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Apr 8, 2024
Early season reminders for grower application timings

Because there have been starts and stops to the Michigan’s 2024 field season due to weather, Michigan State University Extension reminds growers to be on top of management decisions as phenological stages are likely to move quickly and key application timings may be missed.

To begin, the 2023-24 winter has been the warmest on record since temperature data have been collected from 1895-2024. From December through February, the mean average temperature across Michigan during that three-month period was 30.5 degrees Fahrenheit, which was 10.3 F above normal. As a result of these warmer conditions, many fruit crops accumulated their chilling hours well before normal.

San Jose scale on the trunk of stone fruit. Photos by Emily Lavely, MSU Extension.
San Jose scale on the trunk of stone fruit. Photos by Emily Lavely, MSU Extension.


For example, chilling accumulation requirements of Montmorency tart cherries were achieved in December this year, whereas chilling requirements were achieved three months later in 2023 and four months later in 2022. In short, as soon as warm conditions develop in the state, these trees will move quickly. Growers will need to be ready to act to achieve optimal timing of applications.

According to Jeff Andresen, Michigan State University climatologist, the weather is predicted to be cold and rainy/snowy for Wednesday and Thursday, April 3-4, but then clear by Friday, April 5). Over the weekend, temperatures are predicted to warm and daytime highs could hit the mid- or upper 50s by Sunday in Benton Harbor, Hart and Traverse City, Michigan. Warmer than normal temperatures are also expected much of next week. Trees will advance rapidly under these conditions.

Dormant sprays have long been used in agriculture, especially fruit production. They are a very well-named practice as they are applied when the plants are dormant, before green tip or budbreak. Dormant oils, also applied when the plants are dormant, are primarily used to suffocate overwintering insects and their eggs. These sprays can also be applied at delayed dormant timing, close to green tip and until the pink stage; however, oils can often damage new plant tissue so great care must be taken when applying.

Phytotoxicity can occur on buds, emerging leaves, blossoms or fruitlets. As a reminder, oil sprays should not be used 48 hours before or after a frost event or if temperatures are very high (over 85 F) and humid. Additionally, avoid applying sulfur or Captan within several days of oil application as this will also cause phytotoxicity.

Tree fruit

Two pests that growers should have on their radar for early season/dormant applications are San Jose scale and woolly apple aphids; both pests have been on the rise in recent years. San Jose scale has been more prevalent in both sweet cherry and apple while growers have seen dramatic increases in woolly apple aphids in apple blocks across the state.

Overwintering populations of woolly apple aphid in high-density apple.  
Overwintering populations of woolly apple aphid in high-density apple.


Most cherries are still in some stage of dormancy across the state (late green tip in southwest Michigan and bud swell/early side green in northwest Michigan), and oil sprays should be applied for San Jose scale in sweet cherry once this cold weather moves out of Michigan. Apples are also in varying stages of development across growing regions, but most blocks are still at a stage when oil applications would be beneficial.

Horticultural oil applied at the delayed dormant stage will control San Jose scale by smothering the overwintering scale (Photo 1). This application would also provide activity against overwintering European red mites in apple. A common way to use dormant oil in apple is 2 gallons per 100 gallons water per acre prior to green in sweet cherry and at green tip with copper in apple. Some growers will break up their oil applications into two sprays: once at green tip with copper and another at 1 gallon per 100 at tight cluster with an insecticide targeting scale and rosy apple aphid. Rates need to be reduced as the oil is applied closer to pink. Spur and bud damage is a high risk at pink and should be reduced to a rate of no more than a gallon of oil per 100.

Again, woolly apple aphid has become more problematic in apple blocks in recent years. Preliminary data from spring 2024 shows that overwintering woolly apple aphids are present in branches in the apple canopy (Photo 2). Woolly apple aphid typically overwinters in the immature stage on the roots and then moves up into the tree canopy in spring where they form the noticeable “woolly” colonies. From early season sampling, Michigan State University researchers have identified live woolly apple aphids on branches in some problematic blocks. Dormant oil applications may have some effect on these insects; however, because these aphids are covered in a waxy coating, the efficacy of oil sprays may be limited


Diseases are also impacted by dormant applications. Fire blight and apple scab infections have been reduced by early season sprays, and dormant applications have been an important resistance management tool. Michigan State University recommends using copper for fire blight at this dormant timing at a rate of 2 pounds of metallic copper per acre. Adding a spray oil at 1 quart per 100 gallons of spray solution improves efficacy but also increases the potential for phytotoxicity. We also recommend that oil and Captan sprays are spaced out three to four days to help reduce this potential for phytotoxicity.

As mentioned above, this oil and copper combination can be a two-punch for San Jose scale and fire blight. Copper applications have also been helpful for reducing early season scab spores and potential infection. As of April 2, there have been limited spores caught in southwest Michigan and the Ridge area, and more are expected with the coming rains if temperatures are warm enough to result in ascospore discharge.

Read more of the article here.

By Michigan State University’s Nikki Rothwell, Jeffrey Andresen, Cheyenne Sloan and Emily Lavely.


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