Grand Rapids area tree fruit update – May 21, 2019

Apples in the Ridge are in full bloom.

Four scabby leaves
The red arrows are pointing to apple scab lesions found on unsprayed trees on May 17, 2019, near Grand Rapids, Michigan. Photo by Amy Irish-Brown, MSU Extension.

Tree fruit development

Tree development continues very slowly. Most apple varieties have bloom open at some level this week. A little bit of petal fall is just beginning on the earliest apple cultivars. The latest varieties are just in king bloom now.

Peach bloom is still present. Pears are in full bloom to petal fall depending on the variety. Sweet cherry bloom is winding down. Tree planting continues.


Degree day totals for the general west Michigan area are about six or seven days behind normal averages through May 20. From Jan. 1 through May 20, the Michigan State University Sparta Enviroweather station has accumulated 447 degree days base 42 and 194 degree days base 50. Forecasted temperatures are going to be much warmer than we’ve had, but still within average range for this time of year, so degree day accumulations will move at a normal pace and tree growth will move at an even pace and not too quickly.

Tree fruit diseases

There were several wetting events over the past week, and they were quite variable depending on which of the 13 weather stations around Grand Rapids, Michigan, you are following. Please be sure you are checking your weather station data on MSU Enviroweather often as the 13 stations in my coverage area have all been a little different. Some very high apple scab spore numbers were collected with rains both Saturday, May 18, and Sunday, May 19. Keep your fungicide covers tight with the rains predicted for this week as we are right in the height of apple scab for 2019. I estimate we are at about 85 to 90% mature primary spores with at least three weeks of primary scab left.

On Friday, May 17, I started to find the very first apple scab lesions on unsprayed trees, right on the mark of when they were predicted to show up from the major infection period that started April 29. The April 29 wetting event was a major infection with over 2 inches of rain over four days. Scout carefully for any possible lesions. I found them mostly on the underside of the second and third true leaf after the spur leaves. It might take a few more days for lesions to show up in managed orchards with fungicides applied. I was also sent some pictures of primary scab lesions on spur leaves and these were a bit more advanced in development and likely from that very first wetting period right as green tip started. So far, lesions are only being found in unsprayed trees.

Typically, during pink to petal fall is when mildewcides need to be added in susceptible apple varieties to prevent initial infections of powdery mildew. The weather this week will be more favorable for powdery mildew and new tender leaf tissue and future apples are exposed.

The MaryBlyt model called for the possibility of fire blight blossom blight infections with the rains May 18 and 19. Temperatures did not get as warm as predicted, so the risk was lower than expected, but susceptible bloom still needed to be protected. The predicted weather will lead to some high risk potential for blossom blight the rest of the week, so watch your models closely.

Please visit MSU Enviroweather and click on the station nearest your orchards for further model details.

Tree fruit insects

With many tree fruits in the bloom stage, no insecticides are used. Continue to monitor insect activity and start to put together your plan for petal fall applications that will target those pests we know will be an issue (plum curculio, obliquebanded leafroller, European red mite, rosy apple aphid) as well as some that occasionally are pests (San Jose scale, oriental fruit moth and tarnished plant bug). Here’s what’s happening right now that you should be monitoring for.

No adult activity of plum curculio in the Grand Rapids area yet; it’s been too cold and fruits are not yet large enough to be targeted. Warmer weather forecasted will be good for egg laying activity of plum curculio. They prefer nighttime temperatures over 60 degrees Fahrenheit, which we might get this week.

Codling moth adult flight has not begun yet in the Grand Rapids area. First catch is usually at 250 to 370 GDD base 50 (average of 300 base 50) or right around petal fall in apples. First flight could be delayed in low population blocks and due to recent cooler night temperatures. Expect some initial flight with warmer weather this week. Temperatures over 50 F at dusk are most favorable. Codling moth mating disruption pheromone and traps with lures should be in place now.

Small to medium sized obliquebanded leafroller larvae are present. Warmer weather this week will get them moving. Scout for their presence and add management sprays at petal fall for blocks with high numbers—one larvae per tree is a threshold. Traps go up in early June (1,000 GDD 42)

Aphids usually do fine in cooler weather, but they have been hard to find this season so far. Warmer weather and rapidly expanding shoot growth will likely be favorable for aphids to get established.

Overwintering San Jose scale are present. Adult male flight should become active in the Grand Rapids area any day. Consider trapping for male flight in high pressure blocks.

Initial activity of Oriental fruit moth adult activity began in the Grand Rapids area last week with very low numbers in traps. A regional biofix was set for May 17 (294 GDD45). We have accumulated 40 GDD base 45 since that date. Ten percent egg hatch should be expected around Memorial Day. This is the first timing in stone fruits for cover sprays to prevent shoot infestation.

Some egg hatch of European red mite is being reported in the area, but activity is very slow with cooler temperatures recently. Continue to monitor and expect warmer weather this week to activate this pest and petal fall sprays should target it.

Adult black stem borer activity should begin this week with warmer weather. Have your traps out now.

Adult redbanded leafroller flight numbers rose a bit with warmer weather last week. First generation adult flight should wind down soon. Larvae could be found at any time, but this is a very minor pest overall and very uncommon to find in managed fruit trees in this area.

First generation spotted tentiform leafminer adult flight rose slightly last week with warmer weather. Egg hatch should be at a peak. No sap feeders were found yet but should be visible soon. This is another minor pest where management is needed only if damage was high the previous growing season.

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