MSU Extension

Temperature swings have been common during this spring season. Last Friday, May 31, daytime temperatures jumped up into the 80s, but by Sunday morning (June 2), we started the day with temperatures in the mid-40s. Fruit continues to develop slowly with these changing temperatures, but insect and disease development has also been slowed under these conditions. Fruit is starting to size in all tree fruits: apples are 8 millimeters while some varieties of sweet cherries are up to 11 millimeters, and apricots are approaching 20 millimeters. We have accumulated 651 GDD base 42 and 373 GDD base 50, which are comparable to our 22-year average.

We also received some rainfall last week and into the weekend. On Tuesday, May 28, the Northwest Michigan Horticultural Research Station Enviro-weather site recorded 0.26 inches of rain, and on May 30 we had another 0.13 inches. On Saturday and Sunday (June 1-2), we had over a half-inch of rain over this two-day period. This long wetting period resulted in infection periods for the major tree fruit diseases. We also recorded freezing temperatures at two Enviro-weather stations in the northwest early in the morning of June 3; the East Leland Enviro-weather station recorded four hours below freezing and the Bear Lake Enviro-weather station had two hours just around 32 degrees Fahrenheit. The East Leland Enviro-weather station also recorded 31.8 F from 5 to 6 a.m. on June 4. We have heard of isolated reports of hail from the May 30 rainfall event.

Apples. As most apples are entering the target zone for thinning – 10 to 15 millimeters – growers have the carbohydrate model and weather conditions on their minds. Unfortunately, the weather forecast is predicting cool temperatures for the remainder of the week, and the carbohydrate model is showing that trees will be in a surplus situation, which will make thinning more difficult. Most growers will likely wait for temperatures to warm this weekend to begin the thinning. If temperatures do not rise, growers will have to be more aggressive with thinning applications and combinations (see the Michigan State University Extension article “Apple thinning pointers for 2013”).

The apple scab model shows that we are at 99 percent spore maturity here at the Northwest Michigan Horticultural Research Station (NWMHRS) if we were at green tip in McIntosh on April 30. We have 79 percent spore discharge and will need more warmth and rainfall to discharge the remainder of spores. Growers need to continue to cover for primary scab until we know that all spores have been discharged. We have observed some powdery mildew starting to show up in apples; growers need to protect against this disease as we have no fungicides that will eradicate this disease once it is established.

We have not observed any fire blight strikes in the region. Growers should remember that Kasumin cannot be used in late blooming or in orchards with tag bloom – the last day that Kasumin could be used was May 31 (last Friday), even in non-bearing orchards.

As mentioned above, insect development has been slow. We continue to catch spotted tentiform leafminers in fairly low numbers, and we have observed obliquebanded leafroller activity. We caught our first codling moth at NWMHRS on Monday (June 3). These insects likely flew on Friday (May 31) or Saturday evening with the warm nighttime temperatures. Other growers and scouts in the area started to catch codling moths last week. Setting a biofix may prove to be a challenge with these warm-cold temperatures swings. With the predicted cool stretch ahead of us, we will be hard-pressed to set a biofix at NWMHRS yesterday (June 3) as we will likely catch no moths in this coming week. Growers should be trapping for codling moths in their own orchards because of the variable population sizes in different orchards across the region.

Plum curculios were likely active last week when temperatures were warm and there was plenty of moisture to increase this insects’ activity level; however, the cool weather will likely slow plum curculios down again.

Cherries. We are starting to see spottiness on cherry leaves, but the verdict is out if these spots are due to spray injury, bacterial canker showing up from the cold, wet weather earlier this spring or early forming cherry leaf spot lesions. We will be keeping an eye out for mycelial growth on these spots in coming weeks. Most growers are at first cover timing for disease control in tart cherries, and this timing is critical for powdery mildew control as well as continued coverage for cherry leaf spot.

For first cover, the new products (Luna Sensation and Merivon) might be a nice fit as they provide excellent control of both powdery mildew and cherry leaf spot. These materials are rated excellent against cherry leaf spot, and killing all fungi in the orchard early at this first cover timing is the first step in achieving good, season-long control of cherry leaf spot. Again, we do not have any eradicant materials for powdery mildew, so early season control is critical in preventing powdery mildew infection in cherry.

European brown rot has been particularly damaging in some orchards this season, in both Balaton and Montmorency blocks. We will be taking samples of orchards that have been hard hit by this disease, which is favored by cool and wet conditions. If growers would like their orchard sampled, please call NWMHRS at 231-946-1510. Once this pathogen has moved systemically into the spurs, there are no control options available.

With cherries out of or coming out of the shuck, this fruit needs to be protected against egglaying female plum curculio. We have seen sweet cherry orchards with the moon-shaped egglaying scars. Growers should be checking hot spots for these scars. For growers using the P.I.T.S model in tart cherries (not applicable in sweet cherries), we have accumulated 205 GDD since the biofix date of full bloom on May 17. Remember that any eggs laid in tart cherry fruit prior to the 375 GDD base 50 from Montmorency full bloom will cause the fruit or the larvae to fall off the tree, and insects laid at this early timing will not be present at harvest.

We are still finding obliquebanded leafroller larvae in cherries. However, the optimal timing for controlling this pest in sweet cherries has past; growers will have to wait until the summer generation larvae are present to control this population, which is at the pre-harvest timing. Growers that have had problems with this pest in the past and were not able to make a petal fall application should be monitoring for obliquebanded leafrollers prior to harvest to make sure there are no larvae in the tanks at harvest.