Early season reminders for key application timings of dormant and delayed-dormant fruit sprays
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EmailPhenological stages are likely to move quickly when weather warms up this spring.
As is common in Michigan weather, there have been starts and stops to the 2024 field season. As a result, 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. 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
Insects
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. 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
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.
Small fruit
Blueberry diseases
Dormant sprays and oils in a small fruit system are generally most effective against pathogens. Both phomopsis and anthracnose overwinter on dead tissue, allowing for dormant sprays to help clean up the overwintering inoculum and reduce disease pressure at the beginning of the season (Photo 3). Michigan State University recommends using either lime sulfur or Sulforix. Lime sulfur is a contact protectant that, in addition to helping manage phomopsis and anthracnose, can have some control on the hatching eggs and nymphs of scale insects, aphids and mites.
Sulforix controls for similar things to lime sulfur and can be used at a lower rate but it can't be used with oil-containing pesticides. Both chemicals have tight timings when they can be applied since crop injury can occur if applied on expanded foliage. Michigan State University recommends applying Sulforix or lime sulfur at the rate of 1 gallon per acre up through pink bud. Avoiding application right before a rainstorm can aid in efficacy.
Sulforix and lime sulfur dormant sprays have also been shown to reduce mummy berry shoot strikes. The mummy berry model from Michigan State University’s Enviroweather is predicting risk earlier than expected. With mummy berry, if no green tissue exists, there is minimal risk of infection. If you want to treat, use products designed for dormant plant tissue, like Sulforix or lime sulfur, not systemic products.
Grape diseases
Dormant sprays can help in controlling Phomopsis, powdery mildew, black rot and anthracnose. Occasionally, a decline in downy mildew has been shown from dormant sprays, particularly when copper sprays are utilized. Products that can be used as dormant sprays include lime sulfur or Sulforix, copper products, Sulfur (liquid form recommended) and JMS Stylet Oil or other dormant oil.
Since dormant sprays are all about contact, sticky formulations that don’t wash off easily are best. Application during a dry period and not right before a rainstorm can improve efficacy. For Sulforix, Michigan State University recommends 1 gallon per acre before green tissue. For lime sulfur, the rate is slightly higher at 5-10 gallons per acre before green tissue emerges. For copper chemistries, it is recommended to follow the label rate, but Cuprofix is recommended in grapes. Certain cultivars of grapes are sensitive to copper and sulfur once they have leafed out, so make sure and take special care with those varieties.
