MSU Extension

Most fruit crops are moving along slowly and fruit size has not increased much in the past week to 10 days except for MacIntosh apples that made an 8-millimeter jump in size and winegrapes whose shoot length increased with the few warm days last week. Daytime temperatures were up in the 70s over the weekend, and most growers likely tried to thin with these conditions. Apples are at optimal thinning size here at the Northwest Michigan Horticultural Research Station (NWMHRS) except for MacIntosh which are close to out of the window now. Thinning has been a huge challenge with the temperature fluctuations this season. Growers are also trying to put on gibberellic acid applications in tart cherries, but again, most are waiting for a window of a few days of warm conditions. We have accumulated 773 GDD base 42 and 441 GDD base 50. We have not had any substantial rainfall since last weekend, but it has been overcast for the past few days and humidity levels have been on the high side.

Apples. As mentioned above, thinning has been on the docket for most apple growers across the region. Most growers likely thinned late last week or early into the weekend, as we had four days of daytime highs in the 70s. This week is predicted to be in the low 70s and partly sunny, so growers may have a last chance to thin if they did not achieve good results with previous thinning efforts.

With all the potential fire blight infection conditions we had during bloom, very little fire blight is showing up in the region. We have had two scab infection periods this spring and we are starting to see some scab lesions in the orchard. With the little amount of precipitation we have received, we may see another infection period. Growers should be sure to cover up for scab prior to these wetting events. According to the apple scab model on Enviro-weather, if temperatures do reach into the 70s today (June 11), we will be at 100 percent spore maturity and 89 percent spore discharge. We hope to call the end of primary scab with the next substantial rainfall event.

Insect activity has been off to a slow start with the variable weather conditions. We only caught one codling moth again this week, and with such low numbers, we still have yet to set a biofix for this insect. Michigan State University Extension is recommending that growers trap for codling moth in their own orchards as population sizes vary across different blocks of apples. Many growers have set a biofix date two weeks ago while other growers just set one last week.

We also have not caught any obliquebanded leafroller moths. Given the large size of the larvae last week, we predict to catch these moths in the coming week.

Cherries. Fruit size remains in the 11- to 12-millimeter range for all varieties of tart and sweet cherries. We are at the right time for gibberellic acid (GA) applications as most tarts have five to seven leaves or three to five fully expanded leaves. The warm conditions will help with this application as GA does not work well below 70 degrees Fahrenheit.

We have had three infection periods for cherry leaf spot, and lesions are just beginning to show up in orchards. Lesions that are on the leaves now will pose a challenge in keeping these trees clean throughout the remainder of the season. Growers should be out scouting for lesions to know their level of control with their past applications. We have had some challenging weather that has been conducive for disease development. As mentioned last week, the new products like Luna Sensation and Merivon are good products for first cover control as they are excellent against cherry leaf spot and powdery mildew.

We are still investigating the cause of declining spurs in tart cherries. In Balatons, we can confirm that most of these infections are caused by European brown rot. These symptoms are flagging leaves on a spur and eventually these leaves will turn brown and the whole spur will be killed. The symptoms look like fire blight, but rather than kill a whole branch, European brown rot only takes out the spurs. However, if the infection is serious, a lot of fruiting surface can be lost to this disease in a bad year.

We are also finding some European brown rot in Montmorency. The curious new twist in our investigation is that we have observed American brown rot in some Montmorency blocks, and we will put out a full report on this situation as soon as we have confirmed the species of the fungus and reviewed this spring’s weather events. More information on this situation is forthcoming.

Insect activity in cherries is similar to apples, slow and spotty. We have seen plum curculio in cherries and most growers have put on an insecticide spray in both sweets and tarts as soon as the fruit was out of the shuck. Plum curculio numbers seem to be a little higher this season. For those growers using the P.I.T.S model for plum curculio in tart cherries, we bloomed at the NWMHRS on May 17 and we have accumulated 272 GDD since that date. Any eggs laid in the fruit prior to 375 GDD post-full bloom biofix, the larvae will have dropped out of the fruit and will not be present at harvest. This model is only good for tart cherries.

We are still finding large obliquebanded leafroller larvae in cherries, and for most orchards the timing for control of these insects is past. Growers should be trapping for adults to know if a pre-harvest control application is needed. We are catching both American plum borer and lesser peach tree borer in cherries at this time, and we caught an average of 10 American plum borer and 14 lesser peach tree borer in the traps this week.