Northwest Michigan fruit update – May 2, 2017

Development has slowed following the cooler temperatures during the last few days.

Weather report

As is typical in Michigan, we had warm, summer-like temperatures at the start of last week, but those conditions have been replaced with cool and wet ones. Last Tuesday, April 25, we topped out at almost 75 degrees Fahrenheit. We hit a low during the overnight of April 29 at 31.9 F. Cool and wet conditions are predicted for the remainder of the week, though the anticipated daytime high is 60 F on Thursday, May 4.

There is also rain in the forecast for the next few days. We received just over 0.75 inch of rain April 30 – May 1 at the Northwest Michigan Horticulture Research Center (NWMHRC), and we recorded additional rain for the overnight of May 1 into May 2 (just under 0.25 inch). Rain is predicted to continue throughout the day. We are still tracking about “normal” compared with our average; we have accumulated 254 growing degree-days (GDD) base 42 and 104 GDD base 50.

GDD accumulations as of May 1, 2017, at the NWMHRC

Year

2017

2016

2015

2014

2013

2012

27 Yr. Avg.

GDD42

254

216

170

107

150

434

223.1

GDD50

104

81

58

217

40

217

93.8

Growth stages as of May 1, 2017

  • Bartlett Pear – Green cluster
  • Potomac Pear – Green cluster
  • McIntosh – Tight cluster
  • Gala – Tight cluster
  • Red Delicious – Tight cluster
  • HoneyCrisp – Tight cluster
  • Montmorency – Bud burst
  • Balaton – Bud burst
  • Hedelfingen – 40 percent bloom
  • Gold – Early white bud
  • Napoleon – 70 percent bloom
  • Riesling – Early bud swell

Crop report

The crop developed more rapidly than expected last week, likely due to two nights that remained quite warm. Apples moved from half-inch green to tight cluster in the past week. Cherries also moved along in development, where sweet cherries were at early bud burst last week and are now showing early white bud to open blooms, depending on variety. With the recent cold temperatures, development has come to a halt in the last few days.

Planting is ongoing across the region, but the wet weather may slow down progress this week. Growers are also waiting for a warm, dry window for pruning sweet cherries, and some Promalin went on in sweets during the warm spell last week.

Pest report

With the recent rain over the weekend, we found a low level of apple scab spores that were discharged on Sunday into Monday morning, May 1. During the primary scab season, a wetting period begins when measurable precipitation is recorded, and this infection period lasts until the relative humidity drops below 90 percent and leaf wetness proportion for the hour is less than 0.25.

With the rains on Monday and again on Tuesday, we had a continuous wetting period that resulted in a low apple scab infection. On Monday afternoon, the model showed we were at 92 percent toward infection, but the model was still not predicting an infection period. The cool temperatures slowed the development of the scab pathogen, but the extended wetting period triggered an infection period by today, May 2 (see table). Growers should be prepared to cover green tissue when these rains move out.

Apple scab infection periods at the NWMHRC via MSU Enviroweather

Wet period

Station

Start of wetting period

End of wetting period

Duration (Hrs.)

Avg. temp (F)

Rainfall (in.)

Apple scab (leaf)

Wet hrs @ avg temp for first infection

Progress toward infection

1*

Traverse City (NWMHRS)

4/15 5-6AM

4/17 7-8AM

Wet: 35
Span: 51

48.6

0.72

Heavy (Symptoms appear: 5/6)

15

231%

2*

Traverse City (NWMHRS)

4/20 3-4AM

4/21 9-10AM

Wet: 31
Span: 31

40

0.7

Light (Symptoms appear: 5/13)

29

104%

3

Traverse City (NWMHRS)

4/27 2-3AM

4/27 8-9AM

Wet: 7
Span: 7

58.1

0.15

None

10

69%

4

Traverse City (NWMHRS)

4/28 5-6AM

4/28 7-8AM

Wet: 3
Span: 3

36.8

0.01

None

48

7%

5*

Traverse City (NWMHRS)

4/30 4-5AM

Ongoing. Last hour with moisture: 5/1 7-8PM

Wet: 34
Span: 40

39.6

0.77

Light (Symptoms appear: 5/26)

33

107%

*Rows with red font (1, 2 and 5) indicate infection has taken place.

We have some sweet cherry bloom open across the region. The cool temperatures are not as conducive for American brown rot development as warm and wet weather; the wetting necessary for blossom infection is about 18 hours at 50 F. At this time, the conditions are not optimal for American brown rot development. If growers had a high level of inoculum last season, which was not the case in most northwest Michigan orchards, a spray may be warranted.

Sterol inhibitor fungicides are labeled for American brown rot control at the blossom timing, but Rovral is the best material for American brown rot at bloom. Additionally, we have documented that the American brown rot pathogen has reduced sensitivity to the sterol inhibitor fungicides, so growers should only apply a sterol inhibitor fungicide at blossom time under these cool conditions if they had American brown rot infections last season. Growers should maximize their use of Rovral by applying it when trees have substantial bloom, as Rovral is both an effective American brown rot fungicide as well as a material that is excellent for resistance management.

European brown rot has not been detected in sweet cherry orchards, but open tart cherries are susceptible to European brown rot at this time. Growers with open tart cherry blossoms in more southerly areas of the state have been protecting against European brown rot—these cold and wet temperatures are ideal for European brown rot infection. Michigan State University Extension suggests applying the first European brown rot application at the popcorn stage.

Although it was warm later last week, insect flight has slowed with the cooler temperatures. We didn’t catch any oriental fruit moths or American plum borers in our traps. We only trapped three green fruitworm moths this week.

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