Grand Rapids area tree fruit update – June 18, 2019
June drop in apples should appear with warmer weather.
Very little June drop of apples has occurred yet due to cooler than normal weather. Most growers are cautiously optimistic that their crop is setting up quite nicely. All tree fruits are sizing nicely.
Growing degree day (GDD) totals for the general west Michigan area are about five or six days behind normal averages through May 27. From Jan. 1 through June 17, the Michigan State University Sparta Enviroweather station has accumulated 970 GDD base 42 and 510 GDD base 50.
Tree fruit diseases
Primary apple scab spores were caught in very low numbers with rains on Friday, June 14, so primary scab is still on for 2019. We are all anxious for it to be over, but we did start the season about a week or so late, so expect scab to run a bit longer this year as well. There is some primary scab showing up in commercial apple blocks, so take the time to carefully scout before you back off on fungicide rates just yet.
The MaryBlyt model indicates that any blossom blight or trauma blight should now be visible in susceptible blocks. Now that bloom is over, the possibility of fire blight blossom blight infections is no more, but shoot blight and trauma blight are still in the realm of possibilities. Hail or high winds can get trauma blight started if leaves and shoots are still tender—we usually need to get to mid-July and terminal bud set before the risk for trauma blight is reduced.
Tree fruit insects
Plum curculio egglaying scars are lighter than most years. Cool nights have slowed their activity. They prefer nighttime temperatures over 60 F for best activity. We could get another week of last ditch activity along orchard borders if we get some warm nights.
There is quite a bit of variability in codling moth activity this season. Adult males have been flying in high pressure blocks for several weeks, while some low pressure blocks are just beginning to catch a few. I set a regional biofix for moderate pressure blocks for May 25 (237 GDD50), and there have been 289 GDD50 accumulated since then. High pressure blocks should have begun management sprays in the last few days. Low pressure blocks can delay until 350 GDD50 post biofix or by the weekend. Expect trap numbers to increase if we ever get some warm evenings.
Most obliquebanded leafroller larvae are now pupating. Adult flight has just started and a preliminary biofix is set for June 17. Traps should be in place now to set the best biofix for individual orchards, especially if numbers have been high in recent past.
Our MSU trapping network for spotted wing Drosophila has caught very low to no adults in traps this past week. As cherries move to straw color or red stages, consider cover sprays as fruit might ripen quickly. Continue to trap for flies in ripening crops, especially cherries. Earlier than normal trap numbers are leading to a concern for strawberries for 2019. Watch preharvest intervals (PHI) as you plan out SWD management.
Warmer weather and rapidly expanding shoot growth will likely be favorable for aphids to get established. Keep an eye on non-bearing trees where routine cover sprays are lower; aphid populations can get out of hand in these blocks quickly. Woolly apple aphids are now present in apple canopies and they are extremely hard to find, even in blocks with known high pressure in fall 2018. Consider managing woolly apple aphids now in blocks with high pressure in 2018.
Overwintering San Jose scale are still present. Adult male flight continues. A regional biofix was set for May 25 (211 GDD51) with 268 GDD51 accumulated since that date. Crawlers emerge around 400 GDD51 after biofix, estimated for June 26 in the Grand Rapids area with normal temperatures.
Oriental fruit moth adult activity has been declining in traps as expected. We are past peak egg hatch. Very little flagging from first generation is being reported. A regional biofix was set for May 17 (288 GDD45), with 511 GDD45 accumulated since that date. Egg hatch for first generation should be ending in a week or so. Trap numbers will start to increase in a week or so as second generation gets going. Change lures soon to be sure to get good trap numbers.
European red mite nymphs and adults can be found, but numbers are low due to cooler temperatures and petal fall applications. New eggs could be found soon. Twospotted spider mites should begin to be found soon. Continue to monitor carefully. Threshold for all mites is 2.5 mites per leaf for June.
Some activity black stem borer adults was reported in the Grand Rapids area over the past several weeks. Higher numbers of winter injured trees could lead to higher infestation of black stem borer; trunk sprays should be going on now if not already. Monitor blocks along wooded areas for infestation.
White apple leafhopper nymph numbers have increased this week, but still quite low overall. There are some hot spots. Continue to monitor and keep an eye on non-bearing trees for leafhoppers and aphids.
Brown marmorated stink bug adults seem to be active sooner this spring and we are catching them in traps, which is unusual. Continue to monitor.
Several species of borers are now active in tree fruit crops. Dogwood borer could begin flying at any time. Greater and lesser peachtree borers and American plum borer have been flying for a few weeks now. Borer sprays are usually timed for peak adult flight. For greater peachtree borer and lesser peachtree borer, it’s usually late June. For dogwood borer, it is usually around July 4.
Plant bugs are active. Very few tarnished plant bug adults can be seen on weeds in the orchard floor or active in tree canopies. No evidence of fruit feeding is being found or reported. Continue to monitor, especially after mowing, which can push them up into the tree canopy. There have been some reports of plant bug damage to apples, likely from campylomma or mullein bug. They favor Golden Delicious, Red Delicious, Gala, Northern Spy, Empire/Spartan, Cortland and Jonagold. Damage from mullein bug is done during bloom and they will now move to being a beneficial and management sprays will be ineffective.