MSU Extension Fruit & Nuts

Weather update

Temperatures fluctuated over the past week with highs in the low 80s and lows in the mid- to low 30s in the region. June 4 will be very warm and humid, but cooler temperatures will follow for the rest of the week due to a cold front moving across the region. Storms brought over 0.88 inches of rain for Hart over the past week according to the Hart Enviroweather station, and relative humidity increased to 83% at its peak on June 2. Humidity is predicted to remain from 65 – 85% over the next week across the region.

Fruitlets are growing steadily (Figure 1). Based on the Hart Enviroweather station for June 3, current growing degree-day (GDD) accumulation is 941 GDD₄₂ and 474.6 GDD₅₀. Over the next few days, high temperatures will increase. Tuesday, June 4, will be the warmest day this week with highs in the 80s. Toward the end of this week, temperatures will decrease with highs in the mid- to low 70s and upper 60s.

With rain over the past week, soil moisture has ranged from 9 – 18% at 4 inches deep in the soil profile and about 7 – 8.2% at 20 inches soil depth for the Hart Enviroweather station. Steady rain has 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 the 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.

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 approaching the later thinning window where the efficacy of chemical thinners will start to diminish (great than 20 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. Addition of a surfactant may enhance thinner efficacy and is recommended if there is a carbon surplus according to the Cornell Apple Carbohydrate Thinning Model. Some growers are done chemically thinning this year and will rely on hand thinning if additional thinning is needed.

For some apple varieties, most advanced fruitlets range from 20- 24 millimeters (mm) for Honeycrisp and Jonagold in Hart. Some fruitlet thinning occurred naturally this year shortly after bloom. Fruit continue to fall as we approach June drop timing. Growers using the fruit growth rate model or the fruit weight model should continue monitoring fruit abscission with these tools to help determine final fruit set.

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. Additional information and thinning recommendations can be found in the Michigan State University Thinning Guide.

For some varieties, it is helpful to improve return bloom by making summer applications of NAA in low concentrations. This helps minimize the effects of biennial bearing and is especially important for trees that have a heavy crop load this year. Summer treatment of NAA at 5 parts per million (ppm) is important because the application timing is after the thinning window has passed and low concentrations will not induce a thinning response. Fruitlets are about 1 inch in diameter and will not be susceptible to NAA as a thinner at this size. Applications should be made at five, seven and nine weeks after bloom.

While flower bud initiation has already begun, NAA application can enhance this process during the next 30 days after the thinning period ends. Ethrel applications can also enhance return bloom at 200 ppm made at the same timing. Varieties with high biennial bearing tendencies include Golden Delicious, Honeycrisp, Jonagold, Mutsu and Red Delicious. See more information and recommendations about summer applications of NAA from Michigan State University Extension.

Apple scab: With rain showers on June 1 and 4, there have been extended wetting events over the past week. Enviroweather and RimPro models show that almost all spores are now mature, and 97% of the spores have been dispersed as of June 3. From the June 1 rain event, 3.5 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 3–4 and 5–6. Apple scab appearing on leaves and fruitlets has been observed over the past few weeks. 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).

Fire blight: Fire blight symptoms were observed in orchards this week in mature and young trees. 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.

Continue management with an Apogee and Actigard program. The Apogee and Actigard combination as recommended by Michigan State University researcher, George Sundin, is the best combination to use for fire blight protection. This combination works better than other products and has synergistic characteristics, meaning it works better in combination than as individual products alone.

If blighted shoots are observed in orchards, please contact Emily Lavely at lavelyem@msu.edu or George Sundin at sundin@msu.edu. Sundin would like to test infected tissue for antibiotic resistance in the region.

Tart and sweet cherry are growing rapidly. Fruit size for early sweet cherry varieties are 21 – 24 mm (Figure 2). 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 on June 1 and June 3–4, there were cherry leaf spot infection periods according to the Hart Enviroweather station. Cherry leaf spot symptoms have been observed across the region. Pressure will continue to be high with rainy and warm conditions this week.

Peaches are at the green fruit stage depending on the variety. Venture peach fruitlets are growing rapidly with average size of 24-29 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 17–21 mm in diameter for Bartlett pear trees at West Central Michigan Research and Extension Center (Figure 3).

Plums are growing rapidly with high fruit set and average size of 22–24 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 was trapped at one orchard location (out of three) in Oceana County in tart cherry and apple this week. 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. Green peach aphid and green apple aphid were observed in the region this week.

On June 3, 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. Obliquebanded leafroller was trapped for the first time at the Trevor Nichols Research Center. 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. Growers are encouraged to minimize tree stress and wounding when possible. For more information, read this article on the borer pest complex and control alternatives by John Wise and Julianna Wilson from the Michigan State University Department of Entomology.

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. Growers should apply sprays that target adults before egg lay occurs.

Lesser peachtree borer has been observed in the region. An average of seven 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 peachtree 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 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 soon. No adults have been trapped yet in the Michigan State University trapline.

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, but the first generation is winding down. 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.