MSU Extension

Growth stage update

Apples are 10 to 14 millimeters in general. This will be a big week for chemical thinner applications, but growers are advised to be a bit careful. The forecast is calling for very hot temperatures, cloudy weather and slow, drying conditions. The carbohydrate model is indicating these conditions could cause a good deal of stress to apple trees, resulting in an increase in natural fruit drop. This stress will encourage fruit drop naturally and growers need to take this into account and adjust their thinning accordingly and be cautious.

Fire blight

One week ago (as of May 28, 2013) was a serious time for potential blossom blight with full bloom, high temperatures and rain across the Grand Rapids, Mich., area. It does take about 10 days for blossom blight symptoms to appear after an infection, so watch closely for shoot collapse and oozing by the end of the week. The very warm temperatures forecasted for later in the week will push the expression of fire blight symptoms and could lead to further spread if there are any storms that move through and tear up foliage at the same time of the start of shoot collapse. Keep a close eye on your fire susceptible varieties later in the week for any possible fire blight symptoms.

Apple scab update

Last week brought widespread rain across west Michigan’s apple growing regions and resulted in one long apple scab infection period. There were a good number of spores released during this rain (2,310), the highest number so far this year. For the rain that began Monday afternoon, May 27, into Tuesday, there should be another good number of released spores. The general Grand Rapids, Mich., apple area has likely reached 98 percent spore maturity and the farthest north area (Ludington, Mich.) has likely reached 91 percent spore maturity. Once we reach an estimated 100 percent spore maturity, it can take a few rain events to discharge these spores, so Michigan State University Extension advises growers to keep fungicide covers tight for the next 10 to 14 days. It is too early to reduce primary apple scab fungicide rates at this time. Lesions from earlier infections began to be found on May 24 in unsprayed trees.

Powdery mildew

With all the powdery mildew that was present last year, continued addition of mildew materials would be wise for a bit longer. Apple fruits are very susceptible at this stage and the weather this week is great for mildew.

Tree fruit insects

Many growers applied their petal fall-timed insecticides over the weekend. There are several reports of plum curculio being active in stone fruits as well as apples, even with the cooler weather over the weekend. They will most likely really start to move into apple fruits with the warmer nighttime temperatures forecasted for this week and cover sprays are very important to maintain for the next 10 days to two weeks for plum curculio. There have been some unexpected high numbers of codling moth trap numbers in the past week. Not every block is reporting this, but there are hot spots. A regional biofix has not been set yet as data is still being gathered.

European red mites in apples are fully hatched from overwintering eggs and newly laid eggs are being reported.

Oriental fruit moth flight has been very strong with some very high numbers being reported in stone fruits in the last two weeks. The average numbers are 50 to 60 moths per trap per week with some numbers of over 100 per trap in some locations. A regional biofix was set for May 15. We have accumulated 185 DD base 45 since that date using the Sparta Enviro-weather station data. This indicates that egg hatch for first generation oriental fruit moths is just beginning and cover sprays to protect shoot tips and small stone fruits should be considered, especially in blocks with high trap number this season.

A few first codling moths were trapped in the general Grand Rapids, Mich., area on May 17 or 18, but not everyone was catching them that early. A few more were reported last week, but still spotty and not uniform mostly due to the very cool weather over the weekend. The warm weather this week will likely bring them out. All codling moth traps should be in place now in west Michigan and you should be monitoring them closely to determine your own biofix. A regional biofix has not been set at this time as more trap data needs to be collected.

Other minor insect pests are being found in tree fruits. Plant bugs (tarnished plant bug and mullein bug) are active; spotted tentiform leafminers are in the sap feeder stage and controlled well with petal fall timing; aphids can be taken care of at petal fall as well as any obliquebanded leafrollers that might be out there. There are a few white apple leafhoppers present, and they are usually higher in number at the first cover timing.