MSU Extension

Weather report

We finally hit a warm and sunny day over the weekend in Northwest Michigan. These conditions were much welcomed after we had snowfall on the morning of Friday, May 16. As we start off the workweek, we are expected to hit the 70 degree Fahrenheit mark again this Monday, May 26.

Although growers are pleased with the warm and dry conditions, it has been windy for the past three days; most growers have been spraying later into the evening when the winds have died down. Saturday morning, May 17, was cold throughout the region, and the lowest temperatures were reported at Michigan State University’s East Leland Enviro-weather station – 28 F. Based on the slow start to our spring, our fruit crops are not very far advanced, and the cold temperatures had less impact than if we had been in full bloom. Many growers ran their frost fans during the night of May 16.

We continue to slowly accumulate growing degree days (GDD), and as of today, May 20, we have accumulated 274 GDD base 42 and 107 GDD base 50. Currently, we are almost two weeks behind a normal year. We have also had substantial rainfall in the past week and we recorded 1.55 inches of rain here at the Northwest Michigan Horticulture and Research Center (NWMHRC). The rain was spread out over the days of May 13-17, and we had the most rainfall on May 13 at 0.54 inches. 

Crop report

Now that some wine grape bud swell is underway on most varieties, dead buds and weak canes are becoming more obvious due to the lack of bud progress. At the NWMHRC vineyard, most of the hybrid and American varieties are showing bud swell to bud burst throughout the vine.

In Vinifera varieties, the only significant bud swell is on parts of the vine that were under snow cover during the winter. Buds in these areas also receive radiant heat from the soil, so they may also be more advanced in development than buds higher on the vine. However, the bud swell or no bud swell dividing line is very clear: at the snow cover line.

There have been no reports of climbing cutworms or flea beetles to this point, but they should begin their activity soon.

With the recent warm weather, we are starting to see bloom in sweet cherries. In the warmer areas of the region, we saw some open bloom late last week, but the weekend warmth really advanced the blossoms. The weather forecast for the week predicts warm weather, albeit rain on Tuesday, May 20, which suggests good pollination weather. Growers have been placing honey bees into orchards areas over the weekend, and bee activity was high with the warm conditions on Sunday and Monday, May 18-19.

Most growers are protecting against American brown rot blossom blight as bloom progresses in sweet cherries. In and around the NWMHRC, we have seen no white in tart cherries yet. However, with the predicted warm temperatures, we could move very quickly from bud burst to bloom. The weather forecast predicts we will be in full bloom over the weekend into the first of next week.

Green tissue is evident on all apple varieties and growers are actively protecting it against the apple scab fungus. The wet conditions have not allowed apple growers much rest between applications to protect against this disease.

Many growers are still planting across the region. We have heard of many reports of wet orchards, which is not extremely common in our sandy soils in Northwest Michigan. Some growers reported that they have never seen such wet conditions for planting.

Pest report

Dispersal of apple scab spores is ongoing during this primary infection period at our apple scab monitoring field site. The latest potential period for apple scab infection in the Northwest followed rain that began the morning of May 12 and ended mid-afternoon on May 13. During this potential infection period, the average number of spores collected on spore rods made another jump up to 1,855 spores per rod. Moderate temperatures at an average of 58.6 F, 0.52 inches of rainfall, and a long period of wet, humid conditions resulted in the potential for heavy apple scab infection, which is reflected in the number of spores that we collected on May 13 (see table). The snow/rain mix that occurred May 15 into the morning of May 16 was not conducive for an apple scab infection.

Apple scab spore count

New green tissue on apples continues to expand, particularly during this recent warm weather. Keep in mind that new green tissue that has not been protected with a fungicide is susceptible to apple scab infection. Additionally, fungicides may need to be reapplied to green tissue after rain because rain may wash fungicides off of that tissue. At this time, it is important to continue spray to prevent apple scab infection. If predicted temperatures in the coming week remain cool, Scala or Vangard mixed with a protectant are two fungicides with good efficacy against apple scab at cooler temperatures below 70 F.

Powdery mildew infection is also of concern at this time. Growers should consider incorporating management options for powdery mildew into their apple scab program if the orchard had powdery mildew infection last season and the apples have reached tight cluster. Powdery mildew overwinters in the buds and data have shown this fungus is killed when temperatures reach -10 F. This year’s winter temperatures reached much colder than -10 F throughout the region, therefore we may see reduced inoculum due to these conditions. However, we cannot guarantee that all powdery mildew inoculum were killed during those cold weather events. The SDHI fungicides provide efficacy against both powdery mildew and apple scab. Growers who need to spray for both powdery mildew and apple scab can use a SDHI and then move back to the protectant strategy.

Sweet cherries are in bloom and growers in Northwest Michigan have been protecting blossoms from American brown rot blossom blight infection. For resistance management, Michigan State University Extension is recommending that Rovral be used at this bloom timing. 

In tart cherries, several growers have reported that Balaton variety trees are farther along in development compared with Montmorency trees. At the research station, Balaton tart cherry trees are in the tight cluster stage and Montmorency are at late budburst. Development of tart cherry trees will likely move quickly as a result of warm temperatures that are forecasted for the coming week. Therefore, growers should be mindful of this hastened development to adequately time the first European brown rot spray at the white bud or popcorn stage. Progress from white bud or popcorn to bloom will likely move quickly, and a second application may be needed shortly after the first to protect tart cherry blossoms from European brown rot infection.

Bract leaves are unfolded, particularly in Balaton tart cherries, and are susceptible to cherry leaf spot infection. MSU Extension is recommending growers protect bract leaves from cherry leaf spot infection to reduce inoculum and prevent a cherry leaf spot epidemic from occurring later this season.

A slow start in accumulating growing degree days (GDD) this spring has delayed insect activity, but we are starting to see slightly more activity in the region. In particular, we have noticed that wild pollinators and honey bees are actively foraging. Many growers are now receiving honey bee hives. Insect pests are also active, and this is our third week of monitoring for them at the research station.

Green fruit worm moth captures are higher compared to last week with an average of 17.6 green fruit worm moths per trap; this is the third consecutive week of green fruit worm moth capture. This is our second consecutive week of spotted tentiform leafminer moth catch with an average of 12 moths per trap. Plum curculio adults are also beginning to emerge.

We have not captured oriental fruit moth or American plum borer; the first emergence of American plum borer moths typically occurs around 118 GDD base 50 F and adult oriental fruit moths typically emerge around 174 GDD base 50 F, and we have accumulated 114.8 GDD at this time.

Dr. Rothwell’s work is funded in part by MSU‘s AgBioResearch.