May 16, 2017
Freeze impact becoming apparent in Michigan crops

The impact of the May 8 freeze is becoming apparent on fruit crops in Michigan. Some crops were hit hard, others suffered little damage.

Southwest Michigan

Last week started with a freeze on Monday, May 8. This radiation freeze caused wide spread variable damage in southwest Michigan. Every freeze is different and this one was no exception. The temperatures were cold enough to cause damage, but not cold enough to damage all crops or cause severe damage across the region. Damage is quite variable depending on the crop and site. Sites that normally escape injury were hurt and other where you would expect severe damage did not suffer as much as expected. Flowers exposed to the sky and on the tops and tips of shoots were hurt more than protected flowers in the interior or those facing down away from the open sky.

The dry air allowed temperatures to drop quickly. Many people are still assessing the damage from this freeze, which was probably the last spring freeze of 2017 in southwest Michigan.

The rest of the week was warm and pleasant. Scattered showers crossed the region Wednesday night, May 10. Thursday through the weekend was warm and windy with high temperatures in the 70s and lows in the 40s. Dry, windy conditions have dried out the top soil, but there are good moisture levels in the subsoil. Soil temperatures are in the 50s and 60s. About 4 inches of rain fell in April with an additional inch in May.

The weather this week should be warmer with highs into the 80s on Tuesday and Wednesday, May 16-17, then falling into the 70s and high 60s over the weekend. The forecast is for warm, windy, unsettled conditions for the week into the weekend. There is a chance of thunderstorms into the weekend.

The cool conditions two weeks ago almost stopped plant and insect development. Warm conditions have moved plants quickly. Many plants that looked weak now appear healthy. The combined effect of this temperature rollercoaster is that we are close to our average normal development for this date.

Tree fruit

Warm conditions will bring plum curculio feeding and egglaying in exposed fruit, and growers should apply controls. Rain occurred Wednesday night, May 10, and the wetting period was too short for disease infections except fire blight. The impacts of the May 8 freeze are becoming more apparent in low areas and areas away from Lake Michigan.

Apricots are out of the shuck. The crop is generally light.

Peaches and nectarines are at shuck split or out of the shuck. Peaches in good peach sites did not suffer much from the May 8 freeze, and the crop potential for peaches looks good. Insect activity resumed with warm weather. Based on trap catches at Berrien, Van Buren and Allegan counties, oriental fruit moth biofix ranged from April 21 to 24. Enviroweather’s oriental fruit moth model has insecticides targeting larvae timed for May 10 to 15.

Treatment for tarnished plant bug, plum curculio and rusty spot should begin when fruit emerges from the shuck. Long episodes of wetting and temperatures above 50 degrees Fahrenheit favor bacterial spot. After shuck split, Mycoshield is preferred over copper to suppress bacterial spot.

Sweet cherries are out of the shuck. The largest fruit are about 10 millimeters in diameter and the pits have not begun to harden. Sweet cherries are easily visible on the tree. Damage to sweet cherries from the May 8 freeze is spotty.

Montmorency tart cherries are emerging from the shuck. There are clearly two waves of fruit: the larger, 6- to 8-millimeter fruit emerging from the shuck, and smaller fruit in the shuck. Damage from the May 8 freeze is generally light and spotty. Some orchards report significant damage. The tart cherry crop looks about average for southwest Michigan. Protect against plum curculio as the fruit emerges from the shuck. Cherry leaves are susceptible to cherry leaf spot infection.

Montmorency cherries are emerging from the shuck

Montmorency cherries are emerging from the shuck.

Japanese plum varieties such as Shiro and Vanier are out of the shuck. European plums are coming out of the shuck. Crop loads seem light for European plum in several orchards checked so far. We are in black knot infection season when overwintering knots release spores and infect succulent green twigs of the current season’s growth. Use fungicides to protect against black knot infection, which occurs during wet periods with temperatures above 55 F. The period from white bud to shuck split is the most important for control of this disease.

Apple bloom has ended for most varieties. Rapid shoot growth is underway. Some varieties are still blooming on 1-year-old wood. Fruit size diameters range from Gala at 3-5 millimeters, Golden Delicious at 6 millimeters, Red Delicious at 7 millimeters and Zestar at 11 millimeters. Apple fruit set in many sites is light due to poor pollination or damage from the May 8 freeze. Freeze damage in the lower canopy was reported in many apple orchards. A significant fruit drop is underway. The crop should be more apparent as the remaining fruit continue to size.

Cutting this apple fruitlet apart reveals the ovules inside are clear and white, not brown or black that would indicate freezing injury

Cutting this apple fruitlet apart reveals the ovules inside are clear and white, not brown or black that would indicate freezing injury.

Temperatures for this week will be very warm with temperatures in the 80s and 70s. Thinning sprays will be very effective. Growers should assess their crop to determine fruit set and freeze damage. The Cornell University apple carbohydrate modelfor timing thinning sprays and rates is available in the apple section of Michigan State University Enviroweather.

Scab ascospores trap catches are still significant and expected to continue through May and into early June. Apple scab symptoms from the April 28 to May 2 infection period should appear this week. Protect for apple scab through May. Manage powdery mildew on susceptible varieties until rapid leaf growth ceases. Symptoms from the fire blight infection on April 26 should be visible now. Open bloom during this warm period will be very susceptible to fire blight infection if we have any rain. Showers occurred Monday night and are in the upcoming forecast. Newly planted trees often have delayed bloom that requires fire blight control when rain and warm temperature occur.

Pear fruit range from 9 to 10 millimeters in diameter. Fungicide applications for pear scab are ongoing. Growers should check their trees for fire blight from the April 26 infection periods. We are past bloom so streptomycin should not be used for fire blight unless there is hail or strong winds. Treatments for pear psylla are applied post-bloom and target rapid shoot growth and succulent leaves. Fruit at this stage are particularly sensitive to chemicals and weather conditions that can cause skin russet.

Small fruit

Grape shoots are 3-6 inches long with flower clusters exposed in Concord, Niagara and early hybrid varieties. Later hybrids and vinifera grapes have 1-5 inches of shoot growth with flower clusters exposed on the longer shoots. Many exposed shoots were frozen in in the May 8 freeze. Damage is widespread and varies greatly from one site to another. Both juice grape and wine grapes were injured, though damage is most severe in juice grapes.

Healthy ‘Niagara’ grape shoots

Healthy ‘Niagara’ grape shoots (left and center) and damaged shoots (right and background) by the May 8 freeze.

Some vineyards will see no crop reduction; others may see serious crop reduction. Where damage is severe, secondary shoots will grow in a few weeks. These secondary shoots will have flowers on them, but their development will be delayed. This greatly complicates vineyard management with a crop on both primary and secondary shoots.

Juice grapes are past the stage where grape flea beetles and climbing cutworm are a concern, but these pests may still be a concern in varieties with late growth. Protect against shoot infections of phomopsis and leaf infections of black rot. In vulnerable varieties, protection against powdery mildew leaf infections is also occurring. Over the next two weeks, note the beginning of wild grape bloom and record the date it starts to use in the Enviroweather’s grape berry moth model.

Blueberry bloom continues. With the warm weather, many fields are approaching full bloom. Effects of the May 8 freeze are readily apparent. The freeze was cold enough to cause injury to open bloom, but not cold enough to kill less developed flowers. Overhead sprinkler systems seemed to provide good protection. Damage in unprotected fields depends on the site, stage of development and the actual low temperature. The freeze mainly damaged only open flowers and damage to the petals of open flowers was readily visible. Only flowers at the tips of the shoots were open and damaged. Damage to pistils and styles inside these flowers is spotty.

Bloom has progressed rapidly since the freeze and bees seem to be working hard in the field. This freeze will not have a significant impact on Michigan blueberry production since only a small portion of the bloom was lost. Some growers applied gibberellic acid products to save damaged bloom. In flowers where the ovules (that will become the seeds) were damaged, this is unlikely to save these dead fruit. This treatment is likely to help where the style was damaged but the pistil and ovules were not.

Mummy shoot strikes are appearing. Apply fungicides to protect against mummy berry in the fruit if shoot strikes are appearing in your fields. Research indicates that fungicides can harm the health of beehives. Use caution when applying fungicides during bloom. Avoid spraying during the day when bees are foraging. Plan your mummy berry control strategiesrather than reacting to conditions. Fungicides for mummy berry control are recommended at early and mid-bloom.

Cherry fruitworm adults were caught during the warm weather before the freeze. The cold weather shut down trap catch. Trap for fruitworm in your fields so you can set the biofix. We expect a strong flight with this warm weather. Use the cherry fruitworm and cranberry fruitworm models on Enviroweather to target their controls.

Strawberry flower trusses are emerging the crown. Bloom has begun in some fields. Most growers have been using sprinklers to protect their crop. Once strawberry flower buds are above the ground, they can be hurt by temperatures in the 20s. Have sprinklers setup for freeze protection since the flower buds are vulnerable to freeze injury.

Bramble leaves have unfolded on the floricanes. Primocanes are about 6 inches high. Michigan State University Extensionadvises scouting for orange rust; spray to reduce its spread should be applied now.

Northwest Michigan

Daytime temperatures were mostly pleasant for the past week, although the northerly wind made it seem colder than the thermometers recorded. Nighttime temperatures have been cool, and there was even some frost reported on farms the morning of May 15. Some growers ran their frost fans overnight May 15.

We have accumulated 356 growing degree-days (GDD) base 50 and 146 GDD base 42, and these accumulations are behind our 20-plus-year averages. Conditions are quite dry across the region, and we could use rain. We had a small amount of rainfall on Saturday, May 13, and the Northwest Michigan Horticulture Research Center Enviroweather station recorded only 0.08 inch of rain. Rain is in the forecast for this week.

2017 growth stages as of May 15, 2017

  • Bartlett Pear – Fullbloom
  • Potomac Pear – Full bloom
  • McIntosh – Open cluster
  • Gala – King bloom
  • Red Delicious – King bloom
  • Honey Crisp – King bloom
  • Montmorency – Full bloom
  • Balaton – Full bloom
  • Hedelfingen – Petal fall
  • Gold – Early petal fall
  • Napoleon – Petal fall
  • Riesling – Early bud swell

Crop report

A week of relatively cold weather  in northwest Michigan really slowed the development of wine grape buds. Riesling at the research vineyard is mostly in the late bud swell stage. No pest insect activity has been reported from commercial vineyards. Wild vines in our area are more advanced, with some bud break. This is the time grape flea beetle adults appear on wild vines, and occasionally these show up in commercial vineyards. The injury from this insect is usually of minor importance. Climbing cutworm activity may pick up soon if we get some warm evening temperatures. Vineyards with a history of this pest should be monitored for symptoms of feeding injury.

Local saskatoon plantings are at about 50 percent bloom. The first adult apple curculio and saskatoon sawflies have been taken in sweep net samples in the Traverse City, Michigan, area. These insects will not lay eggs until after fruits are set. We are not certain of the prime infection periods for Entomosporium leafspot or saskatoon-juniper rust, but these fungal diseases may now be releasing spores if rainy weather occurs.

Close-up of saskatoon sawfly adult.

Close-up of saskatoon sawfly adult.

Growers are still evaluating the impacts of last week’s frost/freeze events. Growers that had frost fans or irrigation fared better than growers without these technologies. Some areas of northwest Michigan are reporting more damage than other regions. The more southerly areas of the region were harder hit than orchards to the north. Orchards in Manistee and Benzie counties recorded low temperatures, and in some cases, the lows hit 21 and 22 degrees Fahrenheit. Growers in that area weighed their options for Promalin/Perlan applications, but the window for a Promalin/Perlan application has passed following last week’s frost/freeze.

In most situations, the amount of damage seems less than we anticipated last week. Apples in low spots have obvious damage, but apples on higher sites fared well. Growers are still optimistic they will have a good apple crop despite the freeze damage.

Tart cherries seem to have come through the cold events with less damage than we estimated last week, and growers in northwest Michigan feel they will likely have an average size crop for 2017, although it is early to make this determination. Tart cherries are just hitting full bloom at the research center, and we had excellent pollination weather on May 15. Warm conditions are expected today, so we anticipate bees to fly if the rain holds off.

There is a varying level of damage in sweet cherries throughout the region with the most significant damage primarily in orchards on marginal fruit sites. Sweet cherry bloom seems to be hanging around for a long time this year, so we are hoping they were pollinated even though we had cool conditions last week.

Pest report

We received spotty, light rains that were not predicted in parts of the region on the morning of Saturday, May 13. Fortunately, temperatures were cool, we had very little rainfall accumulation and the moisture dried up quickly enough that no apple scabor cherry leaf spot infections resulted from this short wetting event. Although we do not have models on Michigan State University Enviroweather for the brown rot pathogens, it is unlikely conditions were wet or humid for long enough to cause blossom blight.

Although wet conditions did not last long enough for an infection, apple scab spores discharged at a relatively higher rate than we have previously observed (Table 1). While these numbers seem low overall, MSU Extension reminds growers it is not the numerical value or volume of spores that are released, but the relative pattern of release that is applicable. In other words, this pattern of spore release is typical and we should continue to see an increasing trend of spore counts until petal fall, as long as wetting events occur during daylight hours (i.e., spores have a tendency to release more readily in late morning/afternoon rains) and have enough rain (i.e., 0.01 inch) to discharge spores. We had a few scab infection periods in late April, and symptoms should be visible this week if tissue became infected at that time.

As a reminder, in areas with streptomycin resistance, Kasumin is the only effective option to kill the bacteria and knock back fire blight populations. Additionally, Kasumin is unlike streptomycin as it does not have systemic activity and this material needs to be applied prior to rain. Please see the following MSU Extension articles for more information on fire blight management: “A primer for streptomycin, kasumin, and oxytetracycline use for fire blight management,” “Forecasted weather is a concern for fire blight during bloom” and “Apogee application time.” Additional information can also be found in the “2017 Michigan Fruit Management Guide” on pages 108 and 259.In apple varieties with open blossoms, fire blight is a concern this week in predicted warm and wet conditions. Many growers with open blossoms made their first fire blight spray on Monday, May 15, prior to the predicted overnight rains. Following last year’s fire blight challenges, it will be critically important to be proactive with fire blight management this season.

Many growers made their first cherry leaf spot applications last week or early this week, depending on location. As mentioned previously, Saturday’s rain likely did not result in an infection period as temperatures were cool and the period of wet weather was brief. Enviroweather will not report infection periods until after bloom time, but there is susceptible leaf tissue present in most orchards at this time. Growers should consider leaf spot protection this week as the current conditions are predicting warmer temperatures with several days of wet weather that could result in a long infection period if areas do not receive adequate drying time between wetting events.

For growers thinking about American brown rot management in sweet cherries: If warm and extended wet conditions that are predicted do occur, this weather could be conducive for blossom blight. Some growers made an application for American brown rot on Monday, May 15.

Insect activity continues to be relatively slow with a little activity of spotted tentiform leafminers in apples and green fruitworm and American plum borer in cherries at the station (Table 2). We will continue to look for small green fruitworm and leafroller larvae.

Finally, check with processors on acceptable management tools and chemistries if you have not done so already. It is also time to put up traps to monitor for the first male flight of San Jose scale; we have been getting more reports of this pest in older sweet cherry orchards this season. The next timing to manage scale is during the crawler stage, which typically occurs about two weeks after peak male flight. If not already done, mating disruption tools should be in place soon. It is not too early for growers approaching petal fall in cherries to consider their early-season insect pest spray strategies in the context of season-long pest management.

East Michigan


Reports of frost damage and crop loss in apples have been common across most of the eastern Michigan region, with the exception of fruit growers south of the I-94 corridor who saw less damage. For most growers, the coldest morning was Tuesday, May 9, and for others it was the morning of May 8. Over the past week, I also had reports of several mornings with scattered frost events; while not cold enough to damage tree fruit, it was cold enough that strawberries needed to be frost protected for a few hours.

Warmer or seasonally normal temperatures over the last week have resulted in our fruit crops growing again after a week of much below-normal temperatures. Our season is still normal to a few days behind normal in terms of degree-day totals and growth stages.

Most of our region has received rain over the last week, especially last night when thunderstorms moved through. Precipitation totals for the last two weeks very widely over the region, ranging from only a few tenths of an inch to just over 2 inches.

Tree fruits

Apples are 10 to 12 millimeters in the southern parts of the region and 5 to 7 millimeters for most other apple growers. There is still a fair amount of bloom on 1-year-old wood and ragtag bloom in many varieties, causing a concern for possible fire blightinfection if the right set of rain and warmth comes in the next week or so.

Thinning will need to begin in the next few days. Apples have had a very long bloom period, maybe one of the longest growers can remember. Pollinators have been more active this past week, however with the long bloom period of cold temperatures, pollination is still questionable. Combine these factors with the frost and freeze events over the last nine days, and thinning decisions this year are going to be very difficult. Thinning in some varieties is needed, but be conservative in your approach. Consult the new carbohydrate model on our Michigan State University Enviroweather website to use the inactive model to help make your thinning decisions. You will need to click on the fruit tab at the top of the website, then to the Apple Carbohydrate Thinning tab on the left side.

The new insect pests to report this week in apples are plum curculio, rosy apple aphids and codling moth. In the last two days I found my first plum curculio when using a beating tray to scout for them. I have not seen damaged fruit, but with apples sizing quickly and predicted warm temperatures, I would expect to begin seeing stinging soon. Good numbers of rosy apple aphids are being seen in some apple blocks. Codling moth trap catch is high in most non-mating disruption apple blocks. A few green apple aphids and tarnished plant bug adults were found.

Overall, insect pests have been slow to develop over the last two weeks due to cold weather. Oriental fruit moth and redbanded leafroller trap catch continues at a slow pace. Obliquebanded leafroller larvae are on terminal leaves, with leafrolling taking place. Spotted tentiform leafminer adult trap catch is on the decline for the first generation adult flight. Beneficials are being found, and this week I am starting to see brown lacewing.

Apple scab leaf lesions are in just a few apple blocks. Spore discharge continues with each rain event. As discussed earlier in this report, growers need to be concerned about fire blight infections with bloom continuing in apples, especially where fire blight was a problem last season. The Enviroweather website has a great MaryBlyt tool to determine fire blight infection events. Also, look out for oozing fire blight cankers where it was a problem last season.

Pears are mostly 5 to 7 millimeters in size, with a good crop developing. Pear psylla adults are flying.

Peaches are at late bloom to shuck split. I have not seen much frost or freeze damage in peaches. Most growers have a good crop of peaches this season. Oriental fruit moth trap catch continues.

Sweet cherries are at late shuck split to 10 millimeters, with a wider than normal range of fruit sizes in most varieties. Most growers have a nice crop of sweet cherries this season.

Tart cherries are mostly at late shuck split to 3 to 5 millimeters in size. Most growers report they have a good tart cherry crop this season.

Plums are at early shuck split for European types and Japanese types out of the shuck to 8 millimeters in size. Some Japanese varieties have little to no crop on them this season.

Small fruits

Grapes are at bud burst for Concord types and European varieties have about 2 inches of new growth with flower buds just starting to emerge.

Strawberry growth has been very poor this season, mostly due to cold temperatures. Leaves have poor growth and flowers are small in size and vigor. I expect that with warmer temperatures predicted over the next few days that by late this week strawberry fields will look much better. Most fields need a small amount of nitrogen to help bring them around. Early varieties are at 40 to 50 percent bloom, with later varieties at 25 percent bloom. Most growers have frost protected one to two times the past week, some frosts were not predicted and growers have missed the opportunity to frost protect, resulting in some black blossoms.

Raspberry leaves are emerging from the bud with flower bud emergence continuing for summer fruiting types and tip dieback on many varieties. New canes are emerging from the ground in fall raspberries, but many fields appear to have been frosted and are not growing. The longest canes are about 6 inches long.

Blueberries are at full bloom for most varieties. Overall, there are a lot of bloom in blueberries this season. I have not found any signs of mummy berry mummies on the ground.

Michigan State University Extension

75 Applewood Drive, Suite A
P.O. Box 128
Sparta, MI 49345
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