May 9, 2017

Michigan growers assess cold weather impact

Michigan State University Extension has issued an update on frost damage in “The Ridge” area near Sparta, Michigan.

There were two very cold mornings this week on May 8 and 9 leading to some injury to flower buds for apples. There is freeze injury in low lying areas where expected, but also on the tops of hills. Frost fans helped a bit, but there was not a significant inversion due to cold daytime temperature prior to the cold nights. Orchard blocks with micro sprinklers seemed to have had the least amount of bud injury from first observations. While injured flower buds can be seen, there are many that are still green. It is much too early to adequately assess the impact to our potential crop.

GA4+7 is a PGR found in Promalin and Perlan and has the potential to set parthenocarpic fruit is apples where fertilization or frost damage has prevented fruit set. If you have apples that are severely damaged, this PGR won’t resurrect it from death, but there are viable flowers still present and the use of Promalin or Perlan should be considered this season for those blocks with the potential for poor fruit set. The rate is 1 pint per acre applied in 100 gallons of water. You can add your fungicide covers with this PGR for the next rain potential scab infection. Leaving check trees or rows is recommended to best assess how this application might work – we still have much to learn about GA4+7 for fruit set in this type of scenario.

Sweet cherry is in the full bloom to petal fall stage with some damage from the cold, but again, too early to determine the potential crop at this time. Peaches are past bloom and still in the shuck. There appears to be very little cold injury to peaches at this time.

Apple Scab – a week of dry weather has eliminated the risk fofscab infections for a short time. There are still primary scab spores to be matured and released and the next significant rain event could release a big number that have been waiting for rain to release them. There have been 690 DD Base 32 accumulated since first primary scab spores were caught on March 30, 2017. This indicates roughly 90% of primary scab spores are mature, so we have several weeks of primary scab to protect for – don’t skimp on your primary fungicide program now.

Powdery Mildew – the cooler weather of late will be less of a risk for mildew.

Fire Blight – there has been no risk to apples and pears for blossom blight infections with the very cold weather. Warm weather is in the 7-day forecast and there will likely be ragtag bloom still present and vulnerable.

Bacterial Canker in Sweet Cherry – could get a significant foothold with the frosty conditions of late. The frost damages the green tissue and allows the bacteria to enter the tree.

It looks like the cool weather will move out later in the week and, hopefully, bee activity leading to pollination and fertilization of apples will help set fruit.

Insects have been holed up waiting for fairer weather – no significant activity to report at this time. No codling moth flight or plum curculio activity anywhere in Michigan yet. Warm weather next week could bring insects out in earnest – especially is we get night temperatures in the 50’s.

— Amy Irish-Brown, MSU Extension Educator – Production Tree Fruit