West Michigan tree fruit update – Sept. 4, 2018

Heavy rains reduce drought risk and increase disease risk.

Horticultural concerns

Heavy rainfall has been the story for the past two weeks. Areas in my coverage area received anywhere from about 6 inches to over 11 inches around Grant and Kent City, Michigan. Orchards are slick and muddy and hopefully the rain machine will slow down a bit. There are concerns for an increase in fungal pathogens with all the heavy rain and high relative humidity—see my notes below on diseases.

Harvest of early apples continues but has been slowed with the weather. Fruit should begin to size a bit faster with the rain. At this time, we feel the predicted apple harvest dates are still accurate with the data collected so far, but our early sampling is starting to indicate fruit will potentially be ready maybe two or three days earlier than our predicted dates. Our predicted harvest dates are a general guide to help you plan ahead for harvest and your harvest management plan for aminoethoxyvinilglycine, or AVG (ReTain, 1-MCP, Harvista and Blush).

A note about AVG or ReTain use and tank-mixing: The Valent representatives say to be careful with tank mixes or other applications around ReTain. Avoid copper, iron, calcium and Captan specifically within a week of ReTain treatments. NPK (nitrogen, phosphorus, potassium) foliars are not a problem. With all the rain recently, you might be tempted to go out with many things in the tank at once—please be very careful and follow all label and manufacturing recommendations.

Tree fruit diseases

If you remember back to the fall of 2016, we also had a great deal of rain during just about the same period—actually, in 2018, we have had more. In 2016, from Aug. 1 to Sept. 3, the Sparta Enivoweather station recorded 6.48 inches of rain. In 2018, from Aug.1 to Sept. 3, the same station recorded 10.27 inches—far higher with a trend for more rain in the forecasts.

If you also remember 2016, we experienced a very high level of sooty blotch and fly speck along with very high levels of various fruit rots. Add the higher pressure from apple scab in orchards this year and we could see a significant amount of pinpoint scab on fruit coming out of storage.

We can use the model to add up the hours of wetting since fungicide covers have been reduced to help determine timing of cover sprays for summer diseases. The way the model works is once there has been 2 inches of rain, it starts adding up hours of wetting again and when we get to 200 to 225 hours, it’s time for another fungicide cover. For most blocks, that will mean fungicides specifically for summer diseases are needed in the next week. It would be best to cover all blocks soon, unless they will be harvested in the next two weeks.

For fruit rots and apple scab, maintaining an adequate cover of fungicides is the only model we have. Use the rule of thumb that 2 inches of rain removes all pesticides and re-cover as needed. I know you like to box orchards and put the sprayers away, but this fall is shaping up to be good conditions for diseases, so don’t quit just yet.

Tree fruit insects

Apple maggot activity continues in the Grand Rapids, Michigan, area with numbers from some blocks still being very high. Trap numbers had been declining, but we saw a marked spike in activity with the rains 10 days ago, so they are still a concern, especially now that all your insecticide cover would be gone with the heavy rains. For any blocks where apple maggot is still being caught or known problem blocks, a re-cover should be done to prevent stings in any varieties not harvested before Sept. 10.

Brown marmorated stink bugs can be found on various favored hosts in all stages and adults are much more prevalent than nymphs at this time. Last week, we saw a bit of a spike in nymphs and adults and the damage they have done. This week’s traps are all adults with single digit numbers (one to two 2 per trap).

I’ve been watching soybean development closely this season. Due to the late spring, beans went in on the later side in 2018 and they are still very green, but just in the last few days they are starting to change a bit. Brown marmorated stink bugs will quickly move out of beans at this time and move to other more favorable crops nearby. Given the heavy rain recently and a general lack of pesticides left on fruits and leaves, reapplying insecticides for brown marmorated stink bugs should be considered in blocks with a history of brown marmorated stink bug injury or if you are near soybeans or wooded areas. Pay attention to the pre-harvest intervals of all materials.

From my early biofix for oriental fruit moth on May 10, 2,793 degree-days base 45 (DD45) have been accumulated. This indicates third generation egg hatch is just past its peak, but we need another two weeks to get through the end of oriental fruit moth for 2018. Consider cover sprays in peaches and apples where trap numbers have been high (20 or more in peaches and 50 or more in apples). Oriental fruit moth occasionally causes some late season stings in apples after second generation codling moth has ended and insecticide covers get stretched out late in the season.

Woolly apple aphids have been more prevalent across Michigan in 2018. At this time, management sprays are just revenge sprays and it would be best to let the beneficials move in and help. However, pickers generally don’t like to work in high woolly apple aphid populated areas, so you might need to knock them back. The key to good woolly apple aphid management is to slow down, apply at least 100 gallons of water per acre and get excellent coverage. If you don’t, you will be disappointed in the results. Also, be mindful of pre-harvest intervals this late in the season.

Adult trap numbers of codling moth really dropped in the past 10 days, indicating the end of second generation adult flight. I set a regional second generation biofix for July 20 (1,465 DD50) and there have been 930 DD50 accumulated since that date using Sparta Enviroweather data. This indicates we are very nearly to the end of second generation egg hatch and cover sprays are less critical.

We’ve had a great deal of rain and any insecticide covers are gone. If your trap numbers have been under threshold for over two weeks, you are most likely good for the rest of the season for codling moth. If you had accumulated trap catch over threshold (one for disrupted blocks; five for non-disrupted) within the last 10 days, you could potentially get some late season stings through this week.

Small larvae for the overwintering obliquebanded leafroller generation can be found in high pressure blocks. I set a biofix for the summer flight for Aug. 1 (2,517 DD42) and 972 DD42 have been accumulated since that date, indicating all eggs are hatched. While obliquebanded leafrollers have been low overall in my area, there are some hot spots. This is a good time to make note of where obliquebanded leafroller populations are easier to find, and target those spots next spring. Only blocks with high pressure might need a cover spray right now to knock back larvae to reduce the overall population.

San Jose scale crawlers for second generation started to be found in problem blocks two to three weeks ago, right on time with the model predictions. I set a new regional biofix for July 20 (1,382 DD51) with 885 DD51 accumulated since that date for Sparta. Second generation egg hatch has ended, and crawler activity should be all but over. We are really past any good window for effective management of San Jose scale at this time. This is a good time to scout carefully for problem spots for scale that can be targeted in early 2019.

Japanese beetles are still around and numbers seem to be dropping off quickly. Maybe their eggs and larvae in the soil will drown with all the rain.

We should be at the stage now where European red mite and twospotted spider mites no longer need formal management. The heavy rain should have knocked them back a bit to help.

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