Grand Rapids area tree fruit update – July 9, 2019
Hot and humid weather brings on the diseases and insects.
As we move into July, typically rainfall is lessened and supplemental irrigation is necessary. This is not true for all tree fruit blocks in 2019 where ample rainfall seems to be the theme. However, there are some sites with lighter soils and less rainfall that do need to start irrigation. This will be especially true when the hot weather moves back in next week. Trees need water to grow, but they also need adequate water to reduce heat stress through the process of evaporative cooling, which will be very important with the forecast for next week. All tree fruits are sizing nicely and tree growth is more than usual given all the rain this spring and summer.
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 July 7, the Michigan State University Sparta Enviroweather station has accumulated 1,555 GDD base 42 and 935 GDD base 50.
The predicted apple harvest dates are now available on MSU Enviroweather for all weather stations in the Grand Rapids, Michigan area. See the Sparta dates.
Tree fruit diseases
Even though primary apple scab has been over for a couple of weeks now, there is still secondary scab to contend with in many blocks. If you have scab showing up, keep fungicide rates up to primary levels and make sure leaves and fruits are adequately covered for any rain event that lasts more than 6 hours. Secondary scab needs less time for infections to start—about 6 hours in summer temperatures. Once terminal buds set and leaves and fruits get a thicker, waxy cuticle, they are somewhat less susceptible. This also typically coincides with the hotter and drier weather of summer, however, 2019 has not been drier yet and all diseases, including apple scab, will continue to infect.
If fire blight got established in your apple blocks, it is best to cut it out if the strikes are relatively few. Sometimes in young trees that means cutting out the entire tree. Be sure you have a plan in place for how you might deal with potential trauma blight situations if we get any high wind or hail events. Leaves and shoots are still very tender—we usually need to get to mid-July and terminal bud set before the risk for trauma blight is reduced. Tree growth might continue later than usually this season with the ample rainfall and moderate temperatures.
Sweet cherry diseases
Bacterial canker is showing up in the most susceptible cultivars like Summit and Benton. There is very little you can do at this time to combat this awful pathogen. It is present on leaves and fruits this year. Injury from canker plus very warm and humid weather plus ripening cherries equals the perfect storm for brown rot. Keep your fungicide program varied for resistance management and keep it tight as rainfall demands.
Brown rot in sweet cherry has been high in the last week in some cultivars and some blocks. The forecast has a long stretch of hot and humid weather coming our way again, so keep your fungicide covers tight for brown rot in any fruits that are nearing harvest and reapply quickly if heavy rains occur. As always, be aware of preharvest intervals.
Summer diseases in apples
Sooty blotch and flyspeck in apples are now in the window where fungicides need to be applied to prevent their appearance. Captan alone will not manage this complex of pathogens and you need to add a sterol inhibitor or a strobilurin fungicide to your program to effectively prevent them.
Tree fruit insects
There continues to be quite a bit of variability in codling moth activity this season, but even the low pressure blocks have been catching a few adult males over the past few weeks. I set a regional biofix for moderate pressure blocks for May 25 (237 GDD50), and there have been 715 GDD base 50 accumulated since then. We should be past the peak egg hatch for first generation and trap number should be declining. Use your trap numbers to help with decision making. For instance, if you had a week with a higher trap count, look out 250 to 350 GDD base 50 from that week and know that there could be a related higher egg hatch in the date range that fits 250 to 350 GDD50. With the odd flight for codling moth due to the cooler weather, you can use your trap numbers to time cover sprays when egg hatch would be more significant.
Obliquebanded leafroller adults continue to fly in normal to less than normal numbers. I set a regional biofix for June 17 (970 GDD42) and there have been 585 GDD base 42 accumulated since that date, which indicates egg hatch should be occurring and is estimated to be in the 50% emerged range. The first small larvae should begin to be found at any time, and we will see them more readily for sure in seven to 10 days. The threshold for management is one larva per tree. There are other worms in apple orchards too—skippers and other leafrollers—that aren’t really problems to deal with, but could be an indication of poor spray coverage which could increase the risk for obliquebanded leafrollers.
Each week, spotted wing Drosophila trap numbers increase a little bit. Now that many early fruit crops are nearly or at harvest, it’s time to step up your spray coverage efforts to combat SWD.
Apple maggots should begin flight at any time. We have crossed the 900 GDD base 50 from Jan. 1 mark, which indicates they are ready to emerge from overwintering sites in the soil. Often, a heavy rain will be the driver to get them to emerge. Red sticky sphere traps are preferred, and apple essence should be used to get an adequate trap catch for decision making.
The expanding shoot growth has been favorable for aphids in the last few weeks. 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 present in apple canopies and management should be considered now in blocks with high pressure in 2018.
San Jose scale crawlers are fairly easy to find in blocks with high pressure. Adult male flight continues but is declining to a near end for first generation. A regional biofix was set for May 25 (211 GDD51) with 675 GDD base 51 accumulated since that date. Apply contact-type sprays to target crawlers now.
Oriental fruit moth adult activity is starting to increase again in traps as second generation begins. A regional biofix was set for May 17 (288 GDD45) with 990 GDD base 42 accumulated since. Egg hatch for second generation is just beginning with cover sprays becoming again necessary in about a week (July 14 or 15) when we reach 8 to 10% egg hatch.
All stages of European red mite can be found, and in some blocks numbers have exploded and bronzing is easily noticeable. Twospotted spider mites are also present. Threshold for all mites is five mites per leaf for July.
Some feeding indication of adult black stem borer can be found. Higher numbers of winter injured trees in the area 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.
Numbers of white apple leafhopper nymphs are building slowly. Continue to monitor and keep an eye on non-bearing trees for leafhoppers and aphids that can limit shoot growth.
Brown marmorated stink bug adults found now are overwintering adults. A few nymphs could begin to be found at any time. This is the time of year we sometimes see some slight feeding damage from overwintering adults as they are moving through an orchard looking for better food sources. It’s most notable on light colored apples such as Golden Delicious. By the time the damage becomes visible, the culprits have long moved on and are no longer in the orchard.
The best management tool at this point is to be actively trapping in blocks known to have higher populations and when nymphs are caught in a few weeks, a spray program can be initiated based on trap numbers. Brown marmorated stink bug population is building in Michigan, but is still on the low side compared to the eastern U.S. where damage is tremendous.
Several species of borers are now active in tree fruit crops. Dogwood borer is now flying in low numbers. Greater and lesser peachtree borers and American plum borer have been flying for several weeks now. Borer sprays are usually timed for peak adult flight. For greater and lesser peachtree borers, it’s usually late June. For dogwood borers, it is usually around July 4.
The very first Japanese beetle activity is being reported in the Grand Rapids area and other areas of Michigan. They will be found in favored species first—sweet cherries, raspberries and my roses. They will move to apples and other tree fruits over the next several weeks.