MSU Extension

Crop stage and growing degree days (GDD)

After a week of cooler than normal temperatures, warm weather has returned and trees are responding. Apples are mostly at the pink stage, some early varieties with first blooms are opening and later varieties are just at tight cluster. GDD totals have caught up to about normal accumulations after a few days last week in the 70s.

Apple scab

There was a wetting event in the early morning hours of May 4, but there were not enough hours to give an apple scab infection for any Michigan State University Enviro-weather stations. Most stations reported only a few hours of wetting, but the Belding Enviro-weather station had nine hours of wetting at 60 degrees Fahrenheit, so they were just shy of an infection by an hour. Despite the rain coming during the night, I caught 1,120 spores for the Ridge monitoring location and 213 spores for the Standale, Michigan location.

New wetting periods have begun with morning rains today, May 5. It is likely this could turn into an apple scab infection with rain predicted on and off for the next 24 hours. Temperatures will be cooler which will take longer for an infection to begin. Hopefully you are well-covered with fungicide for the rains predicted for today and the remainder of the week.

For the apple scab infection on April 19, lesions could show up soon around May 6-8, so scout closely for this possible infection. Look on spur leaves or the first true leaf on early developing varieties.

Fire blight

We have a had a nice run of dry, cool weather, but of course, now that apple blooms are opening, the rains and warmer temperatures make an appearance. Early varieties are just starting to have some open blooms in the last 24 hours. There is no worry for blossom blight infection of these few open blooms with this rain event – open blooms need at least 24 hours of temperatures over 70 F to build bacteria and we have not had that yet. Temperatures today will not get above 60 F, which will lower or stall bacterial development. The rain predicted for later in the week, after a couple of days near 80 F, will be a different story. The MSU Enviro-weather Maryblyt model is predicting very high risk for blossom blight on any open bloom present with rains predicted for later in the week.

Here are some guidelines to consider for fire blight management, adapted from Kari Peter, Pennsylvania State Disease update, May 3, 2015:

• Apply antibiotics as complete sprays and add an adjuvant or surfactant. Antibiotic sprays are most effective when they are applied the day before and within 24 hours after an infection rain event begins (not ends).
• Where resistance is not a factor, Streptomycin is the best option since it kills the bacteria. It also has partial systemic activity that no other material has.
• Kasugamycin is the tool to use where streptomycin resistance is known. It is different from streptomycin in that it reduces bacterial growth and reproduction rather than killing it directly. It is best applied prior to an infection event, but has effectiveness within 24 hours of the start of a wetting event. It is only labelled for use up until petal fall.
• Oxytetracycline is an antibiotic that reduces bacterial growth and reproduction rather than kills it directly. It is best used before an infection event occurs.
• Copper during bloom will kill fire blight bacteria, but it will likely result in fruit russet.
• Serenade is low on the effectiveness scale, but could be useful in moderate risk blossom blight situations or in combination with antibiotics.

Tree fruit insects

Most apples are nearing the pink stage and this is a great timing to go after rosy apple aphids, obliquebanded leafrollers and spotted tentiform leafminers. Many growers on the Ridge put their pink sprays on last week with their fungicide covers. Michigan State University Extension wants to remind growers that it is best to use insecticides well outside of any bloom present to avoid any issues with pollinators that will arrive when bloom is present. Applying a pink spray and then moving bees in shortly after is not a good plan.

Oriental fruit moths began flying with the warmer weather over the weekend. Mating disruption should be set for this pest and traps should be up with lures in them. Codling moths could begin some initial flight in the next week or so; traps with lures need to be placed soon and mating disruption can go up anytime.