Nursery grower checklist following heavy rain events

Michigan nursery growers may want to review these pest management practices that will reduce the outbreak of diseases and weeds following this week’s heavy rains.

On Tuesday, June 21, many areas in west Michigan received a significant amount of rainfall up to 5 or more inches in a very short time period. The rains were accompanied by small hail and strong winds.

All these factors will increase the potential for plant diseases to sporulate, infect and spread in both container and field grown nursery stock. Also, granular herbicides may be washed out of the soil and will need replacement. Please keep the following items in mind as you review your crops conditions.

Nurseries where hail has occurred should consider applying both a bacteriacide, like a copper-based material, as well as a broad spectrum fungicide that contains chlorothalonil, mancozeb or thiophanate-methyl. This will help prevent the spread of any infections that occur on wounded leaf tissue.

If seed beds or liner beds were impacted by heavy rainfall, these plants will be at risk to soil borne fungi like Pythium and Phytophthora. Consider applying a drench of Subdue Maxx or Truban.

Any place that has standing water for more than 12 hours will show a greater risk for these disease problems. Any container grown crops that are in standing water will also be at an increased risk.

Heavy rains will wash out any fertilizer that was in the soil around the plant’s rooting zone in field grown nursery stock. Reapply nitrogen and potassium as these major nutrients will leach out of the root zone faster than phosphorus will. In water-saturated soils, denitrification can also occur, causing nitrogen to volatize and leave the soil. In container crops, most controlled-release-type fertilizers will still be available to the plant.

Denitrification occurs when nitrate N (NO3-) is present in a soil and not enough oxygen (O2) is present to supply the needs of the bacteria and microorganisms in the soil. If O2 levels are low, microorganisms strip the oxygen from the nitrate, producing N gas (N2) or nitrous oxide (N2O), which volatilizes from the soil. Three conditions that create an environment that promotes denitrification are wet soils, compaction and warm temperatures.

Cultivate or lightly till the surface of saturated fields to increase the amount of oxygen that will get into the root zone following heavy rain events and to help dissipate the moisture faster. However, you will break the herbicide barrier if a preemergent herbicide was applied and this will result in more weed emergence. There is a trade-off on this issue: More weeds versus faster field drying.

Most insects will be washed off plants following the rains we experienced earlier this week, so scouting fields will be important as your plants recover. Stressed plants are more susceptible to insect infestations as some insects can be attracted to plants that give off volatiles from injured foliage.

Know the leaching potential of your preemergent herbicides to determine if you need to reapply them following heavy rains. Dr. Hannah Mathers from The Ohio State University has a herbicide injury document that contains information on that subject.

Nursery growers with any questions on these subjects can contact Tom Dudek at 616-994-4580 for further assistance.

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