Grand Rapids area tree fruit update – Aug. 20, 2019
Late summer pest considerations.
Rainfall in August has been very good for most areas around Grand Rapids, Michigan. It’s amazing how green everything is staying this summer. There are some areas that are drier, but overall rainfall has been adequate. Peaches are slowly moving through their harvest; most growers are reporting peach maturity is behind by about a week. This will likely hold true as we move through apple harvest and we have confidence in our predicted apple harvest dates for 2019. Your buyers will be clamoring for fruit, but it will be delayed due to the cooler weather back in June.
Growing degree days (GDD) for Jan. 1 through Aug. 19 for the Michigan State University Sparta Enviroweather station are 2,790 GDD base 42 and 1,826 GDD base 50. These totals are lined up with the seasonal average based on over 40 years of weather data. Even though the degree days have caught up to normal from the above normal temperatures in July, our predicted harvest dates will remain the same as they are calculated based on the weather shortly after petal fall, which was cooler than normal. As mentioned above, we have confidence in the predicted apple harvest dates for 2019; they will really be about a week behind.
Tree fruit diseases
Apple scab seemed to slow its spread during the hot and dry July weeks, but now that we are nearing the end of August, leaves and especially fruit will become more susceptible to secondary apple scab. You need to maintain a good fungicide cover to protect fruits from now until harvest if you have scab present. Any rain event lasting longer than six hours will be enough to cause secondary scab infections.
Summer diseases in apples—sooty blotch and flyspeck—should be of concern in areas with heavy rainfalls. You could likely need another fungicide cover prior to the busy time of harvest given all the wet conditions lately. The fungicides you use for summer disease should help with fruit rots as well, which we could see higher pressure for this summer and fall.
Tree fruit insects
Second generation codling moth adults should be at peak flight. Egg hatch should also be well underway and good cover sprays are needed to protect from fruit stings. Using the original Grand Rapids regional biofix of May 25 (237 GDD50), there have been 1,589 GDD base 50 accumulated since then, which indicates egg hatch is underway and will reach a peak in the next week or so.
The summer generation of obliquebanded leafroller adults should be slowing in traps. New small larvae should become easier to find soon with the peak activity timed around Paula Red harvest. I set a regional biofix for June 17 (970 GDD42) and there have been 1,819 GDD base 42 accumulated since that date, which indicates the summer larvae will be hatching and feeding soon.
Apple maggot flight continues in normal numbers, with some hot spots reporting high numbers. The heavy rains of late have spurred on flight. Cover sprays now need to be maintained to prevent egglaying stings in all apples. This is a sporadic pest lately and not all orchards need to cover for it, but you don’t know your population if you aren’t trapping properly for it. Place traps along wooded areas if possible. The best traps are the red spheres with apple essence added.
The expanding shoot growth in June and July was very favorable for green apple aphids—their numbers continue to be very high in some blocks. There are a lot of beneficials present in most aphid populations I see; syrphid fly larvae and lady bug larvae and adults are common. There are also woolly apple aphids present in apple canopies and management should be considered now in blocks with high pressure in 2018. Hopefully, the abundance of beneficials will curb the woollies as well.
The second generation of San Jose scale crawlers are now present and good cover sprays will help reduce them. Excellent spray coverage is essential to manage San Jose scale well, so use more water per acre and slow down. Using the original regional biofix date of May 25 (211 GDD51), we have accumulated 1,505 GG base 51, which indicates second generation crawlers are active, just as we are finding.
Third generation oriental fruit moth egg hatch is just beginning. From the regional biofix date of May 17 (288 GDD45), the Sparta Enviroweather station has accumulated 2,121 GDD base 42. Often, several generations of oriental fruit moth begin to overlap in summer, making it hard to judge coverage needs. If you are catching 30 to 40 moths per week in a peach block, you have high pressure. Traps in apple blocks can tolerate higher levels before fruit damage becomes a concern.
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 15 mites per leaf for August. Typically, we let you off the hook for mites around Aug. 15, as any cost of a spray would not help with fruit size prior to harvest. However, we fully expect our harvest dates to be at least a week behind normal and some mite populations are crazy high. The adults will begin to lay eggs in the calyx end of apples soon and this is definitely something you want to prevent. In very high mite populations, please consider a knock down miticide before you put the sprayer away.
Brown marmorated stink bug damage continues to be reported occasionally in apples, but the big damage in known hot spots has yet to be reported. The middle of August is typically when we hear of adults and nymphs feeding on developing apples, so keep looking.