MSU Extension

A warm stretch last week in the 70s and 80s pushed growing degree-days (GDD) along more quickly than we have seen for most of the early spring here in west central Michigan. However, temperatures during the past weekend and early this week dropped back down into the mid-50s, slowing growth down again.

The past week has been defined not only by the fluctuating temperatures, but by moderate to high winds throughout the area as well as intermittent and often unpredictable rains. This has made spray coverage a real challenge and some growers have reported concerns they got behind in some blocks due to these limitations. This leads to some serious concerns about several diseases, particularly fire blight, which are discussed below.

Fire blight on apple blossoms became a major concern last week, with EIP’s (epiphytic infection potential) well over 100 for the entire region for several days. With weather making spraying a challenge, many growers were considering stretching applications of either streptomycin or kasumin out for three days or more.

However, fire blight models indicated that applications approximately every 48 hours were necessary during the worst stretch last week given the high level of risk. Potential gaps in coverage during this period in time may lead to blight symptoms showing up in coming days. In light of several questions that have come in this week, here are a couple of things to keep in mind with sprays for fire blight as we approach the end of bloom.

Oxytetracylene (e.g., Mycoshield) does not actually kill bacteria and will not help you eliminate streptomycin resistance problems in a problem field. While this product is an important and routine mix with streptomycin or kasumin, the outcome of this practice is not the same as mixing two different fungicides that have two modes of action in order to get more reliable kill of fungal spores for better fungicide resistance management. Oxytetracyclene temporarily arrests bacterial cell division, but it does not actually kill streptomycin resistant bacteria. As a result, if you suspect streptomycin resistance, the only appropriate solution in a high risk situation is kasumin, which can be mixed with oxytetracyclene.

Kasumin is like streptomycin—and is unlike oxytetracyclene—because it actually kills bacteria rather than inhibiting them from growing for a short while. Only killing a streptomycin-resistant bacterium truly manages resistance. Anything less and you are just slowing it down for a little while. Continue to use oxytetracyclene as a tank-mix as it an outstanding and necessary partner to streptomycin or kasumin, but do not count on it for resistance management.

Mixing streptomycin and kasumin together in a tank is unnecessary and will not provide any additional management perks. We do not have kasumin resistance in Michigan, so this product should kill any fire blight bacteria that it contacts. As a result, whether you have streptomycin resistance or not you get no additional benefit from mixing the two together in a single application. Use either a streptomycin-oxytetracyclene or a kasumin-oxytetracyclene.

Early cherry leaf spot lesions were reported in the past week. As this wet weather continues, continue to be vigilant and stay protected to prevent this pathogen from getting established early in the season in orchards. The period of cool, wet weather that occurred during bloom in many areas is likely responsible for scattered early reports of European brown rot. While no major damage has been reported, isolated areas have reported small amounts of damage from this pathogen.

The fallout from the frost events that occurred two weeks ago is becoming clearer across west central Michigan. In general, apples have surprised growers in many sites by setting a light to moderate crop, particularly in sites that were sprayed with Promalin at the appropriate timing. Damage is still moderate to severe in many locations, but more apples than originally expected have emerged across the region.

Tart cherries, sweet cherries and peaches are still very sporadic throughout the area. The tart cherry crop in particular has looked continually lighter in passing days where crop is still present. In many blocks, fruit that originally appeared to make it into the shuck following the frost have displayed short, stunted stems and poor growth, typically a sign that they will not survive to maturity and will fall off in coming days. Many growers are beginning to weigh their options for management in these blocks with light crops, trying to determine whether or not a program designed for harvest and consumption will be economically feasible given crop loads and predicted tart cherry pricing.

Heavy parthenocarpic (seedless) apple set has been observed in many locations, particularly in blocks that were sprayed with Promalin following the frost events. Many blocks have a mix of parthenocarpic fruit and seeded fruit, which presents thinning challenges for growers who are looking to apply thinners in coming days. Many are waiting until 10-15 millimeter fruit stage to make a decision, as it is possible that some of the damaged fruit that has set parthenocarpically may self-thin in coming days.

Parthenocarpic ‘Idared’ fruitlets from an orchards sprayed with Promalin following the frost. Note that while the outer ring of tissues (the ovary) is green, the inner part where you would normally expect seeds to develop is browned and devoid of any development.

Oriental fruit moth and codling moth are flying in west central Michigan. The first reports of plum curculio damage came in around May 21, which we would expect given the warm spell last week. Management has begun. San Jose scale has been trapped at the Trevor Nichols Research Center and the Northwest Michigan Horticulture Research Station.

The first female spotted wing Drosophila of the year was trapped in west central Michigan and identified on May 23. The trapping network in place will be keeping growers updated as the population progresses throughout the growing season. Remember that tart cherries and sweet cherries are not at risk for damage until they are yellow in color or later.