Grand Rapids area tree fruit update - April 16, 2024
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EmailFocus on primary apple scab control this week.
Weather update
This is an exciting week of weather! Rain showers will be starting Tuesday evening, April 16, and continuing through Thursday with a potential for thunderstorms on Wednesday afternoon.
Frost potential this weekend
Cooler temperatures will be following the rain events with the potential for radiative frost events this weekend on Saturday, Sunday and Monday mornings. Lows are predicted to be around 32 degrees Fahrenheit, which should not be damaging in most conditions. Just in case this forecast changes in the next few days, resources are attached below.
- Consider this Michigan State University Extension article on what fruit growers can do if a freeze is coming.
- Double check your growth stages against the critical freezing temperatures. The current stages of interest are in the following table.
|
|
Half-inch Green |
Tight Cluster |
First Pink |
|---|---|---|---|
|
Old Temperature |
22 |
27 |
27 |
|
10% Kill |
23 |
27 |
28 |
|
90% Kill |
15 |
21 |
24 |
- Refer to the Michigan State University (MSU) Enviroweather Temperature Inversion Potential page prior to using frost fans.
For a detailed look at the upcoming weather forecast, Jeff Andresen, MSU agricultural meteorologist, has weekly 15-minute talks posted every Tuesday.
Crop update
Phenology has been quickly developing with the warm weather this past week. Apple buds are currently at tight cluster and moving into first pink in warmer areas. Sweet cherry, peach and plum are currently in bloom in the Grand Rapids, Michigan, area.
The phenology is still one to two weeks ahead of the normal growing season. This can be seen in the comparison between current growing degree day (GDD) accumulations and the five-year average.
|
Enviroweather station |
GDD accumulation since Jan. 1 as of April 15, 2024 |
5-year average GDD |
||
|---|---|---|---|---|
|
GDD42 F |
GDD50 F |
GDD42 F |
GDD50 F |
|
|
Aetna-Fremont |
229 |
86 |
170 |
68.9 |
|
Alpine |
268 |
109 |
200.5 |
83.8 |
|
Belding |
258 |
106 |
192.4 |
79.2 |
|
Conklin |
257 |
102 |
192.4 |
79.2 |
|
Grant |
251 |
100 |
168.8 |
67.9 |
|
Kent City |
247 |
96 |
162.7 |
64.6 |
|
Sparta |
254 |
101 |
170.8 |
68.9 |
|
Sparta-Tower |
257 |
102 |
174.2 |
70 |
|
Sparta-North |
257 |
101 |
189.9 |
78 |
|
Standale |
290 |
123 |
199.5 |
82.8 |
We typically report GDD accumulation starting March 1, but this is not a typical year. We reached the required chilling hours on Jan. 15 to move into ecodormancy and start accumulating GDD. This means the heat we received in February mattered for this year’s crop. As such, I will be reporting GDD from Jan. 1 rather than March 1 this season.
These growing degree day accumulations line up well with their associated anticipated phenology. This information is displayed in the table below.
|
Growth stage |
GDD42 F |
GDD50 F |
|---|---|---|
|
Tight cluster |
242 |
109 |
|
Open cluster |
284 |
129 |
|
Pink |
367 |
174 |
Pest and disease update
Apple scab
The wetting period from 2 p.m. April 11 through 1 p.m. April 12 was considered a light infection risk event in the MSU Enviroweather model. I caught an average of 107 spores per rod following this event.
However, this rain event did not fully discharge the mature ascospores from the leaf litter. With the very warm weather over the weekend, the ascospores have had a chance to recharge prior to this upcoming rain.
Approximately 17% of ascospores in the leaf litter are currently mature according to the Rimpro model. With mature ascospores in the leaf litter, warm temperatures and rain predicted on Tuesday through Thursday this week, prepare for a high infection risk this week.
Keeping a solid fungicide cover on is especially important now as the phenology stages are moving quickly and new unprotected tissue is currently available for infection. I looked at some ascospores from the leaf litter under the microscope and they looked mature and ready to infect. While we know the spores are mature from the models, it is always good to double check.
Powdery mildew
As green tissue starts to come out, so is powdery mildew. The mycelium overwinters in the buds, which can look feathered, shriveled and silvery-white. The dormant fungus will then colonize this first flush of growth. These appear as flag shoots. Powdery mildew will also infect developing blossoms and can cause fruit to abort, distort or russet.
Treatment for powdery mildew should start around 0.5-inch green, which we are quickly coming up on. Keep in mind that while rain benefits other fungal diseases, powdery mildew benefits from dry weather. Be sure to maintain mildew control during dry weather periods too. Plants gain resistance with maturity but will be susceptible to powdery mildew until terminal shoot growth ends.
Early season insects
Keep an eye out for San Jose scale, woolly apple aphid and European red mite this time of year. As the phenology has progressed, we have exited the window for delayed dormant oil applications.
Insects coming soon
Oriental fruit moth, spotted tentiform leaf miner, green fruitworm and redbanded leafroller were caught in southwest Michigan this week. They will likely make an appearance in our area soon. I set out insect traps yesterday, April 15, and will have updates on insect pressure in the Grand Rapids area starting next week.
For more information about regional reports, please visit the Michigan State University Extension website.
