MSU Extension Fruit & Nuts

Weather report

The weather over the past week has been much more typical than the heat wave we experienced at the end of May. Daytime temperatures have been in the 70s and nighttime temperatures dip down into the 40-50s. Most days have been sunny although the past few mornings have started out with cloud cover. Conditions continue to be windy, which has made spraying more difficult. We also have had no rain in the past week, and soils are dry.

There is little rain in the forecast, and we expect soils to dry out more over the coming week. Thus far, we have accumulated 861 growing degree-days (GDD) base 42 and 505 GDD base 50. These accumulations are on average compared to past seasons.

Growth stages at the Northwest Michigan Horticultural Research Center as of June 11, 2018

• Bartlett Pear – 15 millimeter fruit
• Potomac Pear – 18 millimeter fruit
• Mac – 20 millimeter fruit
• Gala – 13 millimeter fruit
• Red Delicious – 15 millimeter fruit
• HoneyCrisp – 15 millimeter fruit
• Montmorency – 12 millimeter fruit
• Balaton – 12 millimeter fruit
• Hedelfingen – 11 millimeter fruit
• Gold – 11 millimeter fruit
• Napolean – 11 millimeter fruit
• Riesling – 10-16 inch shoots

Crop report

Fruit is continuing to size and some early varieties of sweet cherries are starting to lose green color. The sweet cherry crop looks good in most blocks around the region. The tart cherry crop size seems to be shrinking compared to last week’s estimate. Many growers are estimating the tart crop to be about 60 percent of a crop. Last week, the following numbers were estimated for the other regions in Michigan and around the country (all numbers are in millions of pounds):

• Southwest Michigan: 25
• West central Michigan: 60
• Utah: 40
• Wisconsin: 10
• Washington: 27
• Oregon: 3.0

We have heard various estimates for northwest Michigan, and some growers are estimating 130 million pounds, but we have heard even higher amounts such as 180 million pounds. However, most growers are estimating a smaller crop than the numbers that were shared at meetings last week. We will keep growers posted on the crop sizes as the season progresses.

The apple crop is also sizing and fruit at the station is 13-20 millimeters depending on variety. For growers that are still thinning, the window may be closing in the next few days as the fruit will be too big to thin. The optimal fruit size for thinning is 10 millimeters and many orchards are well beyond this size at this time. The carbohydrate model (Table 2) is also predicting thinning conditions to be more difficult, and it recommends increasing the chemical thinner rate by 30 percent.

The model is predicting little to no stress, and the four-day average balance for tree carbohydrate is 30-40g/day. Therefore, consider an aggressive thinning program if you have yet to thin apples. Fruit size and the lack of stress will make thinning more difficult. Aggressive thinning combinations include Sevin + NAA or Sevin + 6BA. Adjuvants will also increase the potential to thin the crop.

Pest report

Pest and disease activity seems to have slowed in the last week during more seasonal temperatures and drier conditions. This weather was welcomed, particularly for growers who took advantage of the warm and dry weather to apply plant growth regulators for apple thinning or bud conversion in cherries. The forecast for the coming week suggests that conditions will remain mostly dry with the highest probability of rain on Wednesday, June 13.

In apples, primary apple scab is ongoing in our region; this is also the case for the Ridge area. Most of the region has not received rainfall since early last week, and hence, spore discharge has not occurred in these areas. Some areas in Benzie and Manistee counties received a little rain late last week, and although spores likely discharged during this event, conditions dried quickly and an infection period was not recorded on Enviroweather.

The RIMpro models (below) show little scab activity as a result of few substantial rain events. However, continue to stay covered for primary scab if/when wet weather is a threat in the coming week. With the potential for a drier week ahead of us, be mindful of powdery mildew as this disease requires very little moisture. Although early management is paramount for successful powdery mildew control, this disease could still get a foothold as apple terminals are actively growing.

• Bear Lake (Biofix 1 May)
• Benzonia (Biofix 1 May)  
• East Leland (Biofix 1 May)
• Eastport (Biofix 2 May) 
• Elk Rapids (Biofix 1 May) 
• Kewadin (Biofix 1 May)
• Northport (Biofix 5 May) 
• NWMHRC (Biofix 1 May) 
• Old Mission (Biofix 1 May) 
• Williamsburg (Biofix 1 May)

Flagging terminals and ooze symptoms caused by fire blight infection have just started to show up in a few orchards. If an orchard has substantial fire blight symptoms and ooze, slow the spread of this disease until trees reach terminal bud set and growth is inhibited. Copper sprays can kill the bacteria, but there is also the potential for fruit russeting and phytotoxicity on foliage, particularly if it is applied in high temperatures in the 80s and high humidity. If infected shoots are removed, pruning wounds will be vulnerable to infection from bacteria that are spread by rain. Contact us if you find fire blight or scab—we are working with George Sundin’s lab to screen fire blight and apple scab sensitivity this season.

Disease pressure in cherries remains low at this time. We have received a few reports of cherry leaf spot lesions showing up on older leaves, but overall incidence is low thus far this season. Previous data have shown that a season-long protectant Captan program has been an effective leaf spot management strategy. However, we caution growers that Captan alone will not provide powdery mildew control, if this disease is a concern. As mentioned in last week’s report, previous research has shown that an efficacious mildew fungicide applied at first cover timing can provide effective seasonal mildew control in cherries. Lastly, like in apples, drier conditions will favor mildew development.

Most apple pests have been relatively quiet in the last week. Since May 28, the Northwest Michigan Horticulture Research Center codling moth biofix, we have accumulated about 222 GDD base 50; egg hatch is estimated to begin 250 GDD base 50 from biofix (i.e., roughly Wednesday or Thursday, June 13 14). The first flight of codling moth is likely ending but ongoing at the station—we found an average of one moth per trap this week (Table 3).

San Jose scale male flight activity also decreased in apples and sweet cherries at the station this week (less than one male per trap in apples and 10 males in cherries); peak flight occurred last week (Table 3). We have not observed crawler activity. Based on degree-day accumulations, crawler activity should begin this week as we reached 500 GDD base 51 Monday, June 11.

Plum curculio activity has slowed and evening/dusk temperatures have been cooler lately. We have not observed new oviposition scars since last week. If evening temperatures warm into the 60s, there could be an uptick in plum curculio activity.

We received a report of rose chafer activity on Friday, June 8. These skeletonizing beetles have an aggregation pheromone and as a result, some orchards often have a hot spot area with high numbers of beetles. While several insecticides are available for rose chafer management, control can be difficult as this pest can quickly re-infest an area.

Drier weather has also been conducive for twospotted spider mites. We have observed twospotted spider mite adults, juveniles and eggs stages in tart cherry at the station. Mite populations appear to be higher on leaves in the interior of the canopy. Many growers have been mowing and managing weeds recently and orchard floor management often correlates with the movement of mites from ground cover into tree canopies. Additionally, some insecticides can also contribute to mite flaring.

The first detection of spotted wing Drosophila in northwest Michigan was Wednesday, June 6. This first fly was a male and it was detected in a trap on the orchard edge of a tart cherry block on Old Mission Peninsula. Spotted wing Drosophila management programs should begin when fruit begin turning straw color, and most cherries are still green at this time.

Potato leafhoppers have not been reported and we have not detected obliquebanded leafroller adults or cherry fruit fly in our traps at the station.