Jun 25, 2021

General crop management notes as Midwest finally gets rain

With a prolonged period of rain in the forecast, Michigan State University thought it would be good to cover a few things related to spraying and fertilizing around rain events.

For mechanical cultivation, a little bit of moisture is good for working through any soil crusts that might exist. But too much moisture makes for poor soil movement through the tool, and weeds can recover faster afterwards. Additionally, with any equipment, compaction and degradation of soil structure can follow cultivation in wet soils, especially tools with deeper action. With increasing soil moisture, a shallower tool would be more effective without as many negative effects on soil structure.

How long do postemergence herbicides need to dry before rainfall? Postemergence weed control with herbicides is still possible with a window of drying time before rains or between rains. Here is a table showing rain free periods for postemergence herbicides from the Michigan State University Extension E0433, “Weed Control Guide for Vegetable Crops.”

As it comes to disease prevention, Dan Egel, from Purdue University wrote this article, “Fungicide Applications During Rainy Weather,” which summarizes the timing and rainfastness attributes of several types of vegetable fungicides. He writes, “As a general rule, apply contact products 24 hours before a rain if possible, but an application of a protectant even eight hours before a rain may be effective. Apply systemics one to two hours before a rain. Some product labels have specific information.”

Which products are systemic, and which are contact? That is not always obvious either, and Purdue has another publication summarizing that too, titled “Fungicide Mobility for Nursery, Greenhouse, and Landscape Professionals.” One important note: There is good reason to have fungicides applied prior to an extended period of rain, as leaf wetness is necessary for infection in many pathogens.

For insects, John Wise at MSU wrote this article, “Rainfast characteristics of insecticides on fruit,” which has a lot of crossover products with vegetables. He writes, “For most insecticides, a drying time of two to six hours is sufficient to ‘set’ the compound in or on the plant. With neonicotinoids, for which plant penetration is important, drying time can significantly influence rainfastness. For neonicotinoids, up to 24 hours is needed for optimal plant penetration, thus the time proximity of precipitation after application should be considered carefully. Spray adjuvants, materials intended to aid the retention, penetration or spread on the plant, can also improve the performance of insecticides.” A handy reference for the contact or systemic nature of different insecticides is also included in his article.

For additional nitrogen applications with rain in the forecast, topdressing within a few days before a good rain (about 0.5 inches or more) is the ideal situation, serving to incorporate fertilizer into the soil, washing errant prills off leaf surfaces, and reducing potential for volatilization losses from urea-containing fertilizers.

However, particularly heavy rains can bring concerns about nitrogen losses. How much rain is too much from this standpoint? It depends on a number of factors, including soil texture, soil moisture content, crop rooting depth, and rainfall intensity.

As a general rule though, rain exceeding 3 inches in three days or 4 inches in seven days has the potential to lead to meaningful nitrogen leaching losses. Granted, on heavier textured soils that can hold more moisture, even higher precipitation amounts may be required before substantial nitrogen is moved below the root zone. Keep in mind though that soil and fertilizer nitrogen can also be lost through runoff/erosion at high rainfall intensities or through denitrification if soils remain too wet or flooded for extended periods.

Forecasts (especially of precipitation amounts) can be fickle, so don’t stray too far from recommended topdress timings or rates for your crop based on how much rain might fall. In general, you should only consider making an additional supplemental application of nitrogen (about 30-40 pounds nitrogen per acre) when both significant rainfall meeting the above criteria has occurred and signs of crop nutritional status in the following week (slowed growth, yellowing older leaves) support doing so.

– Benjamin Phillips, Benjamin Werling, Michael Reinke, Mary Hausbeck, Sushila Chaudhari, Zsofia Szendrei, Zachary Hayden, Daniel Brainard, Jeffrey Andresen and Jaime Willbur, Michigan State University Extension