Northwest Michigan fruit update – July 9, 2019

Spotted wing Drosophila numbers are on the rise as cherries are ripening. Early sweet cherry harvest will begin this week.

Figure 1. American brown rot on sweet cherry variety ‘Sam’ at the Northwest Michigan Horticulture Research Center.
American brown rot on sweet cherry variety ‘Sam’ at the Northwest Michigan Horticulture Research Center.

Weather report

Conditions have changed over the past week, and we have had much higher daytime temperatures, with highs in the 70s to low 80s. Nighttime temperatures remain relatively cool: mid 50s to low 60s. The region has also seen relatively high humidity, and we have had a couple of pop up type, variable showers. The last significant rainfall recorded at the Northwest Michigan Horticulture Research Center was on June 28 when we received 0.68 inch of rainfall. However, we recorded minor amounts of rainfall on July 2, 3, 4 and 5: 0.12 inch, 0.01 inch, 0.1 inch and 0.07 inch. These rain events did not bring substantial amounts of precipitation, but enough to provide wetting and infection periods to drive disease development and growth. Humidity levels continue to be high as a result of these random showers.

We have also seen significant amounts of dew and fog in the mornings, which is contributing to disease development. According to Jeff Andresen, we will continue to see hot temperatures with high humidity. We have accumulated 1,290 growing degree day (GDD) base 42 and 747 GDD base 50. We are still over 200 GDD behind our 30-plus-year averages: 1,555.8 GDD base 42 and 945.2 GDD base 50. We recorded 4.76 inches of rainfall at the Northwest Michigan Horticulture Research Center for June, and our 38-year average for June is 3.12 inches.

Crop report

Growers have been challenged to keep fast-growing tissues covered with all of the rain, moisture and humidity, and diseases are the main concern in tree fruits at this time.

We have tremendous shoot growth in both apples and cherries. Keeping this tissue covered has posed challenges this season. Sweet cherries are coloring, and birds are a problem with ripening fruit. Some growers estimate to start brine cherries around July 21 and canners around July 24. Sweet cherries are susceptible to spotted wing Drosophila (SWD) at this time. We have seen trap counts at the Northwest Michigan Horticulture Research Center rise in sweet cherry blocks. We caught SWD in all of our traps that are placed in sweet cherries. Protect sweet cherries from SWD.

Tart cherry harvest is underway in southwest Michigan, and the quality of fruit is looking good. The tart cherry crop is variable in northwest Michigan and some blocks have a bigger crop while others have less. Some growers are concerned about ripening the crop with the severity of leaf spot in some blocks. We have also heard of leaf spot showing up on fruit and concerns if that fruit can be harvested.

Strawberry harvest is underway and quality is excellent.

Disease report

Overall, this season’s wet conditions have put disease pressure at the forefront of management programs. As sweet cherries have ripened, we have been increasingly concerned about the potential for American brown rot to get a foothold. Most fruit with canker damage have dropped from the trees at the station, but there are fruit damaged by birds will be a good host for brown rot. Humid and hot conditions over the last week were especially favorable for brown rot development, and brown rot is not difficult to find in the variety ‘Sam’ in our pathology block at the station (see photo). Furthermore, the brown rot fungus seemed to be growing on fruit that were intact or undamaged prior to becoming infected.

The week ahead is looking drier, which will be helpful for keeping good fungicide coverage as well as to lessen the chance of brown rot spreading and further developing. Unfortunately, materials for excellent brown rot efficacy are limited, and if using the product Indar, there is a Special Local Need (SLN) label to apply Indar at a higher rate, up to 12 fluid ounces per acre. Previous testing has shown that brown rot has reduced sensitivity to Indar, and therefore, it is suggested that growers use the 12 fluid ounces per acre rate to ensure the best possible control of this disease. Keep in mind that under the SLN label, growers cannot apply more than 48 fluid ounces per acre per season which equates to a total of four applications at 12 fluid ounces per acre per season. Growers using an SDHI fungicide for brown rot should use the highest labeled rates and tank mix with a broad spectrum such as Captan.

We have been observing substantial leaf drop in trees with severe cherry leaf spot infections. Some of these blocks also have lesions on stems as well as black spots on the fruit that are a result of leaf spot lesions. While many growers have been doing a good job staying ahead of rainy weather, this has been one of the most challenging leaf spot seasons that we can recall. Since early June, every rain event has resulted in an infection period. The four most recent infection periods: June 24-25, June 28-29, July 2-3 and July 4-6, have each spanned 29-46 hours at the research station with average temperatures in the mid-60s to 70s. In some of these events, rainfall did not accumulate too much, but there was enough moisture and humidity to support disease progress. Growers should keep an eye out for new lesions that could begin showing up this week from late June infections.

In apples, old fire blight strikes that remain on apple trees are evident at the station. There are a few newly infected shoots starting to flag and produce ooze this week. Fortunately, the week ahead is looking drier, which will help slow the spread of ooze containing bacteria to adjacent tissue. We will keep up with our copper program at the station until the trees reach terminal bud set; at harvest, we plan to rate the Gala and Honeycrisp blocks for russeting that could have been caused by these copper applications.

Insect report

Spotted wing Drosophila (SWD) numbers are on the rise. After several weeks of catching very few flies, we found a total of 25 SWD in 40 traps at the station yesterday. This increase in SWD activity has coincided with ripening of several susceptible fruit crops and alternate hosts in the region. Many sweet cherries are vulnerable to egg-laying and need to be protected from this pest. Tart cherries that have reached straw-color or later are also at risk and should be protected from this pest.

We have not detected cherry fruit fly at this time.

The Northwest Michigan Horticulture Research Center set biofix for obliquebanded leafroller for June 30 following two consecutive trap catches on July 1 and July 8. The moths have been active in both apple and cherry blocks at the station. Since biofix, we have accumulated 260 GDD base 42. Egg hatch is estimated to begin at 400 GDD base 42 after biofix and insecticides (depending on the product that is used) target 350-450 GDD base 42 after biofix. Based on the current forecast, we should reach this timing by the end of the week and into early next week.

Catalpa trees are blooming across the region and most areas have reached or surpassed the 600 GDD base 51 mark that indicates peak San Jose scale crawler emergence. There were no detections of male scales on traps at the station this week.

Codling moth flight is ongoing at the station. Since biofix on June 8, we have accumulated about 500 GDD base 50 and we are approaching peak first generation egg hatch at about 550 GDD base 50. Depending on trap catch numbers since the last treatment was made, some orchards may need to take action against this pest this week. At the station, we have had a consistent catch of codling moths since biofix.

Spotted tentiform leafminers, green peach aphids and leafhopper nymphal populations are on the rise in our apple blocks this week. Wooly apple aphids have also been on the move and can be found in cracks on tree trunks and branches.

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