By Scout Nelson
Diseases are widespread in parts of Nebraska corn this season. Extended humidity, frequent rainfall in some areas, and warm temperatures have favoured many pathogens, especially fungi and some bacteria. Several regions have seen record southern rust again. As cooler weather approaches, southern rust should slow, but tar spot may become more active, raising new questions for fields across the state.
With rapid increases of southern rust in some fields, many are weighing late fungicide sprays, and in a few cases, second applications. University data summarized by the Crop Protection Network generally do not recommend fungicide use at the dent (R5) stage because the chance of economic return drops sharply, especially for second sprays. Late-season treatments have sometimes improved yield in field trials under extreme situations, such as susceptible, late-planted corn under heavy disease pressure, but those cases are exceptions.
Most foliar fungicides provide about three weeks of protection. If a second treatment is warranted, it should occur roughly three weeks after the first to maximize benefit. Even then, second applications are less likely to pay unless disease pressure is high.
Black spots can be confusing late in the season. As conditions shift, rust fungi that cause southern and common rust begin producing black, pigmented teliospores instead of the usual orange or tan urediniospores. Teliospores form on or near old rust pustules, often in a ring or horseshoe shape. This pattern helps distinguish them from the black stromata of tar spot, which tend to become pointed or diamond-shaped as they enlarge. Mature tar spot stromata are usually visible on both the upper and lower leaf surfaces.
Risk of stalk rot diseases and lodging may rise this year. Substantial leaf disease can reduce photosynthesis during grain fill, triggering stalk cannibalization that hollows and weakens stalks. Nutrient imbalances, such as post-rain nitrogen loss, can further stress plants and weaken lower internodes.
Evaluate lodging risk with a “push or pinch” test on more than 100 plants sampled across the field. Calculate the share that lodge below the ear or crush easily. Prioritize fields with more than 10% weakened stalks for earlier harvest to reduce losses from late-season lodging.
Photo Credit: gettyimages-songdech17
Categories: Nebraska, Crops, Corn