Regional reports on Michigan fruit - May 31, 2011
MSU Extension educators’ pest and fruit updates for Michigan.
This week’s regional reports:
- Southwest Michigan - Mark Longstroth, Bill Shane, Diane Brown-Rytlewski
- Southeast Michigan - Bob Tritten
- Grand Rapids Area Tree Fruit - Amy Irish Brown and Phil Schwallier
- Grand Rapids Area Small Fruit – Carlos García-Salazar
- West Central Michigan - Mira Danilovich
- Northwest Michigan - Nikki Rothwell, Duke Elsner, Erin Lizotte
Southwest Michigan – Mark Longstroth, Bill Shane, Diane Brown-Rytlewski, Michigan State University Extension
Last week was cool and rainy with highs in the 60s. There were long wetting periods that were infections for many diseases. A strong storm passed through the region Sunday (May 29) with high winds and heavy rain. Highs rose to 90 on Monday (May 30). Rainfall totals across the region ranged from 2 to 3 inches for the week. This week will be warmer. High temperatures will be in the 80s. Soils are saturated and water is standing in many fields. Most sites have received over 12 inches of rain since March 1. Soil temperatures are in the mid-60s. We are about a week behind average for fruit development. Check for the closest weather station at: enviroweather.msu.edu.
|Southwest Michigan Growing Degree Day Totals from March 1 through May 29|
|Location||GDD 42||GDD 45||GDD 50|
The fruit set is more apparent. Plum curculio egglaying has begun. Oriental fruit moth was biofixed in early May (May 8) and we are about 390 GDD base 45 after biofix, and egglaying has peaked and is declining. Tarnished plant bug will migrate from mowed cover crops to trees and feed on developing fruit. We are catching codling moth adults and have set biofix as May 30.
Apricot fruit are 20 mm in diameter.
Peach fruit are about 11 mm in diameter and completely out of the shuck. Peach leaf curl symptoms are common. Growers should switch to Mycoshield or Syllit to suppress bacterial spot. Controls for plum curculio should be on to protect the fruit. Oriental fruit moth will be attacking the shoot tips.
Tart cherries are in 10 mm in diameter and the pits are hard. The crop is not heavy, but looks better than average. Cherry leaf spot is the major cherry disease in Michigan cherries and there have been many cherry leaf spot infection periods this season. Growers need to protect against plum curculio.
Plums are 10 mm in diameter. Plum curculio is the major post-bloom pest of plum. Growers should maintain black knot controls during shoot growth.
Apples are 10 to 13 mm in diameter. In some orchards, a heavy drop is underway. Many growers applied thinning sprays early last weekend. The hot conditions will make chemical thinners very effective and over-thinning may be a problem. Growers who have not applied thinners can apply them this week. Apple thinning will be good this week due to high temperatures. Growers who have already applied thinners should not reapply thinning materials.
Storms over the weekend probably spread fire blight trauma infections on Sunday. Fire blight blossom blight symptoms from the May 11, 12 and 13 infections should be visible and ooze can spread the disease in a storm. Many growers used Kasumin during bloom and it appears to have been effective. Streptomycin symptoms on the leaves have reported. Apple scab symptoms have been reported in sprayed orchards. According to the apple scab spore maturity model, not all the spores have been discharged – ascospores are still being caught in significant numbers in recent rains. Growers still need to maintain fungicide coverage to protect against primary infection. Growers can use the scab infection tool on the Enviro-weather website to track infection periods. Powdery mildew is a concern on susceptible varieties.
Insect activity is now the primary concern in apples as the fruit are vulnerable and warmer temperatures will increase insect activity. Codling moth biofix was Sunday or Monday (May 30) for most of the region. Plum curculio egglaying is occurring. Tarnished plant bug will be feeding on the fruit. Aphids and leafrollers can be found in many orchards.
Pear fruit are 12 mm in diameter. Pear psylla egglaying was heavy during the hot weather.
Grape shoots are 10 to 16 inches and the flower clusters have elongated and the smaller clusters are separating. Bloom could begin as soon as this weekend. Prebloom disease controls should be applied soon. There is no need to apply insecticides until after bloom. Growers should watch for and note bloom in wild grapes to set the biofix for their grape berry moth sprays for the second and third. Second generation grape berry moth egglaying starts at 810 GDD base 47 and third generation GBM at 1620 GDD base 47 after biofix.
Grape growers planning on attending the Grape IPM meeting at Cronenwett Farms on June 8 need to RSVP for the meeting. Register in advance with Linda Gustafson at theBerrien County MSU Extension office, 269-944-4126.
Blueberry bloom is ending. Early varieties such Bluetta have small green fruit. Bluecrop is at late petal fall. Jersey and Elliot are past full bloom with about 25 percent of the bloom remaining. Water is standing in many fields. As bloom ends, growers need to protect against anthracnose fruit rot and fruit worms. Both cranberry fruit worm and cherry fruit worm adults have been trapped and a region wide biofix for cranberry fruit worm was probably last Friday, May 27. Growers who trap for this pest can determine biofix on their farms and use the Enviro-weather models for fruitworms. Obliquebanded leafroller are causing distorted shoots.
Strawberry bloom is ending. The largest berries are thimble-sized. Growers should be monitoring for tarnished plant bug. Feeding by tarnished plant bug causes deformed fruit (button berries). Leaf spot diseases from all the rain are showing up.
In brambles, bloom in wild blackberries and black raspberries has begun. Early summer bearing red raspberries, such as Prelude, have begun to bloom. Do not use insecticides during bloom. Fungicides to control fruit rots and cane diseases are worthwhile during the wet spring. New primocanes in fall bearing raspberries are almost a foot tall.
Precipitation is the major story over the region. We have not had a great deal of insect and disease activity, although with warmer temperatures yesterday (May 30) and today I would expect that we will see some insect populations begin to explode. Our season is still running about a week behind normal in terms of flowering stages and degree day totals.
We have recorded precipitation at many of our Enviro-weather weather stations an average of 11 days during the last two weeks. Locally heavy rainfall totals for the last two weeks that have been shared with me by fruit growers has averaged 4 to 8 inches of precipitation. This has been a particularly troublesome time for fruit growers that have not been able to plant either tree or small fruits yet, although most have found a couple of short windows to get things planted or at least ground worked in preparation of planting.
The raining weather has also made for a challenging apple scab and fire blight season, and many diseases are starting to show up such as black rot leaf symptoms in apples and leaf diseases of strawberries.
A word of caution for fruit growers during mowing operations in orchards: due to excessive rains and wet soils, most orchards have not been mowed for the last month or so. The result is tall, lush growth of sod. There are several insects, including plum curculio and tarnished plant bug, that tend to feed in this grass as well in tree fruits. These insects can move up into trees when mowing operations occur. I encourage growers to be vigilant in their pest control operations to be sure that they are covered for these insects when mowing.
|Southeast Michigan Growing Degree Day Totals for March 1 to May 30|
|Emmett (St Clair)||631||502||329|
Apples are around 10 to 12 mm in the southern part of the region and, for the most part, 5 to 8 mm in the Flint area. It is still difficult to assess the apple crop as there has been a good amount of fruit drop over the last few days. Some blocks, particularly Red Delicious, have seen extensive amount of fruit drop and have little to no crop left. My observation that where blocks were cropped a little bit heavy last year – this is a relative term because most people had a very short crop last year – fruit set is very light in those blocks. I’ve had a report this morning (May 31) that return bloom is very poor in the Thumb.
I am starting to see some good numbers of our typical springtime pests. Most growers applied a petal fall insecticide in the last three days. Plum curculio is now stinging fruit in fence row trees and I started to see a few stings in commercial orchards as of yesterday. Codling moth is starting to emerge, although we have just had two warm mornings with temperatures in the low 60s. I have heard of one grower who biofixed for codling moth mid-last week, but most are catching one to three per trap. I expect to see a large flight of codling moth in the next two days with these warm morning temperatures. I received a report that oystershell scale crawlers have emerged. If growers have this species of scale, now is an ideal time to control them. Oriental fruit moth populations also have been fairly high with average trap catches in the range of 30 to 40 per trap. Tarnished plant bug continues to be seen in a few blocks of apples, I have seen very little feeding damage. Aphid numbers are pretty much under control at most farms. At a few farms I have seen the larvae of green fruitworm, obliquebanded leafroller or redbanded leafroller causing damage to terminals. I continue to get a few reports of mullein bug being found, with no damage being seen.
As mentioned earlier, the sod is very tall in many blocks of apples. I encourage apple growers to do a good job of scouting prior to mowing and to be sure that fruit is protected during mowing because of the potential insect’s damage to fruit.
Apple scab has been a challenge for most fruit farms across the region. At many farms we have had as many as 20 wetting events this spring with 13 apple scab infection periods. Some have had eight infection periods since May 13. Apple scab spore discharge continues, although the numbers that I am catching on spore rod traps are diminishing quickly. We are still in primary apple scab season. I am continuing to see apple scab lesions in many abandoned or unsprayed blocks of apples and have seen a few lesions in commercial blocks.
Fire blight continues to be an issue where bloom is present. We may have also had some trauma blight at some farms due to high winds from storms that have moved through the region. Many growers report using more streptomycin this year than they have in the last previous five seasons combined. Most growers have made four to five applications of streptomycin over the last two and a half weeks. I am also seeing more streptomycin burn on apple leaves than I have seen in a dozen years or so. I have not seen any fire blight strikes. I have had several reports and have seen one orchard where black rot leaf symptoms are starting to be seen. Another name for this disease is frog eye leaf spot. Powdery mildew symptoms are starting in a few apple blocks.
Pears are mostly at 9 to 10 mm. We have had a fair amount of fruit drop but an OK crop remains. Pear psylla activity continues to be low this year. Again, no fire blight has been seen in pears.
Peaches are at shuck split for Red Haven and several of the PF varieties are at 7 to 9 mm in size. I continue to catch oriental fruit moth in fairly high numbers in several blocks. Remember, the regional biofix for oriental fruit moth was set for May 11. We are approaching the time where control measures need to be taken for egg hatch of oriental fruit moth in peaches. I am also starting to see some bacterial spot leaf symptoms in several blocks of peaches.
Sweet cherries are at 7 to 12 mm in size and there is a wide spread of fruit size this year. There has been some drop in sweet cherries; however, most clusters still have three to five fruit per cluster. Black cherry aphids are just starting to be seen. I am starting to see some leaf spot disease in sweet and tart cherries.
Tart cherries are at 8 to 10 mm in size and there has been a lot of fruit drop in the last week. In many blocks of tart cherries, singles are the only fruit that are left.
Plums are at about 6 to 9 mm in size for European plums and Japanese plums are a little bit larger, however, there have been some varieties that have had an extensive amount of fruit drop in plums.
Strawberries have had a long bloom period at many farms across the region resulting in a long harvest season. Most farms in the south have thimble to thumb tip size fruit. Growers to the south are trying to predict when their opening day or first pickings will be. Many growers are looking at June 10 or 13 for their first picking. Some strawberry varieties in Flint are still at the tail end of bloom. Slugs continue to be an issue in most blocks of strawberries and growers have done a good job of applying slug bait. I am seeing a fair amount of leafspot disease and have had a number of calls from strawberry growers who are reporting angular leaf spot symptoms on foliage. Copper applications have been applied in many strawberry fields to angular leaf spot. In new plantings weeds are particularly a problem over the last week or so. Many of our strawberry herbicide application have washed through the soil profile and are no longer effective.
Raspberry flowers are just beginning to expand and are ready to open in the next week for summer black raspberries. The winter kill to the tips of canes should be pruned out immediately to eliminate the potential for disease spread in summer raspberries, particularly in this wet season. Fall raspberries have about 12 to 15 inches of new growth. Now is a good time to watch for raspberry saw fly damage in fall red raspberries. This season it is also necessary to keep fungicides current to prevent spread of cane diseases.
Blueberries are at full bloom to small fruit being around 5 mm in size. Watch the disease issues on blueberries this year, particularly during bloom. Unfortunately, I have visited many blueberry farms that have standing water in the fields, making spraying operations very difficult.
Grapes have about 16 inches of new growth for Concord types with the flower clusters developing. The leaves of many Concord types look tattered due to windy conditions early in the season. French hybrid grapes have fruit clusters that are now visible with about 8 inches of new growth. I have not seen any powdery mildew symptoms in grapes as of yet, but expect to see it soon. Grapes need to continue to be protected for powdery mildew with this wet season.
Grand Rapids Area Tree Fruit– Amy Irish Brown and Phil Schwallier, Michigan State University Extension
Tree fruit growth stages
Most apples in the region are at 8 to 15 mm, depending on variety and site. Apples appear to be thinning down quite nicely on their own. Due to the very hot temperatures, most growers have not done their chemical thinning yet.
Tree fruit pests
Apple scab. There have been several different rain events over the past two weeks depending on the weather station site. Many of these rain events resulted in apple scab infections. Apple scab ascospores continue to be caught and numbers appear to be tapering off, but primary scab season is still on for at least another week. Despite all the rain events this season, there is very little scab showing up in commercial orchards and even in non-sprayed trees. All of the rains that occurred in May should start to produce lesions any day, so the next two weeks will be very important for close scouting for primary scab.
Fire blight was a concern over the weekend for any apples with open bloom left. Symptoms from the blossom blight infections on May 21 should start to show up any day. Trauma blight is very much a concern with the potential for severe weather today (May 31), or any day this week, if storms move through that rip or tear foliage from high winds or hail.
Insect development in tree fruits has responded to the warmer weather. Most growers applied petal fall insecticides late last week. Plum curculio damage started in stone fruits about a week ago and in apples over the weekend. European red mite egg hatch is complete and there are a few adults present. Rosy apple aphids are rolling leaves in apple and green apple aphid are increasing, but at normal levels so far. There are green peach aphids present in peaches and black cherry aphids are just starting in sweet cherry.
Spotted tentiform leafminer tissue feeders are just starting to be found. Obliquebanded leafroller larvae continue to be found in apple terminals in low to normal numbers – there are some pupae present, but no adult flight as of yet. Codling moth flight really made a debut on Memorial Day (May 30). There are a few white apple leafhoppers being found. Oriental fruit moths continue to fly in normal numbers for this region. A biofix was set for the general Grand Rapids area for May 12 and 262 DD Base 45 have been accumulated, which indicates early egg hatch is well underway and it’s an important time to protect stone fruits to prevent shoot flagging from oriental fruit moth.
Weather conditions in the region have improved with respect to temperature. For the last seven days, minimum daily temperatures were in the low 50s with an average temperature of 51°F. Maximum temperatures during the same period averaged 69°F. These temperature conditions increased the accumulation of temperature. Through May 30, the growing degree-day (GDD) accumulation in Ottawa and Allegan counties reached 348-357 GDD base 50°F and 667-684 GDD base 42°F. So far, minimum temperatures have remained above the freezing point and no spring frost damage has been reported.
Thunderstorms in the area produced abundant precipitation that left up to 3 inches of rain in some locations.
Summer raspberries are in full bloom and fall raspberries are in 18- to 24-inch cane stage. There is no evidence of insects or disease problems and their growth is very healthy.
For strawberries, late season varieties are in 50 percent bloom. Early varieties, on the other hand, are in 100 percent bloom. No diseases or insect problems are present.
Blueberry growth stages range from full bloom to petal fall and fruit set in early season varieties like Duke or Bluecrop. As the season progresses, problems related to winter damage and early season diseases are more evident. Mummy berry shoot strikes are evident in most fields where growers are having problems getting to in time to apply the control measures. Those fields require attention to prevent future fruit infections.
Regarding winter damage, mid to late season varieties like Jersey and Elliott are showing more dieback than previously observed. In affected fields, winter damage will reduce substantially the size of the 2011 crop. Photo 1 illustrates the type of damage observed, which includes shoot and flower bud kill. It is important to apply the recommended fungicides against mummy berry and phomopsis canker and twig blight. Products recommended for early season mummy berry and phomopsis control are Indar 2F (6 fl oz) or Indar 75 WSP (2 oz) or Orbit (6 fl oz). For a complete list of products and recommendations for mummy berry and phomopsis control, please follow MSU Extension bulletin E-154, 2011 Michigan Fruit Management Guide.
For insects in blueberries, the cherry fruitworm flight is well established, with 10 to 25 moths per traps in the Allegan-Van Buren area. For fields with history of cherry fruitworm damage, insecticide applications are recommended if the crop is in petal fall. Otherwise, the application may be delayed to coincide with the cranberry fruitworm application. The cranberry fruitworm started emerging during the past week in Van Buren and Allegan counties.
Growers should start preparing for the beginning of cherry fruitworm and cranberry fruitworm control season. As a reminder, timely detection of sustained moth catch is necessary to synchronize our pest control measures with the cherry fruitworm and cranberry fruitworm egglaying period. It seems that the application against the cherry fruitworm will coincide with the timing for the first application against the cranberry fruitworm.
Compared to the rest of the state, our weather has been rather uneventful all of last week. Temperatures in the high 50s to low 70s with the exception of yesterday (May 30) when it climbed to just below 86 in Mason and Oceana counties and hit almost 89 in Manistee. Rains kept missing us. Last week, we had three rain events in Mason County and two rain events in Oceana and Manistee counties with total precipitation from 0.79 to 0.55 inches. Overall, precipitation in the area is below normal. The new forecast is promising more rain for this afternoon as a result of two weather fronts colliding above central and northern Michigan. Besides rain, there is a possibility of hail and very strong, damaging winds.
Trees are developing nicely. It is becoming easier to see what the crop potential is, at least for some of the fruits. Sweet cherriesTart cherries are just getting out of shuck and predictions are not coming out yet. Apple fruit is from 5 to 10 mm in diameter, depending on the variety. It appears that some varieties are thinning by themselves. The carbohydrate model is showing that the apple trees have experienced elevated stress over the weekend and were very prone to thinning. At this point they are on their way to recovering from it. The next two to three days will be suitable for thinning applications. are well out of shuck with fruit 11 to 13 mm in diameter. It is obvious that the fruit set is not as heavy as initially anticipated. Growers and crop consultants are speculating that there is about 50 percent of the potential crop.
West Central Michigan Growing Degree Day Totals since March 1 as of Monday, May 30
|Last Week||Since 4/1|
After a frosty night on May 27, a few warm days brought the insect activity to a new high. Scouts are reporting catching lesser peach tree borer, although the numbers are still low. American plum borer trap catches are very high. The averages are in two-digit numbers. Plum curculio numbers have gone up slightly. The first egglaying scars have been reported in sweet cherries in the outside rows bordering some wooded area (high pressure area). There are still high numbers of obliquebanded leafrollers in the blocks. A few speckled green fruitworms are found throughout the area. Aphids (green apple aphids, rosy apple aphids and black cherry aphids) are building their colonies. Mites are still below threshold, averaging two mites per leaf. Plant bugs have been spotted as well as weevils (brown and metallic green). Codling moth trap catch is steadily going up. Many individual blocks have been biofixed already. The regional biofix for Oceana County is set for May 29 at 273 DD50 and Mason County is set for May 30 at 280 DD50.
Last week, there were few rain events throughout the area. Based on the weather data from the local Enviro-weather weather stations, Mason County did not have any apple scab nor cherry leaf spot infections while Manistee and Oceana counties are showing infections. There are moderate apple scab and low cherry leaf spot infections for May 28-29 in Manistee County and low apple scab infections for May 25 and May 29 and low cherry leaf spot infection for May 29 in Oceana County.
Fire blight is still presenting a threat due to the presence of tail-end bloom. Epiphytic infection potential numbers are very high and well above the threshold. With forecasted storm (May 31) bringing high winds and possible hail, there is very high potential not only for late blossom blight, but for the trauma blight. If this forecasted event materializes, Streptomycin or other antibiotic applications will be necessary up to 24 hours of the event.
A big thunderstorm rolled through the region on Sunday night (May 29), but the amount of rainfall was variable at each regional Enviro-weather weather station. May rainfall totals at the NWMHRS are 2.24 inches. Temperatures have been on the rise and we hit the mid-80s on Memorial Day (May 30). We have accumulated 502GDD base 42 and 232GDD base 50. We are still behind our 21-year average by about 7 to 10 days. We did have reports of spotty frost in the early morning hours of May 27. Humidity levels have been relatively high in the past four days. Weather forecasts are predicting a major storm today with very high winds and large hail.
Plant development remains slow across the region. A bit more green has started to show in apples and cherries, and plums and apricots are at bud burst. As ground has begun to dry out with the recent dry days, growers have been planting trees. Brush removal is also underway as tractors can move more easily throughout the orchards.
Wine grape report
Bud development is anywhere from bud break to 6-inch shoots, depending on cultivar. No signs of disease or insect injury at this time. Less than one grape berry moth per trap has been found.
Apple. The apple scab model has quieted down with the cool weather last week but as things heat up and rain is in the forecast, it is likely that we will see infection periods over the coming days. According to the apple scab model, over 90 percent of overwintering spores have matured and 51 percent have discharged. A preventative scab program is critical, particularly with the recent discovery of fungicide resistant scab isolates identified in Michigan apple orchards. Early in the season when fruit scab is not a concern and temperatures are cooler, Scala and Vanguard are good options to tank-mix with EBDCs for scab control.
As we move into warmer weather and the risk of fruit scab increases, growers should be considering second generation sterol inhibitors (Indar and Inspire Super) or Captan tank-mixed with EBDCs. As many orchards are in some stage of bloom and there is potential for trauma blight, it will be important to keep an eye on the weather and the fire blight model. Based on the weather and crop development, many area growers have already made at least one antibiotic application. When the epiphytic infection potential (EIP) reaches 100 (or is forecast to do so) and the average temperature is greater than or equal to 60°F, the Enviro-weather model will show the corresponding boxes on the chart turning red, which indicates that rain or trauma (high winds or hail) is needed for infection to occur. Based on the weather forecast, the EIP will be extremely high in the coming week with forecasts predicting EIP levels well over 200. Additional instructions for utilizing the fire blight model can be found on the Enviro-weather website.
Things have picked up in terms of insect activity and we likely hit the peak of spotted tentiform leafminer adult flight last week with trap catches dropping this week to an average of 125. Oriental fruit moth adults were continued to be trapped this week with an average of nine per trap. A total of six codling moth were trapped this week, following a trap catch of one last week. Codling moth management will begin in the coming weeks as most treatments target egg hatch, a predicted 250 DD50 after sustained catch. Growers looking to use an ovicide and delayed larvicide program should look to apply ovicidal materials around 100-150GDD following sustained catch. Growers can track their progression using the codling moth model.
Small obliquebanded leafroller (OBLR) larvae are prevalent in area apple orchards. Widespread obliquebanded leafroller resistance to Guthion and pyrethroids exists in northwest Michigan, so growers should be applying non-organophosphate materials to control this insect this season. In apple, management of obliquebanded leafroller often occurs later in the season and populations are likely kept low by lepidopteron materials commonly used to combat organophosphate resistant codling moth. Adult obliquebanded leafroller moths are easily trapped with pheromone baited delta traps, and larvae are also relatively easy to locate due to their leaf-rolling behavior. At this time of the year, look for larvae in the terminals.
Cherry. Area scouts are reporting some obliquebanded leafroller activity, but larvae are still small and hard to spot in cherry. Overwintering larvae feed inside bud clusters prior to bloom, and begin feeding on fruit after petal fall. Targeting this overwintering generation is critical in cherry because they are small and easier to kill. At early petal fall, growers should scout their orchards by looking at 20 clusters per tree in five trees per orchard for larvae or feeding sites. An insecticide should be applied if they observe more than two larvae or feeding sites per tree. The materials that target this life stage are Delegate, Belt, Altacor, Voliam flexi, Entrust and Bts. Growers in northwest Michigan should not expect organophosphates or pyrethroids to provide effective control because of insecticide resistance. Obliquebanded leafroller do not feed internally in fruit, but are problematic as contaminants in tanks at harvest.
Plum curculio continue to migrate into orchards from neighboring wood lots with high trap counts this week, particularly in the edge rows of blocks at the NWMHRS. Plum curculio migrate from their overwintering sites to orchards in the spring with migration lasting about six weeks with peak activity occurring two to three weeks after shuck split as young fruit develops. If growers are utilizing insecticides other than the organophosphates (Guthion, Imidan) the treatment timing is petal fall or shuck split depending on the mode of action. Consult the insecticide label and MSU Extension bulletin E154 Michigan Fruit Management Guide for more information. Based on a full bloom biofix on May 21, we have accumulated 124 DD50 towards the 375 DD50 window for organophosphate application.
American plum borer is out and active in higher numbers this week. American plum borer larvae attack the cambium and cause severe damage or tree death due to their feeding habitats. American plum borer has two generations per season with trunk applications of Lorsban targeted during adult flight.
As leaves unfurl and petals drop, the potential for cherry leaf spot infections increase. Despite the wetting events over the past week, no cherry leaf spot infection periods were recorded because minimum rain or temperature thresholds were not adequate to initiate infection. Cherry leaf spot overwinters in fallen leaves on the orchard floor and produces apothecia (sexual spore-bearing structures) in the spring. Ascospore (sexual spore) dispersal occurs during the drying period that follows a wetting event and when temperatures are between 60 to 85°F.
Following infection, acervuli (asexual spore-bearing structures) develop on the underside of the leaf and produce a visible mass of asexual spores called conidia. Spores are dispersed from leaf to leaf by wind or rain and this secondary infection cycle can be repeated several times within a season, depending on conditions. Cherry leaf spot is resistant to sterol inhibitor fungicides (Indar, Elite, Orbit) in all the major fruit producing areas of Michigan. Petal fall and shuck split application of chlorothalonil are recommended. As orchards move into first cover, products that contain strobilurin fungicides (Gem, Pristine, Adament) are recommended as they provide excellent leafspot and powdery mildew control. Remember to alternate the use of fungicide classes during the season to manage against resistance development.