Northwest Michigan fruit update – June 25, 2019

The weather is more summer-like, so fruit is sizing and sweet cherries are starting to color. Some diseases, like fire blight and cherry leaf spot, are a major concern at this time.

Weather report

The region finally had some nice, summer-like weather over the weekend. Daytime temperatures were in the high 70s and even into the low 80s. Overnight temperatures are still down in the low 50s. Conditions were fairly dry last week, and rain moved back into the region on Monday, June 24. The Traverse City (NWMHRC) Enviroweather station reported just under a half inch of rain. We have accumulated 883 GDD base 42 and 452 GDD base 50. We are still considerably behind our averages; our 30+-year averages are 1183 GDD base 42 and 681 GDD base 50. We do not anticipate making up growing degree days for the remainder of the season. We anticipate harvest to be quite stretched out this season.

Crop report

The crop continues to size, particularly with all the rainfall this season. Sweet cherries are also starting to color, and many varieties at the Northwest Michigan Horticulture Research Center are at straw color. Some varieties are starting to take on a blush. Cherries have not increased in size much since we measured last week.

Apples have gained 8 to 10 millimeters since last week, and the apples at the Northwest Michigan Horticulture Research Center are out of the optimal thinning window at this time. However, growers are still thinning north of the station. Honeycrisp apples do not appear to be thinning as well as we anticipated, and we will likely be doing hand-thinning in all blocks of Honeycrisp at the Northwest Michigan Horticulture Research Center. Gala, on the other hand, seemed to thin much easier this season, and in fact, one block of Gala may have been overthinned here at the station. The carbohydrate model is showing that we have some deficit in carbohydrates, and is calling for standard rates of thinners.

The sweet cherry crop looks good. There is still some fruit that were not pollinated that are dropping, but the overall crop appears to be nice. The tart cherry crop is variable in some blocks, but the crop looks to be slightly less than the 2018 crop. In talking with regional growers, one unknown at this time is the number of acres that have been removed and the number of acres that are coming into bearing. A sizeable number of acres of tart cherries have been taken out in the last year, and this removal will impact the carrying capacity of the region’s tart cherry crop. Additionally, we have considerably more moisture this year than last year, and the amount of rain/water will influence the size of the fruit, which will ultimately impact the overall crop size. Orchards that were set heavy last year have a very light crop this season. Crop estimates are in the 100-130 million pound range for northwest Michigan.

It is still too early to determine the size of the apple crop as some growers are still thinning. Strawberry harvest is expected to begin next week.

Pest report

It has been a long primary apple scab season in this year’s generally cool conditions. In other areas of the state, the end of primary has been called. For the northwest region, Enviroweather is still showing that there is the potential for spores to discharge in upcoming rains this week. RIMpro also suggests that very few spores are remaining in the Benzonia and Williamsburg area following rainfall on Monday, June 24, and a slightly higher number of spores remain in East Leland. Due to historically low scab pressure in nearby blocks, we are not actively monitoring spore discharge this season. Hence, we are relying on the model predictions to help us determine when all primary spores have discharged.

Our colleagues, Amy Irish-Brown and Dave Jones, are also using RIMpro in addition to monitoring for spore discharge. According to their observations, the RIMpro spore discharge predictions have corresponded well with the number of spores on monitoring equipment this season. Therefore, we will wait until RIMpro suggests that all remaining primary spores are discharged before we call the end of primary this season. We have not found secondary scab lesions at the station, but we have observed and received reports of infections on susceptible varieties in the region. Scout carefully for signs of scab that could show up in warmer conditions this week from a few long infection periods that occurred over the last two weeks.

Fire blight infections began showing up over the weekend. If fire blight blossom blight occurs, symptoms are estimated to show up approximately 103 GDD base 55 following infection. According to MSU’s Enviroweather, the few blossom blight infections on ragtag bloom that we are observing at the station could be from an infection that occurred on June 9-10 when EIP values were over threshold (EIP>70) and we received rainfall. However, we did make a bactericide application on the morning of June 9.

We are also observing a considerable amount of shoot blight at the station and have had reports of shoot blight across the region. Blocks that have not been treated with an Apogee/Kudos program this season seem to be especially impacted by shoot blight. There has been tremendous tender new growth in some blocks and this growth can be very susceptible to fire blight infection. Discerning when shoot blight occurred is not as simple this season as we have not had serious weather events like hail. One possibility is that the shoot blight we are observing are a result of trauma blight that could have occurred during one of the few windy days over the one or two weeks. At the time, however, those conditions did not seem overly alarming for trauma blight. Another scenario is that the bacteria entered through natural openings such as micro fissures in new, tender growing points on the terminal ends of shoots.

The rainfall and windy conditions that we received Monday, June 24 were troubling as this weather spread the ooze and bacteria produced from the infected tissue to other tissues. We have also received reports of fire blight symptoms in commercial orchards. Where fruit finish is not a concern, some growers have applied copper to reduce the fire blight bacteria population in the orchard. Growers are also using Apogee/Kudos programs to prevent the further spread of the bacteria from the infected branch to the tree trunk. Cutting infected shoots out should be done soon if growers decide to use this strategy. It is best to cut during dry conditions and place cut shoots in the orchard row middle to dry out. If growers with fire blight would like Michigan State University to test fire blight infected trees for bactericide resistance, please contact the Northwest Michigan Horticultural Research Center at 231-946-1510.

We are also observing clusters and shoots impacted by nectria twig blight, a fungal pathogen that manifests with symptoms similar to fire blight (i.e. collapsed flower clusters and flagging terminal ends). Shoots infected with nectria are girdled at the base and the branch collapses from the base toward the tip. This disease is unlike fire blight, which collapses from the terminal end of the shoot downward. Nectria does not produce ooze like fire blight, but instead bright orange or pink fungal bodies appear in mid-summer at the base of the infected branch.

Lastly, nectria is often associated with a twig that was winterkilled or a fruit stem from the previous season. This fungus is widespread this season, but it is considered a minor disease and pesticides are not suggested for management. Infected or dead branches can be pruned out when trees are dormant.

Since our codling moth biofix of June 8, we have accumulated 173 GDD base 50 and we are predicted to reach 250 GDD by Thursday, June 27 or Friday, June 28 this week. This timing, 250 GDD base 50 after biofix, is the suggested treatment timing in blocks that are over threshold. In blocks without mating disruption, the treatment threshold is a cumulative catch of five to seven moths in one trap in a block. In blocks with mating disruption, the threshold is much lower – one moth in a trap. Additionally, we observed an uptick in codling moth activity at the station and moths were likely active in warm evenings over the weekend.

Mite activity continues to be quiet and rainy weather has likely helped to slow mites. Other sucking pests like rosy apple aphids and leafhoppers populations continue to rise at the station.

This season has continued to be a challenge for cherry leaf spot. While we were fortunate to have a stretch of (mostly) rain-free days, much of the region experienced a good amount of rainfall on Monday that likely washed off fungicides. We are continuing to find conidia in lesions at the station, but we are not observing any new lesions on new leaves at this time. Preventative control measures are critical to effectively manage this disease and protecting new tissue from infection will be especially important to minimize the amount of defoliation in blocks that have moderate or high leaf spot incidence already this season.

Signs of bacterial canker have also continued this week. MSU tree fruit plant pathologist George Sundin’s lab has started investigating alternative strategies for bacterial canker management. In an effort to help the Sundin lab collect bacterial canker isolates, we are asking commercial growers with substantial canker infection showing up on fruit to contact the Northwest Michigan Horticulture Research Center. Cherries are still pretty green, and we have not observed brown rot getting a significant foothold in fruit at this time.

Spotted wing Drosophila (SWD) trap numbers continue to be low across the region. We have not found SWD at the Northwest Michigan Horticulture Research Center so far this week and regional on-farm traps will be monitored later today and throughout the week. We suggest that growers wait to begin management programs until SWD detections become more consistent across the region and fruit are at susceptible stages.

Plum curculio activity is ongoing, and we are finding small larvae in fruit that were infested earlier this season.

San Jose scale male numbers on traps have continued to decline this week. We have not observed crawlers at this time, but females are large under the protective scale. Peak crawler emergence (~600 GDD base 51) is estimated to occur in early July.

American plum borer activity has continued to decline, lesser peachtree borers peaked last week, and we have not detected greater peachtree borers at this time.

It is still a bit too early to begin catching obliquebanded leafroller moths, but traps have been deployed at the station.  

With a slower start to SWD populations, monitoring for cherry fruit fly could be an important strategy this season. In previous seasons, we have been hard-pressed to catch cherry fruit flies due to overlapping SWD management programs when cherry fruit flies would normally be active.

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