MSU Extension

Weather report

The warm weather has increased the number of growing degree-days (GDD) at all Michigan State University Enviroweather stations in northwest Michigan. As of Monday, May 7, we have accumulated 178.1 GDD base 42 and 80.2 GDD base 50; these jumps are significant compared to last week’s accumulations. We are still behind GDD accumulations of the 2014 season, when we had only accumulated 130 GDD base 42 and 31 GDD base 50.

We had a sizeable rain event on Friday, May 4, where the northwest Michigan Enviroweather station received 1.4 inches of rainfall. We also had a minor rain event on Sunday, May 6, and temperatures were cold and wet, which was a concern for bacterial canker infections in newly pruned sweet cherries.

Temperatures were into the 70s on Monday and Tuesday, and more rain is in the forecast for Wednesday and Thursday, May 9 and 10, and into the weekend. Temperatures will also drop from the 70 down to the 50s over the weekend.

Growth stages at the Northwest Michigan Horticulture Research Center as of May 7, 2018

• Bartlett Pear – Bud burst
• Potomac Pear – Bud burst
• Mac – 0.25-inch green
• Gala – 0.5-inch green
• Red Delicious – Early tight cluster
• HoneyCrisp – 0.5-inch green
• Montmorency – Side green
• Balaton – Early bud burst
• Hedelfingen – Early bud burst
• Gold – Early bud burst
• Napoleon – Early bud burst
• Riesling – Scale crack

Apple and cherry crop report

More spring-like temperatures have hastened tree development across the northwest region. Green tissue is evident on apples and cherries, and growers are starting to make applications to protect the new leaf tissue in apples. Pruning is still ongoing, but many growers are finishing up. Growers are trying to find a good window for sweet cherry pruning as we have had cold and wet weather interspersed with warmer and drier conditions. Many growers applied Promalin late last week to try and break buds in sweet cherries; conditions were warm but on the edge if they were warm enough for good plant growth regulator (PGR) activity.

We have had some reports of damage in young tart cherries, even those on good sites. We hypothesize that some of these orchards lost leaves early as a result of the many wetting events and cherry leaf spot infections last season. This early defoliation combined with cold temperatures in December and January are the likely culprits of young tree loss in the region. Those damaged trees are easy to spot as the bark color is dark to blackish, and the damage is apparent as tip and branch die back. When we cut into the trees, there is damage to the cambium layer but also in the heartwood of the trees. Other than those young isolated tart cherry blocks, we have observed very little bud damage in tree fruits.

Pest report

Following rain and relatively warmer temperatures, several apple varieties at the station were at green tip by the end of last week, and our green tip biofix date (110 GDD base 42 after Jan. 1) for McIntosh was on May 1. Our team is using two web-based apple scab models, RIMpro-Venturia and Enviroweather, to assist with management decisions this season. While Enviroweather provides a good summary of when wet weather could have resulted in an infection, RIMpro-Venturia provides a detailed look at forecasting apple scab spore releases during a predicted rainfall. Therefore, the predictive capability of RIMpro allows growers to better assess their management strategy prior to possible infection periods.

Recent warmer weather has matured more apple scab ascospores, and RIMpro is currently forecasting the potential for high spore release in this week’s predicted wet weather beginning Wednesday, May 9, in the evening. While evening rains do not typically result in high spore discharge, mature spores will be ready to discharge following this recent stretch of warmer temperatures.

The weekend is also predicted to be wet and as a result, RIMpro is predicting the possibility of a major scab infection event. However, we anticipate spore discharge and maturity to slow this weekend with the cool temperatures predicted. Continue to monitor the weather to assess if and when to make applications to protect growing tissue from scab infections. In areas with predicted prolonged periods of wet weather, consider using a tank-mix of a fungicide with systemic activity plus an EBDC.

RIMpro-Venturia Outputs

• Bear Lake (biofix May 1)
• Benzonia (biofix May 1)  
• East Leland (biofix May 1)  
• Eastport (biofix May 2)  
• Elk Rapids (biofix May 1)  
• Kewadin (biofix May 1)  
• Northport (biofix May 5)  
• NWMHRC (biofix May 1)  
• Old Mission (biofix May 1)  
• Williamsburg (biofix May 1)  

Black stem borer are tiny (less than 2 millimeters), boring beetles that can attack all tree fruits grown in Michigan as well as ornamental and forest plant species. These beetles typically only infest small diameter trees (less than 4 inches) and while they can infest trees that appear healthy, the beetles are attracted to scent cues (i.e., ethanol) produced by stressed trees. In northwest Michigan, we have observed black stem borers infesting young/dwarf apple trees in recent years; trees that are infested often appear unhealthy, but if populations are high enough, they can attack healthy trees.

The best option to mitigate black stem borer infestation is to identify and remedy tree stress. In some cases, growers may want to treat trees for black stem borer, which can be difficult because the beetles spend a majority of their life cycle inside of the tree. Historically, the beetles have been reported to begin emerging from trunks at around 100 GDD base 50 starting from Jan. 1, which often aligns with forsythia bloom. If you are planning to take action against this pest, this emergence timeframe is the best chance to target black stem borer.

We are trapping for black stem borer and we have not caught any flying beetles as of yet; these beetles were found in traps in southwest Michigan. We anticipate flight to begin with the warm temperatures, and forsythia is starting to bloom.

In tart cherries at the station, Montmorency was at early bud burst and bract leaves were visible on Balatons on Monday, May 7. Remember, bract leaves are susceptible to cherry leaf spot infection, and if these leaves are present, they should be protected prior to the coming rain. If these leaves become infected early and the fungus gets a foothold, battling leaf spot for the remainder of the season will be a challenge. Last season was a tough leaf spot year, and some orchards defoliated early. We have observed young orchards with substantial tree losses this season, and we suspect that early defoliation played a significant role in reduced winter hardiness resulting in tree death this spring.

Sweet cherries buds developed quickly last week making delayed-dormant applications for San Jose scale a challenge for some growers. Growers had to weigh their options to determine if the possibility of some phytotoxicity with specific chemistries was worth the risk to take advantage of the first opportunity to manage this pest. The next opportunity will be to target crawlers.

Based on GDD accumulation estimates, we could be in cherry bloom next week. The forecast is for warm temperatures in the upper 60s and 70s with slight chances of rain. If we receive wet weather in these warm temperatures, these conditions will be ideal for American brown rot. Last season was very challenging for brown rot near harvest time and some orchards have a substantial inoculum load going into bloom.

The fungicide Rovral continues to be the best option for American brown rot during bloom, both for resistance management and also because new data suggests that Indar is not as effective as it used to be even at higher rates allowed by the special use label.

Grape crop report

Across the Leelanau and Old Mission regions, and as of May 8, most basal buds of V. vinifera cultivars are proceeding through the dormant stage with few (1-20 percent) apical buds in early bud swell stage. Generally, bud burst stage for V. vinifera is expected to occur May 16-22.

During the week of April 30 to May 4, American and European interspecific hybrid cultivars including La Crescent and Frontenac gris were at 25-75 percent early bud swell stage, and as of May 8 the majority of hybrids were at 75-100 percent early bud swell stage and, in response to recent warm days, are proceeding into mid- to late bud swell stage quickly. Generally, bud burst is expected to occur May 12-18, slightly earlier than V. vinifera cultivars.

Please review the grapevine stages of development from dormant bud to harvest stages to ensure consistent monitoring and recording of your grapevine development, and to help prepare your vineyard operations of the coming growing season that are based on the grapevine developmental stages.