West central Michigan tree fruit update – May 29, 2024
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EmailApples are growing rapidly. The chemical thinning window is closing for some varieties in the region.
Weather update
Weather over the past week has been mixed with periods of warm and cool temperatures and scattered showers bringing high winds and heavy rains. High temperatures reached the upper 70s on Friday, May 24, at the Hart Enviroweather station, and temperatures cooled down over Memorial Day weekend with highs in the low to mid-60s. Storms across the region brought cloudy and rainy conditions. Temperatures will be moderate this week with highs in the 60s and lows in the 40s. Temperatures will increase toward the end of this week with highs reaching the upper 70s by June 3. Dry conditions are forecasted for the remainder of the week before another storm front moves across Michigan on Saturday, June 1.
Storms brought over 0.5 inch of rain in Hart over the past week, and relative humidity increased to 89% at its peak on May 27. Humidity is predicted to remain from 60 – 80% over the next week across the region.
Fruitlets are growing steadily across the region (Figure 1). Based on the Hart Enviroweather station for May 28, current growing degree-day (GDD) accumulation is 832 GDD42 and 409 GDD50. Medium range forecasts call for warmer and near normal or above normal precipitation over the next few weeks.
With rain over the past week, soil moisture has ranged from 8 – 11% at 4 inches deep in the soil profile and about 7.2 – 7.9% at 20 inches soil depth for the Hart Enviroweather station. With fruitlets growing, shoot length increasing and high winds, water loss from soil is increasing and evapotranspiration is high. Soil temperatures at a depth of 2 inches have ranged from the low 60s to low 70s. Soil temperatures at 4 inches deep have been in the 60s over the past week and reached the low 70s on sunny days with high solar radiation.
Weather data was gathered from Enviroweather.
More information and reports on normal weather conditions and departures from normal can be found on the NOAA Climate Prediction Center website, NOAA U.S. Climate Normals website, NOAA Climate Normals Quick Access Page (which may be searched by region) and Midwest Regional Climate Center website.
Crop update
With warmer temperatures, tree growth and fruitlet development has been steady. For most apple varieties, fruitlets are approaching the later thinning window where the efficacy of chemical thinners will start to diminish (greater than 15 millimeters). Thinners that are effective for larger fruit include ethephon and ACC. Thinners can also be mixed with carbaryl, 6-BA and oil to enhance efficacy where needed. Adding a surfactant may also enhance thinner efficacy and is recommended if there is a carbon surplus according to the Cornell Apple Carbohydrate Thinning Model.
For some apple varieties, most advanced fruitlets range from 16 – 20 millimeters (mm) for Honeycrisp and Jonagold in Hart. Some fruitlet thinning is occurring naturally this year, and fruit set may be around 50 – 60% for some varieties. According to the Cornell Apple Carbohydrate Thinning Model, apple trees have a carbon surplus and will remain in a surplus over the next few days. Some blocks may be sufficiently thinned by this time if target crop load has been reached. For other blocks with heavy fruit set, additional chemical thinner applications may be needed to manage crop load. Check block by block to determine if additional thinner applications are needed.
Thinning early can be particularly important for varieties, such as Honeycrisp and Minnieska, that initiate flower buds for the following season 45 days after bloom. Early thinning for varieties like Honeycrisp can result in higher return bloom. More information on thinning strategies for Honeycrisp can be found in the Michigan State University Extension article, “Honeycrisp crop management for 2022.” Additional information and thinning recommendations can be found in the Michigan State University Thinning Guide.
Apple scab. With scattered storms in rainfall on May 20 and a long wetting event from May 26 – 28, there have been a couple high risk scab infection periods. Enviroweather and RimPro models show that almost all spores are now mature, and 95% of the spores have been dispersed. From the May 26–27 rain event, 21.5 spores per rod were trapped. From rainfall on May 28, six spores per rod were trapped. I will continue trapping spores released during upcoming rain events to determine when primary scab season has ended.
RIMPro forecast is predicting that apple tissues are at a low risk for apple scab infection on June 1 and June 3–4. Apple scab appearing on leaves and fruitlets was reported last week. Continue scouting orchards to determine if some blocks have apple scab symptoms showing from primary scab infections. With rapid tree growth in our region, adequate fungicide coverage is key to minimizing apple scab infection. Coverage is also important for pear blocks in the region for pear scab (Venturia pirina).
Fire blight. Fire blight symptoms were observed in orchards this week. Scout in blocks with susceptible varieties or blocks that had fire blight in the past and remove diseased tissue as quickly as possible. The fire blight bacteria can move rapidly through shoot tissue with an average rate of 5 centimeters per day.
Tart and sweet cherry are growing. Fruit size for early sweet cherry varieties are 15–20 mm. Tart cherry fruitlets are about 11–13 mm in size. Cherry leaf spot management is needed to protect leaves against infection. Due to warm temperatures and rain events on May 26–27, there was a cherry leaf spot infection period according to the Hart Enviroweather station. Cherry leaf spot symptoms have been observed across the region, and pressure will continue to be high with rainy and warm conditions this weekend.
Peaches are at the green fruit stage depending on the variety. Venture peach fruitlets are growing rapidly with an average size of 15–18 mm at the West Central Michigan Research and Extension Center.
Pear fruitlets are growing rapidly for varieties such as Bartlett, Gem and Cold Snap. The largest fruitlets were 13–16 mm in diameter for Bartlett pear trees at West Central Michigan Research and Extension Center (Figure 2).
Plums are growing rapidly with high fruit set and average size of 18–22 mm for some varieties.
Pest and disease update
Consider scouting reports and block history to target hot spots for early season pests in the orchard according to bud stage. Plum curculio damage and codling moth adults have now been observed in the region, and warmer weather will encourage more activity from orchard pests.
On May 28, the Trevor Nichols Research Center in Fennville, Michigan, trapped pests such as codling moth, oriental fruit moth, lesser apple worm and San Jose scale adults. American plum borer and lesser peachtree borer were also trapped at the Trevor Nichols Research Center and West Central Michigan Research and Extension Center.
Plum curculio damage has been observed in some orchards. Plum curculio are more active with warm temperatures above 75 degrees Fahrenheit. When young fruit are exposed, plum curculio deposit eggs under the skin of the fruit in a crescent-shaped slit. Scout for damage to determine if a curative spray is needed.
Rosy apple aphid is a common pest in our region. Scout for colonies in fruit clusters to determine if treatment is needed.
Obliquebanded leafroller larvae have been observed in Oceana County. Nontarget adult leafrollers may end up in obliquebanded leafroller traps since they are also attracted to the same pheromone blends. Be sure to positively identify obliquebanded leafroller adults when they arrive, likely in June.
Oriental fruit moth was trapped at three locations in Oceana County with an average of three oriental fruit moths per trap. Adult moth catches have declined. For the West Central Michigan Research and Extension Center, oriental fruit moth was first observed in peaches this season, and a biofix date of April 29 has been set. They are now present in traps in apple blocks. The biofix date can be adjusted in Enviroweather according to orchard site and site-specific trapping.
For pest and disease management recommendations, please refer to the Michigan Fruit Management Guide (E-154) for product guidelines.
For more information about regional reports, please visit the Michigan State University Extension website.
