MSU Extension Fruit & Nuts

Weather

The first half of June has come with more cool weather and rain, and the remainder of June is currently forecasted to be below average temperatures. Growing degree day (GDD) accumulations continue to lag behind five-year averages, and at this point are unlikely to catch up to normal this season. The slow accumulation of growing degree days means we will see a late harvest for sweet and tart cherries, and if we don’t see a heat wave to catch us back up, we will likely be looking at later harvest dates for peaches and apples as well.

Crops

Apples are sizing rapidly and are currently in the 6-10-millimeter range at most locations. June drop is ongoing, and some growers are reporting orchards are thinning heavily, even in spots they have not applied thinners. We are in the “sweet spot” for thinning in much of the region right now. Growers at earlier sites applied standard rates of thinner in the middle of last week during a nice span of carbohydrate stress, but we have been heading in to a trend of low carbohydrate stress in the first half of this week, so growers looking to thin this week are either waiting until near the end of the week when carbohydrate stress increases on trees or are increasing their rates by 20-30%.

Fruit set is highly variable by variety and orchard this season. Poor return bloom in Honeycrisp has resulted in many growers choosing to skip thinners for this variety altogether. Poor pollination weather during early king bloom has also resulted in some orchards setting few (less than 50%) king blossoms.

Fire blight EIPs rose to high risk levels (greater than 100) over the weekend, so growers with bloom left needed to cover orchards. Most growers were well past bloom when these conditions arrived, but growers with significant rag-tag bloom or late sites needed to be on the alert. MaryBlyt indicates that cold weather moving in today, June 11, and for the rest of the week will push EIPs well below the range where we would consider even light to moderate risk by Thursday, and it is highly unlikely we will have any bloom left even at the latest sites by early next week when it finally warms up again, so this is likely going to be the last span of blossom blight risk this season.

No symptoms of shoot or blossom blight have been detected yet, but nectria twig blight has shown up in the past week.

Primary apple scab is very nearly over in the region—local spore rods have did not catch any ascospores with the most recent round of rains. RIMpro indicates there will likely be one final release of a few ascospores with the next rain, but most growers covered ahead of the Sunday rain system with high quality scab material and should still have good residue left for rain forecasted to arrive in the middle of this week, so the cover that goes out after this next round of rains will see many growers transitioning over to summer fungicides throughout the region.

Tart cherries are out of the shuck and sizing rapidly. Fruit set is extremely poor at many sites this season, and a large number of orchards are looking light at this time. Frequent periods of extended wetting have triggered several cherry leaf spot infections in the past two weeks, and more is likely on the way with rains forecasted to arrive as this week goes on.

Growers looking at a first cover spray this week are reminded that this is the time to target powdery mildew; SDHIs and strobilurins are good materials for this purpose and provide good leaf spot management. Syllit does not have good powdery mildew activity, so avoid it at first cover timing. Copper also does not have good powdery mildew activity and should not be applied at the acidic tank mix pH’s that sprays for plum curculio will mandate. Coppers in west central Michigan are best saved for second or third cover timing. Captan also does not contribute much to a powdery mildew management plan, so mix it with either an SDHI or a strobilurin to get good control of both powdery mildew and cherry leaf spot.

Many growers looking to cut costs due to concerns over pricing and a home for the crop will be considering whether a mildewcide is worth it this season. If you do decide to skip a mildewcide, it should not be on young orchards. Powdery mildew on young trees can be a major setback to the growth and development of a tart cherry orchard.

Sweet cherries are out of the shuck and sizing rapidly. The crop looks heavy at most locations. A few orchards have reported some frozen blossoms/fruit from cold nights in May, but this is generally minimal in the area.

Peaches are out of the shuck. The crop looks generally good this season and growers should expect a full crop that will require some thinning. Peach leaf curl is causing heavy damage on unsprayed trees in the area, but orchards that were treated this season generally look very good. We had several significant infection events for this disease this year, which does not always happen—peach leaf curl has a narrow set of parameters for successful infection. This is definitely a year where we can directly see why those early-season peach leaf curl sprays are so critical.

Insects

Codling moth flight of the first generation is ongoing, with a regional biofix estimated for June 1. This is a particularly important year to be setting your own biofix for individual blocks based on trap catches, the intermittent cold weather we have been seeing means that catch is highly variable from block to block this year and should not be estimated by region when possible. Expect cold weather at dusk this week to slow down codling moth activity, this pest favors activity when temperatures at dusk are 60 F or higher and we will not be seeing that for five to six days. The first treatment for early egg hatch is recommended at about 250 GDD base 50 from biofix for an orchard.

Oriental fruit moth flight of the first generation is ongoing, with an estimated regional biofix of May 20. The first treatment for oriental fruit moth is recommended to be timed around 200 GDD base 45 from the biofix date you set for an orchard.

Plum curculio was highly active late last week as we finally had some warm days above 70 F and night temperatures of 60 F and higher. Expect plum curculio activity to drop off this week as the weather gets cold, but it is highly likely we will see another wave of plum curculio next week when the weather warms up again. The late start to spring and the intermittent cold weather likely means that plum curculio will be active for longer than its traditional period of activity this year. All orchards should have had a cover out for plum curculio last week, and another will likely need to be timed prior to the next warmup, probably sometime early next week.

Obliquebanded leafroller overwintering larvae will likely be pupating in the next couple of weeks. The first flight is predicted to begin at about 900 GDD base 42 from March 1.

Spotted wing Drosophila has been captured in south and northwest Michigan. Traps set last week in west central Michigan will be counted and reported by the end of the day today. Sweet and tart cherries are not susceptible to spotted wing Drosophila yet, so no applications should be considered even where spotted wing Drosophila is caught this week.

Various aphids were highly active last week with the warm weather and were particularly visible on young apple orchards where no insecticide was applied at pink. Activity should slow down somewhat this week with the cold.

Lesser peach tree borer is in flight right now and greater peach tree borer is just beginning its flight. Trunk applications of Lorsban can go out any time.

Black stem borer damage has been observed in several orchards over the past two weeks. Damage is limited mostly to orchards neighboring wood lots and is particularly obvious on trees that had other issues prior to 2019 (winter damage, fire blight, etc.). It is too late to do anything about this pest once the characteristic sawdust toothpicks and bore holes are visible, but trees that are heavily damaged should be removed and destroyed and areas of blocks that are high pressure areas should be considered for treatment next season.

San Jose scale male flight has started, so crawlers will likely be targeted in west central Michigan in late June and early July.