MSU Extension

Weather report

Warmer temperatures are finally hitting the north region. The weather was beautiful over the weekend and perfect for pollination: sunny and warm. Warm weather is predicted to continue this week, and on Monday, May 23, we hit a high of 81 degrees Fahrenheit. The Northwest Michigan Horticulture Research Center has accumulated 457 growing degree days (GDD) base 42 and 204 GDD base 50.

Rain is predicted for many days this week. Conditions have been extremely dry across the north. The last rainfall event was May 13, and at that time no Michigan State University Enviro-weather station recorded above 0.5 inch of rain. These dry conditions have been good in terms of the lack of disease development, but the region could use the rain. There is variability among weather forecasts, but some percentage of rain is predicted for Wednesday, Thursday, Friday, Saturday and Sunday at this time. Growers are planning to cover up prior to these rain events.

Crop report

Most sweet cherries are finished with bloom, and tart cherry bloom is also starting to wane. Apple bloom varies throughout the region and by variety. We expect to move quickly through apple bloom with the predicted warm weather. Bee activity was good over the weekend, and we anticipate good pollination for trees that were in bloom.

All wine grape varieties in the research center vineyard are now at bud burst through 3-inch shoot stage, and the warm weather ahead will push some rapid growth. The rains predicted for later this week will likely result in the first significant infection period for powdery mildew on vinifera cultivars and a few hybrids that are somewhat susceptible. MSU Extension advises getting on a protectant spray for powdery mildew, especially in vineyards that had a significant infection in 2015. For growers with some of the table grape varieties that are prone to Phomopsis, these rains could also start the season for that disease and a protectant spray is in order. No significant pest insect activity was noted this week.

Most saskatoon sites are in petal fall. Insect activity has really picked up due to recent warm weather. Sweep net samples at the research center planting caught lower numbers than last week of saskatoon sawflies, first detection this year of apple curculio, tarnished plant bugs, leaf-feeding weevils and several types of small moth caterpillars. There were also many beneficials in the sampling, including spiders and parasitoid wasps. Unfortunately, the threat of fruit losses from sawflies and curculios outweigh the current value of the beneficials, so it is advisable to protect the fruit with an insecticide at this time. It is also time to start protecting fruits from rust and entomosporium spot disease, especially with the threat of rain later this week.

With the predicted warm temperatures, apple bloom may be over quickly this year. Growers should be sure to use the plant growth regulator Apogee, particularly on fire blight-sensitive varieties. The epiphytic infection potential (EIP) will be high this week with rain in the forecast, which is optimal for fire blight. Under these conditions, using Apogee is even more important as this plant growth regulator is a growth inhibitor that provides excellent control of shoot blight.

The first timing for an Apogee spray is around king bloom petal fall when there is less than 3 inches of new shoot growth. This timing coincides with the beginning period of rapid shoot growth of the tree. However, in some years, shoot growth is accelerated around the king bloom petal fall timing. In this case, when Apogee is applied to trees that have shoot growth longer than 3 inches, it does not work as well. Growers should pay particular attention to the timing of Apogee to ensure they obtain maximum effectiveness from the Apogee applications. Please refer to “Apogee application time in apples” by MSU Extension for more information.

Some growers are starting to think about thinning apples, particularly if they are using the nibble thinning approach. According to the carbohydrate model below, we are entering a time of stress in the coming days with warm and cloudy days in the forecast; these are conditions where thinners will work better than under cold and sunny conditions when the trees have a surplus of energy. Growers should try to thin at this time, especially if they are at petal fall, which is a less sensitive time to thin and the deficit will benefit this thinning timing. Conditions during and after the thinner applications will influence the activity of the thinners. Growers will likely have good results from thinning early under these deficit conditions. We are recommending growers consider mild to normal rates of thinners under these conditions.

On an additional note, we have had droughty conditions across the region, and trees that are stressed from drought may thin easier than trees with adequate water. However, we are expecting significant rainfall this week, so trees will not likely be stressed from drought for much longer, if the weather forecasts are correct.

Carbohydrate model for the Northwest Michigan Horticulture Research Center for May 20, 2016.

Pest report

So far this season, the region has been fortunate to have conditions that are not conducive for cherry leaf spot and apple scab infections. Earlier spring conditions were cooler and not favorable for American brown rot during sweet cherry bloom. Although some areas had cool and wet conditions that favor European brown rot infections in tart cherries, most growers were covered and protected for European brown rot.

Many cherry orchards are at petal fall, and early sweet cherries are starting to come out of the shuck. In anticipation of rain this week, growers are covering for cherry leaf spot. Prior to shuck split, chlorothalonil is an excellent early season material for leaf spot control. Good coverage will be critical as the leaf spot ascospores have had ample time to develop in recent dry and warm weather, and the region could have several warm days with rain this week, ideal conditions for leaf spot development. Some of the early season’s rain events did not trigger leaf spot infections because conditions dried quickly and temperatures were cool; however, leaf spot infection periods will progress much quicker with the warm temperatures this week.

Chlorothalonil has excellent leaf spot efficacy, but reapplication of a fungicide may be needed if there are several days of rain. Chlorothalonil is not a systemic fungicide, so if we have multiple days of rain or a single rainfall event over about 1 inch, we recommend coming back in with another fungicide. Captan is also rated excellent for leaf spot and is recommended if a fungicide is needed before the 10-day retreatment interval of chlorothalonil is met.

The coming rain will also be a concern in apples for possible fire blight and apple scab infections. The MSU Enviro-weather fire blight model is currently reporting high potential for fire blight infection for all weather stations in the region, and most growers have applied or will apply a bactericide prior to the predicted rain. Due to the warm and wet conditions that are predicted, apple growers need to be actively managing fire blight at this time. The output for Northport, Michigan, currently shows an EIP of 73 and although this EIP does not exceed 100, there is a high potential for infection if apples are in bloom. Therefore, growers in northerly Leelanau County should also apply a bactericide to susceptible and high value varieties, and in orchards with a history of fire blight. Applications made Monday-Tuesday this week, May 23-24, will provide up to three days of control.

Since conditions will continue to be warm, fire blight bacteria will grow rapidly; hence, if conditions later in the week are favorable for fire blight infection, growers may need to reapply a bactericide. Some orchards could also be approaching the timing (1-3 inches of new shoot growth) for application of the plant growth regulator Apogee, an effective tool that inhibits shoot growth and also prevents shoot blight by thickening cell walls to stop the movement of fire blight bacteria through plant cells. The MSU Extension “2016 Michigan Fruit Management Guide” also contains information on Apogee use on page 257.

Primary apple scab season is ongoing, and mature spores will readily discharge in the coming rain. The Northwest Michigan Horticulture Research Center biofix for primary apple scab is April 17, and at this time the center is at 91 percent spore maturity and 60 percent discharge. The MSU Enviro-weather apple scab model is currently predicting 100 percent maturity and 81 percent discharge by May 29; hence, primary season will continue into June and possibly end by early to mid June.

There has been substantial growth since the region’s last rainfall, and this new tissue will need to be covered prior to rain. As mentioned previously for cherry leaf spot, many apple scab spores are mature and will be discharged, and good coverage will be critical especially if conditions are wet for several consecutive days. The SDHI fungicides have very good to excellent efficacy against apple scab, and they are the best available fungicides for scab control. Aprovia is a newly registered SDHI and like Fontelis, it is a single chemistry fungicide; Luna Tranquility and Merivon are premix fungicides. These SDHI fungicides also provide powdery mildew control. We remind growers to use the maximum label rate for SDHI fungicides and to tank-mix with a protectant for all fungicides that are at risk of resistance development. A maximum of two applications of an SDHI per season is recommended to preserve the longevity of these materials for scab control. The sterol inhibitor fungicides Indar and Inspire Super have good efficacy against scab, and tank-mixing these materials with a protectant is also recommended.

This is the third week of American plum borer moth activity, and we found an average of 12 moths per trap in cherries. Lesser peachtree borer activity has not been detected; lesser peachtree borers could begin flying later this week. Larval activity of leafrollers and green fruitworm is ongoing. We remind growers that the diamide insecticides Belt, Altacor and Exirel are rated excellent against obliquebanded leafroller, codling moth and oriental fruit moth larvae. Delegate also has excellent efficacy for obliquebanded leafrollers. In the case of insects that have multiple generations per year, obliquebanded leafrollers and codling moths, it is recommend that different modes of action are use for the first and second generations to minimize the potential of resistance development. Anecdotal evidence suggests Belt works better early for targeting the overwintering obliquebanded leafroller larvae at the petal fall timing rather than for second-generation larvae that come out at or near harvest time.

In apples, a total of two oriental fruit moth were found in traps at the center this week; this was the Northwest Michigan Horticulture Research Center’s first detection of oriental fruit moth this season. We have not detected codling moth at the center, but the first codling moth in the region was detected late last week, and we continue to receive reports of activity in known hot spots. Spotted tentiform leafminer moth activity is ongoing, and trap numbers remain low at around 20 moths per trap. Control of spotted tentiform leafminer larvae is often achieved with materials that are commonly used for other key apple insect pests around petal fall timing.

We have not observed plum curculio activity, and we expect these beetles will show up in orchards soon as small cherry fruit are beginning to come out of the shuck and nighttime temperatures are above 60 F. Some growers are planning to apply an insecticide for plum curculio within the next five to seven days or following rain this week. Growers with fruit at susceptible or exposed growth stages should protect fruit from plum curculio oviposition.

Dr. Rothwell’s work is funded in part by MSU’s AgBioResearch.