MSU Extension Fruit & Nuts

Weather update 

Moderate weather has led to steady changes in bud development over the last few days across the west central and Grand Rapids, Michigan, regions. Sweet cherry (Figure 1), plum and some peach varieties are in full bloom. Moderate daytime temperatures and cooler nighttime temperatures in the west central region means there is a lot of new leaf growth and opening flowers. Tart cherries are also in early bloom, and apple and pear bloom will be starting this week (Figure 2). In early apple varieties, such as Zestar and Idared, king bloom is opening in the Hart, Michigan, area. Honeycrisp, SweeTango and Jonagold are at pink. In the Grand Rapids region, Honeycrisp is in bloom this week.  

Based on the Hart Enviroweather station for May 5, current degree day (DD) accumulation is 326 DD42 and 136 DD50. For the Sparta Enviroweather station, current DD accumulation is 431 DD42 and 204 DD50.  

Temperatures have been in a holding pattern over the past week, and forecasts seem to change more regularly than normal, which makes planning orchard tasks difficult. Michigan State University’s (MSU) climatologist, Jeff Andresen, says there is a weather pattern that has been causing this called a “cut-off low.” A cut-off low is a persistent low-pressure area that becomes isolated or cut-off from the main airflow, according to NOAA. These weather conditions are notoriously difficult to model and forecast and can lead to extended periods of cooler temperatures, precipitation and cloudy conditions. Watch a full weather update from Jeff Andresen. 

Degree day accumulation is similar to the five-year average for the west central region (Table 1), and the Grand Rapids region is slightly ahead of the five-year average (Table 2) according to the MSU Enviroweather stations.  

For the rest of this week, Hart temperatures are forecasted to have highs in the low to mid-60s and lows in the upper 30s and low 40s. The forecast for Sparta is similar but with a few days reaching up into the upper 60s and low 70s. Warmer weather is in the forecast for this weekend and into next week, and temperatures are predicted to be 5-10 degrees above normal next week.  

Table 1. West central region: Current and five-year average DD summary from Jan. 1- May 5. Numerical integration was used for DD calculations. 

Table 2. Grand Rapids region: Current and 5-year average DD summary from Jan. 1- May 5. Numerical integration was used for DD calculations. 

Weather data was gathered from MSU 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 

Apple varieties in Oceana County range from pink to early king bloom. Early varieties such as Zestar and Idared are at early king bloom, and later varieties such as Gala and Honeycrisp are at pink. In the Grand Rapids area, varieties such as Smitten and Red Delicious are in bloom, and Honeycrisp in early bloom. With extended cool conditions during bloom, Retain (AVG) applications can help extend flower viability if there are concerns about delayed fertilization. Promalin applications can also be used at this timing to help preserve fruit production if borderline freeze conditions occur during bloom and cause slight damage to flowers.  

Do not apply Promalin to freeze damaged tissue. As with most plant growth regulators (PGR), temperatures should be warm (65-plus degrees Fahrenheit) before application. Promalin can also help increase cell division, increase fruit weight, increase apple typiness, increase fruitset, and start the thinning process. With any PGR, applications should be made under slow drying conditions. 

Sweet cherry are in full bloom, and tart cherry are about 50% bloom. For tart cherry, bract leaves are starting to emerge, and tissue will become susceptible to cherry leaf spot. When the bract leaves emerge, stomates are mature enough for infection. With cooler temperatures, brown rot infection can occur with optimal infection temperatures of 64 to 78 F, with 4-plus hours of wet conditions. Brown rot infection can occur under a wide range of temperatures (40 to 86 F).  

Peaches are at pink or full bloom depending on the variety. Venture peach at the West Central Michigan Research and Extension Center are at about 50% bloom.  

Apricots are at petal fall and fruit are starting to swell in the shuck in Oceana County. Like other stone fruit, brown rot infections are favored by temperatures in the 60s and 70s with rain. 

Plums are at full bloom for European and Japanese varieties. Petal fall is a critical time for fungicide control for black knot (usually mid- to late May) to late June (10-inch new shoot growth). Knots should also be removed through pruning under dry conditions.  

Pear varieties are at the popcorn state or first bloom for Bartlett pear at the West Central Michigan Research and Extension Center. Pear scab sprays are needed to protect from infection as more green tissue is present. Fire blight management may also be needed in pear if temperatures are about 70 F and rain occurs during bloom.  

Pest and disease update 

West central area 

Diseases. With warmer temperatures, leaves continue to grow. New green tissue is emerging in apples, and this tissue is at risk for primary apple scab infection. Since temperatures have been more moderate with highs in the 50s and 60s, periods of leaf wetness have not been long enough to have high infection risk, although low or moderate risk occurred on May 1-2 in the west central region. Only a small number of spores were trapped at the New Era, Michigan, site during this rain event with 12 spores per rod. There is a slight chance of rain today on May 6, but little rain is forecasted for the rest of the week. 

The powdery mildew fungus grows well in warm and humid conditions. Powdery mildew does not need wet conditions to infect leaf tissue like apple scab does. Fungicides will be needed this week to cover tissue and protect against powdery mildew.  

Sprays may be needed for fire blight this week with so many open blossoms. Cooler weather will keep blossoms open for an extended period of time, which means they will be susceptible to fire blight infection for an extended time. Watch the fire blight model through MSU Enviroweather (Maryblight). The fire blight pathogen grows well at temperatures over 65 F, and the fire blight model uses 65 F as a base temperature to calculate degree hours for infection risk. Other factors that are included in the model are if the trees are in bloom and wetting events (rain, dew, fog or spraying).  

If apples are in bloom from May 7-11, there is low risk of fire blight infection since conditions are predicted to be dry. As temperatures increase early next week, infection risk will increase slightly. To review some helpful reminders about fire blight management, check out “A primer for Streptomycin, Kasumin and Oxytetracycline use for fire blight management” from George Sundin, PhD, MSU tree fruit pathologist.  

Insect pests. In Oceana County, green fruit worm (eight adults per trap) and red banded leafroller (46 adults per trap) were trapped this week. Black stem borer were trapped this week in Oceana County. First catch of oriental fruit moth occurred this week with an average of two moths per trap.  

Grand Rapids area 

Information provided by Amy Irish-Brown, Valent U.S.A 

Tree fruit diseases. Widespread rain started before dawn on May 1, resulting in a moderate infection for much of the general Grand Rapids area. I caught 474 spores from the morning rain and another 68 spores from the afternoon rains for a total of 542.  

There was a separate wetting period Sunday night, May 4, across the area with areas south getting more hours wetting than to the north, so some stations had another infection period and most did not. I counted an average of 16 spores per rod from that rain event. There could have been more, but the rods were very dirty, and it was hard to find things. 

We are approaching the spore maturity stage where the next few weeks could give us big numbers. Scab lesions from the first infection on April 18-19 could show any day now, so keep an eye out for them on the first early leaves that emerged in the timeframe. There is more rain in the near forecast and that seems to be changing daily, so stay ahead of rain events and always keep new foliage covered well.  

We are also moving into the key window to manage powdery mildew. Temperatures have been too cool and relative humidity too low for mildew development so far. If temperatures increase to 65-75 F and some light rains and damp conditions stick around, mildew could get a foothold. Make sure you are including mildewcides over the next few weeks, especially if you had mildew last year and likely have higher inoculum and potentially higher pressure. 

Apple kings are starting to open around Grand Rapids. Keep an eye on the fire blight model on MSU Enviroweather to stay on top of potential blossom blight situations. If temperatures stay on the cooler side of normal, we might have some marginal risk, but we know how fast the weather can change around here so keep your guard up and be ready to cover up. 

Tree fruit insects. Most growers have their pink sprays on late last week. For more northern sites in Grant and Fremont, you might still need to apply pink. Know your pest complex and use the material(s) that cover you for that pressure. Insects typically targeted at pink in apple include aphids (all species), San Jose scale, apple rust mites, green fruitworm (which has been higher in recent years), redbanded leafroller (minor pest) and spotted tentiform leafminer (very minor pest).  

Oriental fruit moth will start to fly soon, so get traps placed, but controls come later. Codling moth traps should be up by the end of this week, too. Bloom is a good time to have them ready to not miss the start of flight and a good biofix. Small, overwintering obliquebanded leafroller larvae are feeding in terminals. If you had more damage than expected, be sure you target this overwintering generation of obliquebanded leafroller to keep the summer generation down. Traps for obliquebanded leafroller don’t need to be placed until late May or early June. 

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