MSU Extension Fruit & Nuts

Weather update 

A storm front brought cooler weather on June 9 this week. Fruitlets are growing steadily, and growth rates have been very dependent on weather. Warm weather last week led to fruitlets growing about 1 millimeter (mm) per day or more. Cooler temperatures this week have slowed growth rates slightly compared to last week. Tart cherry, sweet cherry and peach have set well in many orchards across the region. Apples and pears are sizing well and some varieties like Gala have a heavy set in many orchards (Figure 1). June drop is starting in early cherry varieties. 

This week, the highs across the region will be in the upper 60s and 70s. Nighttime temperatures will be cooler with lows in the mid to low 50s according to the Hart and Sparta Enviroweather stations. With warmer temperature last week, tree phenology and fruit growth rapidly changed such as faster fruitlet growth and more canopy development. Based on the Hart Enviroweather station for June 9, current degree day (DD) accumulation is 832 DD42 and 419 DD50. For the Sparta Enviroweather station, current DD accumulation is 1013 DD42 and 552 DD50.  

Current degree day accumulation is lower than the five-year average for the west central region (Table 1) and for the Grand Rapids region (Table 2) according to the Enviroweather stations.  

For the next few days through the end of the week, temperatures will be slightly cooler with chances of rain on Thursday and Friday across the regions.  

Get the full weather update from Jeff Andresen 

Table 1. West Central Region: Current and 5-year average DD summary from Jan. 1- June 9. Numerical integration was used for degree day calculations. 

Table 2. Grand Rapids Region: Current and 5-year average DD summary from Jan. 1- June 9. Numerical integration was used for degree day calculations. 

Weather data was gathered from Enviroweather. 

Storms on June 8-9 brought light rain and some hail across the regions. The Hart Enviroweather station recorded 0.2 inches of precipitation. For the Sparta Enviroweather station, precipitation totaled 0.19 inches. Some areas across the state also had hail during the storm on June 9. 

For the Hart Enviroweather station on June 9, maximum soil temperature at a 2-inch depth was 68.9 degrees Fahrenheit and 66.4 F at 4 inches. Optimal soil temperatures for root function is about 64-77 F. Maximum soil moisture at 4 inches is 7.5% and at 20 inches it is 7.1%, which is still quite dry. For the Sparta Enviroweather station on June 9, maximum soil temperature at 2 inches depth was 75.1 F, and 71.1 F at 4 inches. Maximum soil moisture at 0-12 inches is 44.8% and at 12-24 inches it is 55.4%, similar to last week. 

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 

Apple varieties in Oceana County range from fruit set to about 30 mm for early varieties. Orchards close to the lake shore are 7-10 days between orchards further inland. In Hart, Zestar have an average fruitlet size of 29 mm. Later varieties such as Gala and Honeycrisp grew rapidly since last week, especially with the warm weather, and are at an average of 19 and 17 mm respectively.  

There is a wider range of phenological stages on trees this season which is likely due to changes in temperature with mixed conditions of warm then cool temperatures. Fruitlets are also beginning to differentiate more in size, which gives an indication of fruit that may persist and fruit that may fall. This may be helpful for thinning as the largest fruit in the cluster will be more competitive than smaller fruitlets, and the smaller fruitlets will be more affected by chemical thinner.  

Some orchard blocks may need an additional thinner application or hand thinning shortly. The Cornell Apple Carbohydrate Thinning Model shows that Hart and Sparta have a slight carbohydrate surplus for the six-day weighted average predicted for June 10. The model suggests that thinning efficacy will be mild. If you look at the carbohydrate balance graph, the trend of the carbohydrate status has gone up and down over the past few weeks and is now above the balance for six-day average threshold of 0. For June 10, the calculated six-day average is 14.94 for Hart and 21.84 for Sparta.  

According to the Apple Thinning Guide from Michigan State University Extension, the apple fruitlet thinning sensitivity is greatest at 10 mm, and then quickly becomes insensitive as the 25 mm stage is approached. There is variation in fruit set this year by block and by variety. Scout orchards block by block on foot to determine if a chemical thinner is needed.  

Also, measure fruit daily or every few days to determine if fruit are continuing to grow or if they stop growing. Use the fruit growth rate model to determine fruit set and efficacy of thinner applications. Fruit that continue to grow steadily will persist, and fruitlets that are growing at a rate of 50% or less than the fastest growing fruit will abscise.  

Learn more about the fruit growth rate model.   

If fruit has grown past the size for effective chemical thinning or if crop load is being evaluated to decide if another chemical thinner is needed, consider using tools to help determine optimal crop load such as the Equilifruit disc or the trunk gauge. This Equilifruit disc was developed in France as a hand-thinning gauge for centrifugal systems, but research at Penn State University showed that it is also an effective tool for tall spindle systems with renewal type pruning. The disc can be placed on a limb at 3 centimeters (cm) from the tree trunk using the notch size that fits snuggly around the branch. The corresponding F-value on the selected notch shows the number of fruit that should be left on the limb of that size using limb cross sectional area (cm2, LCSA) after hand or chemical thinning.  

If the number of fruit is greater than the F-value, additional thinning is necessary. To adjust crop load on an entire tree, start at the lowest limb on the tree and systematically work to the top of the tree.  

A trunk gauge developed by researchers at Cornell University and Valent can also be used to adjust tree crop load and indicate how many fruit a young tree can carry for a given trunk size. The trunk gauge measures cross sectional area (cm2, TCSA). It also provides guidance for varieties with biennial bearing tendencies like Honeycrisp and Fuji and more annual varieties like Gala, Empire, McIntosh and Delicious.  

For more information about using the Equilifruit disc and other hand-thinning tools, check out these articles from Penn State and Washington State: 

• Penn State University: Apple Crop Load Management - A Hand-thinning Gauge 
• Washington State University: Green Fruit Thinning with the Equilifruit Disc 

Using these tools can enhance fruit size and quality. These are also helpful gauges to train crews and recalibrate crews who are doing tasks like hand thinning to see what the target crop load should be for your blocks and varieties. 

Sweet cherry are growing steadily. Average fruit size is 16 mm for sweet cherry. Average fruitlet size for tart cherry was 14 mm. June drop is occurring in some blocks. 

Peaches are out of the shuck and fruit set looks adequate this year for many orchards. Venture peach at the West Central Michigan Research and Extension Center had an average fruitlet size of 12 mm.  

Apricots are growing steadily. Average fruit size is 30 mm in Oceana County. 

Plums have a good set for European and Japanese varieties. Stanley plums measured 14 mm on average.  

Pear fruitlets are growing steadily and differentiating in a cluster. Average fruitlet size was 17 mm for Bartlett pear at the West Central Michigan Research and Extension Center.  

Pest and disease update 

Diseases 

Warmer weather means more insect activity, and we have seen more pest activity and trap catches in some orchards, especially for pests like plum curculio, oriental fruit moth and codling moth. There will be risk for apple scab and cherry leaf spot this week with the chances for rain. Primary apple scab spores are still being trapped in the Grand Rapids area, and four spores per rod were counted after the June 9 rain event. In New Era, no spores were trapped during the June 9 rain event.  

I will continue monitoring for apple scab spores through the next rain. If primary scab spores are being trapped (even in low numbers), there is still risk for infection, and continued protection of tissue is important. The Enviroweather apple scab model for Hart and Sparta is predicting that 100% of spores are mature and 95% of spores have been released in Hart and 97% of spores have been released in Sparta. 

Symptoms of bacterial canker have been observed again this week with lesion development on fruit and leaves. The best approach for managing bacterial canker is to prune out dead, decaying wood and cankers during a stretch of dry weather. 

Insect pests 

In Oceana County, no green fruitworm or redbanded leafroller were caught. Oriental fruit moth was trapped with an average of six moths per trap.  

American plum borer was trapped this week at the West Central Michigan Research and Extension Center (four moths per trap) and in Mason County. Lesser peach tree borer was trapped in Oceana and Mason counties with an average of 11 borers per trap. Plum curculio have been trapped throughout the state, and egg laying damage was observed on tart cherry.  

Codling moth was trapped in Oceana and Mason counties with an average of three moths per trap. One lesser apple worm was trapped this week.  

In Sparta, codling moth was trapped at six moths per trap and no oriental fruit moth were trapped. Obliquebanded leafroller adults and dogwood borer adults have not been trapped. 

For more information about regional reports, please visit the Michigan State University Extension website.