Northwest Michigan fruit regional report – May 12, 2015

Northwest Michigan fruit growers are continuing to protect for possible apple and cherry disease infections during recent wet weather.

Weather report

Our warm and dry, summer-like weather last week was replaced with cool and damp conditions over the weekend. On Friday evening, May 8, temperatures dropped by more than 20 degrees in a matter of minutes, and since that time, daytime temperatures have been in the mid-50s and low 60s; this cool weather was accompanied by mist and fog.

Wet and cool conditions are expected through today, May 12, but the sun is forecasted to return Wednesday and Thursday, May 13-14. Rain is predicted to move back into the area for the coming weekend. Although conditions have been wet, we have received no considerable rainfall at the Northwest Michigan Horticulture Research Center (NWMHRC). In total, we have received less than 0.25 inches of rainfall from this last extended wetting period (Saturday-Tuesday morning); perhaps more substantial rain will fall today.

Our growing degree day (GDD) accumulations are almost exactly where we have been for our average: 341 GDD base 42 and 157 GDD base 50. Overnight temperatures are expected to reach into the mid-30s tonight and Wednesday, but frosts are not as likely in good fruit growing sites and wind tonight should help prevent development of an inversion layer. However, the middle of the state and low-lying areas may dip below freezing.

Crop report

Many grape varieties are showing bud swell. In the NWMHRC vineyard, Cabernet Franc is the most advanced vinifera cultivar at bud burst with several leaves showing in the lowest portion of the vine. On all of the vinifera cultivars, there is very little bud swell above about 1.5 feet from the ground – the snow cover line back during the worst winter cold. At some commercial vineyards, we have seen significant bud swell on Chardonnay vines much higher in the trellis.

There is still time for dormant sprays against powdery mildew in many sites. No pest activity has been reported. Vineyards with a history of climbing cutworms should be monitored closely for their activity during the next couple of weeks.

We still have a good deal of pruning and training to do at the NWMHRC vineyard. We would greatly appreciate volunteers to help get this done – we plan on working in the vineyard this Thursday and Friday, May 14 and 15, starting about 9 a.m. both days. If the weather is questionable, give Duke Elsner a call at 231-357-8353 to make sure it will be a work day before heading over to the vineyard.

Saskatoon bloom is underway in the area. Unfortunately, the weather is pretty poor for pollinator activity. I have not seen any symptoms of entomosporium leaf spot or rust disease, but with the wet weather of late we have probably had some chance for early infections.

If you do not have a copy of the 2015 pesticide recommendations for Michigan saskatoon production, just send an email request to Duke Elsner at

The warm weather last week really moved crop development, and most sweet cherries began to bloom toward the end of last week. Tart cherries in warm sites also began to bloom last week. Fortunately for those orchards with open blossoms, conditions during Thursday and Friday, May 7-8, were warm and sunny, and open blossoms had good potential to be pollinated. However, as blossoms continued to open over the weekend, there was little bee activity with the cool, wet weather. Pollination is a concern, particularly in orchards in cooler areas of the region. Wednesday and Thursday, May 13-14, are our next best days for pollination as sun is expected but temperatures are only predicted to be in the mid-50s to mid-60s. After Thursday, rain is predicted to move back into the region for the weekend, and there may be little opportunity for good pollinating conditions.

Most apple orchards are not yet in bloom, and we are hoping for warmer and dryer weather for apple bloom.

Pest report

Cloudy, periodic rain and cool weather have been consistent since Saturday, May 9, and growers were busy spraying fungicides last week to protect blossoms and new tissue prior to wetting events that favor disease development. We received very little rain overnight Friday and into Saturday, May 8-9, and the MSU Enviro-weather apple scab model did not report any apple scab infections at northwest Michigan stations. There was an increase in spore discharge in the block where we are monitoring for scab; we caught an average of 112.5 spores per rod following the Friday-Saturday rain.

We have also received spotty rain in the region since Sunday evening, which has initiated ongoing scab infection periods on the MSU Enviro-weather model. We collected fewer spores (24 spores per rod) following the Sunday evening and Monday morning rain. The last possible scab infection occurred April 20-22 and if infection occurred in orchards with showing green tissue that was not protected, scab symptoms should be appearing at this time. Rain is possible Friday through Monday, May 15-18, and growers will need to continue protecting green tissue to prevent possible scab infection.

Cool temperatures in the last few days have slowed the development of fire blight bacteria, and as a result the current risk of fire blight infection for the coming week is low. Additionally, apple blossoms are not susceptible to fire blight infection until flowers are open and few apples have open blossoms in the region at this time. Fire blight could be a concern if temperatures increase into the mid- to upper 60s when apples begin blooming.

Currently, the epiphytic infection potential (EIP) values on MSU Enviro-weather’s fire blight model are low, and at zero if apple bloom or biofix is May 9-15. However, EIPs can change quickly because these values are based on degree-hour accumulation rather than the typical degree-day accumulations used in other disease models like apple scab and cherry leaf spot. Therefore, Michigan State University Extension would like to remind growers to check this model often as predicted weather conditions can change frequently and these changes would impact EIPs. The EIP values are higher for Saturday and Sunday at this time. Some orchards in the Benzie-Manistee area could have open blossoms by the end of the week.

The figure below is the May 11 Maryblyt fire blight output for the Benzonia station that currently shows a moderate risk of infection with an EIP of 74 on Sunday.

Benzonia fire blight chart
May 11, 2015, Maryblyt fire blight output for the Benzonia Enviro-weather station. See online at Enviro-weather website.

Conditions may have been conducive for American brown rot and European brown rot pathogens over the weekend, but with the cool temperatures, European brown rot is more likely to become a problem in area orchards. Current and predicted conditions will continue to favor European brown rot infection of cherry blossoms from popcorn through bloom. Cherry development, in particular tart cherries, is variable across the region where in some areas many blossoms are open and in others, there is little white present on trees.

Tart cherries are susceptible to European brown rot infection during the popcorn stage and through bloom. Balaton are more susceptible than Montmorency to European brown rot infection and should be protected from European brown rot infection. However, Montmorency can become infected in optimal conditions like the current cool and wet weather we have been receiving. MSU recommends growers who are planning to protect from European brown rot pathogens use Indar at the popcorn and seven- to 10-days-later timings to prevent infection. If blossoms were not protected at the popcorn stage, a later Indar application during bloom will help prevent European brown rot infection.

Green tissue expanded late last week in tart cherries across the region. Fully expanded leaves, including bract leaves, are susceptible cherry leaf spot infection. Although cherry leaf spot develops and infection occurs more slowly during cool temperatures, cherry leaf spot spores can discharge and initiate possible infection periods in wet weather with temperatures in the low to mid-40s. The northwest region is currently in the midst of ongoing cherry leaf spot infection periods that were initiated by wetting events that began May 9-11.

Spotted tentiform leafminers and green fruitworm moths are active, and green fruitworm larvae could also be active at this time. Apple flea weevil was detected in high densities in an organic apple orchard in Leelanau County last week. Cool conditions do not favor insect activity.

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