MSU Extension

Temperatures remain on the cool side across the northwest, and fruit continues to size slowly. Tart cherries have not increased in size in over seven days, and apples have just grown a little. Despite the accumulation of growing degree days (GDD), we are not far off our 20-plus year average, but fruit development is moving very slowly. So far this season, we have accumulated 927 GGD base 42 and 539 GDD base 50.

No rain was in the forecast last week, but most of the region had a surprise shower that amounted to 0.92 inches of rain at the Northwest Michigan Horticultural Research Station (NWMHRS). Most growers were likely covered for this event, but since this rainfall was not predicted, we hope this event will not result in increased disease incidence as it was an infection period for both apple scab and cherry leaf spot.

Apples. Apple scab lesions are starting to show up across the region from early infection periods. According to the apple scab model on Enviro-weather, we are at 100 percent maturity and 95 percent discharge. From this last rain, in most years we would typically call the end to primary scab, but since they are still catching spores on the Ridge and in southeast Michigan late last week, we think growers should keep covered up through the next rain event. Additionally, growers that have lesions on their leaves will need to be protected beyond primary scab to prevent fruit scab later in the season.

Fire blight also started to show up last week in many different varieties. Most of this disease is a result of those perfect fire blight conditions during bloom when we had warm temperatures coupled with rain or fog events. However, some of the blight is a result of a hail event. Growers with a history of fire blight should be looking for the typical shepherd’s crook that shows up when terminals have been infected.

Again this week we have caught very few codling moths here at NWMHRS—only an average of 2.5 moths per trap. Because these numbers are so low, we are hesitant to set a biofix although we have caught moths on three successive dates. Many growers throughout the region have already set a biofix date, and some were set at the end of May. Population size plays a huge role in the numbers of moths captured at each farm, which is why Michigan State University Extension recommends growers should be trapping on individual blocks on their own farms.

Very few other insects were caught in apples this week: zero obliquebanded leafrollers, seven spotted tentiform leafminers, and an average of two oriental fruit moths.

Cherries. Cherry diseases have also become apparent across the region late last week. First, we are starting to see more bacterial canker showing up in both tart and sweet blocks. Anecdotally, more leaf symptoms are appearing in tart blocks while both leaf and fruit symptoms are showing up in sweet cherry blocks. Growers with canker on the fruit need to be scouting for American brown rot fungus. This fungus is an opportunist that will move into fruit that has been damaged, such as bird pecks or, in this case, fruit that has been infected with bacterial canker.

American brown rot is a disease that is difficult to control as we approach harvest, and sporulating green fruit now presents an even bigger challenge for growers. Gem, a strobilurin, is an anti-sporulant. In blocks where cankered cherries are infected with American brown rot and are sporulating, we are recommending growers come in at the next cover with a full rate of Gem. This material is also good on cherry leaf spot and American brown rot, but the anti-sporulant properties will minimize the spores on already infected fruit. Growers should save their Indar applications for closer to harvest.

We are also recommending Gem for orchards that have been infected with the European brown rot fungus. From samples collected around the region, we have confirmed many collapsing spurs with and without leaf flagging is in fact European brown rot. This disease favors cool and wet conditions, which have not been in short supply so far this season. The orchards hardest hit are near the tips of peninsulas and orchards that do not have good airflow. These orchards did not dry off quickly with all of this spring’s fogs and wet conditions, and the European brown rot fungus was able to infect Montmorency spurs.

European brown rot is a close relative to American brown rot, and both fungi are in the genus Monolinia. We have not observed European brown rot to infect the fruit, but we have no reason to suspect this disease does not have the capability of causing fruit rot; therefore, we are recommending growers with substantial European brown rot infection use Gem to minimize sporulating spurs, leaves and pedicels with the intention of minimizing fruit infection later in the season. Gem at the full rate will be a good next application to minimize European brown rot sporulation and because this material is also good against cherry leaf spot and American brown rot.

Cherry leaf spot lesions have been observed in northwest Michigan orchards. As we head into second cover, coppers are a good rotational fungicide, particularly as temperatures remain cool. Coppers used at a rate of 1.2 pounds of metallic Cu have provided excellent efficacy against leaf spot. Syllit (dodine) is also a good option at this time. If growers used one of the new fungicides at first cover, an alternative mode of action should be used at this second cover timing.

Insect activity is minimal in cherries. We caught our first obliquebanded leafroller in traps here at the station, but very few area scouts have caught obliquebanded leafrollers this week. Given the size of the larvae we have been collecting, we expect pupation of these larvae in the coming week – we just need a bit of warm weather.

For growers using the P.I.T.S model for timing insecticide applications against plum curculio in tarts, we have accumulated 365 GDD since the full bloom date of May 17. Any larvae laid in cherries prior to the 375 GDD base 50 from full bloom will drop out of the cherries and will not be present at harvest.

American plum borer flight is on the decline, but lesser peach tree bores were still caught in relatively high numbers this week: average of 10 moths per trap. We have captured no peach tree borers (greater peach tree borers) yet this season.