Grand Rapids area tree fruit update – April 28, 2020
Rain every day raises the risk for apple scab infections this week.
Continuous cooler than normal weather has moved degree day totals for the general west Michigan area to behind normal averages. The Michigan State University Sparta Enviroweather station has accumulated 200 degree days base 42, which is 3.5 days behind the average for Jan. 1 to April 27. Degree days base 50 at Sparta are 63, which is 10 days behind normal from Jan. 1. More seasonable temperatures are in the forecast and tree growth will take on a nice even pace. The very cold nights over the past two weeks have caused very little visible bud or tissue damage at this time.
Tree fruits will start looking a little pale and sickly if this cool weather continues. Adding in foliar nutrition is a good idea to prevent any stress to developing flowers and tissues.
Tree planting continues and soils have been on the drier side to allow getting this task done earlier than usual. Check out this MSU Extension article for pointers on best practices for apple tree planting.
There have been several short duration apple scab wetting events since the first green started showing up in the first week of April. For the Sparta Enviroweather station and many other nearby stations, our first official apple scab infection of the season was on April 7, 2020. Some stations have had more wetting events, some have had less. Please visit MSU Enviroweather's apple scab model to see the information for the station nearest you.
For the latest wetting event that started in the afternoon of April 27, many of the Grand Rapids area weather stations have now had just enough hours of wetting to be a light infection, but there are a few that are only an hour or two short. We need 8 hours of drying time—no rain, low relative humidity and no leaf wetness—to end one wetting event and start another. There is more rain in the forecast for later today, April 28, and it’s quite likely wetting periods could be strung together into one long wetting event.
Apple scab spore numbers increased significantly in the rain on April 22, indicating that mature spores are ready to infect green tissue. Fungicide cover sprays are critical over the next several weeks and should be adequately maintained with new tissue expansion and rain events.
Insect activity is nearly nothing with the very cool weather of late. A few spotted tentiform leafminer are flying as well as some green fruitworm. These are minor pests and the numbers are well below thresholds. If you need to apply a pink spray this year for rosy apple aphids or San Jose scale, it will be best timed as close to bloom as you can as insects are going to be slow in their development this year. As always, use your scouting reports and orchard history to make the best management decisions.