MSU Extension Fruit & Nuts

Weather update 

Temperatures have been cool over the past week with highs in the 60s and 70s and lows in the 40s according to the Hart Enviroweather station. Temperatures will increase through the end of the week with highs reaching the 80s by the beginning of next week. June 8 brought rain across the region and 0.22 inches of rain in Hart, Michigan. Over the past few days, weather conditions have been dry and very windy. Relative humidity was in the 60s and 70s this week, and evapotranspiration has been high. More rain is forecasted for June 12 and 13 this week.  

Fruitlets are growing steadily (Figure 1). Based on the Hart Enviroweather station for June 10, current growing degree-day (GDD) accumulation is 1,079 GDD42 and 559 GDD50. Over the next few days, high temperatures will increase and will reach the 80s by early next week.  

Soil conditions are similar to last week. Soil moisture is about 11% at 4 inches deep in the soil profile and about 9% at 20 inches soil depth for the Hart Enviroweather station. Steady rains over the past few weeks have enhanced soil moisture deeper in the soil profile. Water has steadily infiltrated into the soil profile and is readily available for the majority of fine roots, which are produced in the top 1.5 – 2 feet of soil. These roots are responsible for taking up water and nutrients. This water availability is critical to support rapidly growing fruitlets and shoots this time of year.  

With the consistent rains this spring, early applications of nitrogen fertilizer may not be available to trees and may have moved past the root zone. Consider soil test results and nitrogen needs for this season to determine if another application of fertilizer (particularly nitrogen) may be needed.  

With moist soil conditions and continued chances of rain in the forecast, more frequent applications of fertilizer are recommended in smaller rates compared to one or two applications in larger rates during the growing season. Consider nutrient availability in the soil, nutrient analysis of leaf tissue or sap, and fertilizer source. 

Soil temperatures at a depth of 2 inches have ranged from the low 60s to upper 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. Deeper in the soil profile, temperatures fluctuate less and have ranged from the low to mid-60s at 8 and 20 inches deep. 

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 now passed the effective chemical thinning window (more than 20 millimeters). Some growers are done chemically thinning this year and will rely on hand thinning if additional thinning is needed. 

For early apple varieties, most advanced fruitlets range from 40 – 44 millimeters (mm) for Zestar and 35 – 40 mm for Idared. Honeycrisp fruitlets ranged from 31 – 35 mm and Gala were 24 – 28 mm. Minnieska fruitlets were 35 – 41 mm in Hart. Some fruitlets are falling because of June drop, and strong fruitlets are continuing to grow rapidly.  

Apple scab: With rain showers on June 4 – 8, there have been extended wetting events over the past week. The Enviroweather model shows that almost all spores are now mature, and 98% of the spores have been dispersed as of June 8. From both the June 5 and June 8 rain events, no spores per rod were trapped at a commercial orchard in Oceana County. The June 8 rain was a steady, slow rain. This would have been a good opportunity for final primary scab spores to be released. Primary apple scab season is likely done for some orchards. Consider additional fungicide applications in orchard blocks where high apple scab disease pressure has historically been present or if primary scab lesions are present.  

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 as primary scab season continues. Coverage is also important for pear blocks in the region for pear scab (Venturia pirina).  

Tart and sweet cherry are growing rapidly. Fruit size for early sweet cherry varieties are 23 – 25 mm. Tart cherry fruitlets are about 14 – 16 mm in size. Cherry leaf spot management is needed to protect leaves against infection. Due to warm temperatures and rain events over the past week, there were cherry leaf spot infection periods on June 4, 5 and 8 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 week.  

As a reminder, chlorothalonil, copper products and Captan are effective materials against cherry leaf spot. Note that chlorothalonil has a 10-day retreatment interval. New leaf tissue should be protected before the potential for rain later this week. 

Powdery mildew has been observed in apple and cherry orchards in the region. Powdery mildew is caused by the fungus Podosphaera leucotricha. Powdery mildew has been reported in other fruit growing regions in Michigan over the past few weeks. Note that sterol inhibitors, strobilurins and succinate dehydrogenase inhibitors control powdery mildew. Captan, Scala, Vangard and EBDC fungicides will not control powdery mildew.  

American brown rot: Cracking from rain or wounding due to bird damage or bacterial canker infection in sweet cherries leads to higher likelihood of brown rot infection. As fruit gets closer to harvest, susceptibility to brown rot increases. Consider if brown rot management is needed. Using Indar may not be effective, even at the highest labeled rate of 12 ounces per acre. Previous research and resistance screening of American brown rot isolates in west central and northwest regions found that most screened fungal isolates had functional resistance to Indar. Find more information about this study. Effective materials for managing American brown rot include Merivon, Miravis and Cevya. Full coverage of all rows will be important to manage this disease. 

Peach fruitlets are growing steadily with many fruit setting this year across varieties. Venture peach fruitlets are 30 – 35 mm in size at the West Central Michigan Research and Extension Center. Growers are beginning to hand thin peaches in the region.  

Pear fruitlets are growing rapidly for varieties such as Bartlett, Gem, and Cold Snap. The largest fruitlets were 23-25 mm in diameter for Bartlett pear trees at West Central Michigan Research and Extension Center.  

Plums are growing rapidly with high fruit set and average size of 22-28 mm for some varieties.  

Pest update 

Consider scouting reports and block history to target hot spots for orchard pests according to fruitlet stage. Plum curculio was not trapped in Oceana County in tart cherry and apple this week, but fruit assessments for damage are still being made. Codling moth adults have been consistently trapped over the past few weeks, and a biofix of May 15 has been set at an orchard location in Ludington, and May 17 for the West Central Michigan Research and Extension Center. The treatment threshold for codling moth is five adults per trap.  

On June 10, the Trevor Nichols Research Center in Fennville, Michigan, trapped pests such as codling moth, oriental fruit moth and adult obliquebanded leafroller. Dogwood borer was trapped for the first time at the Trevor Nichols Research Center and was also trapped at two locations in Oceana County this week. Adult obliquebanded leafroller has not been trapped in Oceana County yet with the Michigan State University trapline, but adults will likely be flying shortly. 

American plum borer, lesser peachtree borer and greater peachtree borer were also trapped at the Trevor Nichols Research Center. Only American plum borer and lesser peachtree borer have been trapped at West Central Michigan Research and Extension Center so far this season.  

Borer management options are limited, but insecticide applications can be made when multiple adult borer species are active. A well-timed trunk spray is key for optimizing the value of the spray as well as management of borer larvae. Furthermore, taking special care to spray the base of the trunk will help with borer control. While chlorpyrifos is available for use this season, alternatives materials may be used to manage borers as well. Assail is a recommended material for a trunk spray once adult borers are active. Follow label recommendations for rates and application timings. Mating disruption is recommended for greater and lesser peachtree borer in orchards with at least five acres and trees that are at least 3 years old.  

American plum borer has been observed in the region. This week, an average of 0.5 adults per trap was observed. Our tools to manage American plum borer are limited with the loss of Lorsban (chlorpyrifos). Continue to monitor borer populations. American plum borers are attracted to wounded trees, and wounds allow easy entry for borers into the tree cambium tissue. Minimize tree stress and wounding when possible. For more information about the borer pest complex and control alternatives, read the Michigan State University Extension article, “Control alternatives for borer pests of tree fruits” by John Wise and Julianna Wilson, Michigan State University Department of Entomology.  

Dogwood borer was observed this week in Oceana County. Adult emergence is typically in mid-June, and egglaying occurs over a four- to sic-week period. Trees with burr knots (adventitious roots) are highly susceptible to dogwood borer infestation. Burr knots typically develop on the above ground portion of the clonal rootstocks. Dogwood borer larvae feed inside the burr knots, and if larvae continue to feed below the bark, damage can eventually girdle the tree. For more information about the borer pest complex and control alternatives, read “Control alternatives for borer pests of tree fruits.” 

Greater peachtree borer (peachtree borer) has not yet been trapped in the region.  

Green apple aphid nymphs and adults were observed feeding on the underside of leaves this week. Scout to determine if management is needed. Many aphid predators such as lady beetles and lacewings can help keep aphid populations in check. In general, established orchards can sustain thresholds of an average of three to four infested leaves on terminals before fruit damage occurs from honeydew.  

Green peach aphid was observed this week. Adults and nymphs were present and caused leaf curling and yellowing from feeding on the undersides of leaves. Monitor orchard blocks to determine if a treatment is needed.  

Lesser apple worm was observed in low numbers this week. Two adults were found in a trap at an orchard in Oceana County. Lesser apple worm larvae feed on small fruit or can borer into succulent shoot terminal this time of year. Apply sprays that target adults before egglay occurs. 

Lesser peachtree borer has been observed in the region. An average of 1.3 borers per trap were counted at the West Central Michigan Research and Extension Center. Similar to greater peachtree borer, mating disruption is recommended for lesser peach tree borer in orchard blocks with at least five acres and trees that are at least 3 years old.  

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 and woolly apple aphid are common pests in our region. Scout for colonies of rosy apple aphid nymphs in fruit clusters. Woolly apple aphids can be found at pruning scars and nodes of interior and upper branches. They are starting to become more prevalent in orchards. Treat blocks as 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 soon. No adults have been trapped yet in the Michigan State University trapline in Oceana County; however, obliquebanded leafroller adults have been trapped in other parts of Michigan. Sustained adult catch is expected at 900 GDD42. Scout orchard blocks in a few weeks to determine if a spray is needed. Consider Bt sprays in problem blocks. Larvae have been found in Oceana County. 

Oriental fruit moth was trapped at two locations in Oceana County with an average of two 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. The biofix date can be adjusted in Enviroweather according to orchard site and site-specific trapping. 

Spotted wing drosophila have not been reported in Oceana County, but it has been found in other parts of Michigan. With cool and wet conditions so far this season, spotted wing drosophila emergence may be earlier than in previous years. Since cherries are already starting to turn red, fruit will become attractive to spotted wing drosophila. Read more information can about spotted wing drosophila. 

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.