwest. 2018 was the first year when significant yield losses were recorded. Losses were the result of premature plant death and harvest time standability issues. Both issues arise from the larval stage infecting the stems of plants just above the soil level and moving higher. Swollen stems and discolored soybean canopies are visual symptoms that warrant further investigation; symptoms usually begin around field edges. Since Soybean Gall Midge expanded its range in 2018, it has been identified as a new species that goes through multiple generations during the growing season. Currently, management practices show no correlation to infection rates. Preventing the adult female from laying eggs is thought to be the most effective method for preventing infestations, but specific recommendations and strategies are still being discussed. While we are still learning about host habitat and overwintering, observations this year will be critical to help understand why there was a sudden explosion in the area of impact. In 2018, I made observations of impacted plants as early as the last week in May. — Nate Mayer, CCA - Field Agronomist

PEST: REDBANDED STINK BUG

You’ve probably never heard of the Redbanded Stink Bug (Piezodorus guildinii) unless you’re from the South, but this is an insect that is starting to cause major trouble for farmers in Beck’s southern marketing area. It is an invasive pest from the Caribbean island of St. Vincent that has been causing serious economic damage to South American soybeans for many years. It was first reported in the U.S. in 1970, but didn’t become a major economic pest here until 2002. Like most other stink bugs we deal with, this pest does not do much damage to soybeans until they reach their reproductive stages, specifically pod and seed set. It can be difficult to control because of its ability to rapidly infest a field. The Redbanded Stink Bug overwinters in the far south but can typically be found in clover patches early in the spring. It looks similar to the Common Green Stink Bug and the Red Shouldered Stink Bug, but can be distinguished by its size and dull finish to its dorsal surface. Because this pest overwinters so far south, you most likely will not see it in your fields anytime soon unless you are located in Beck’s farthest southern territories. But just in case it makes it to your fields, it is important to keep the sweep net handy and monitor threshold levels. Using a 15 in. diameter net, the threshold level would be six or more pests caught in 25 sweeps. The Redbanded Stink Bug is harder to kill than other common stink bugs we deal with so bifenthrin or a high rate of acephate would be the control option of choice. — Austin Scott, CCA - Field Agronomist

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Author: Samantha Miller

Categories: CropTalk, 2019

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