As I write this just after noon on Saturday, May 10, it looks like this long stretch of rain we have been experiencing for the past week is wrapping up a bit earlier than expected. Over the past seven to ten days around the state we have seen rainfall totals from less than two to over five inches, and rain or at least drizzly conditions every day. Even when there were gaps in the rain, apple scab spore germination can continue in dry periods under 24 hours, so for most orchards there has been essentially a continuous scab infection since May 1 or 2. Many growers have sprayed twice during this infection, but we can expect for most orchards that fungicide residue sprays applied even yesterday morning may be considered gone or at least substantially compromised. I recommend that growers go in with an appropriate kickback material as soon as possible now that the rain has largely stopped, looks like it will stay stopped, and wind conditions for spraying look good through Monday. Post infection activity starts from the time of application, so delaying application for a day or two will reduce the time period that the material will be active on spores that germinated in the past couple of days. The materials that have kickback activity fall in the FRAC codes 3, 7, 9, and 11. The strobilurin materials in FRAC code 11 tend to have less kickback activity and are more useful for summer diseases so I do not recommend those be used at this time. The DMI fungicides in FRAC code 3 tend to have longer kickback activity as well as have excellent activity against powdery mildew and cedar apple rust which are also active at this time. As always, I recommend tank mixing single site fungicides with a protectant fungicide like mancozebs or captan. We are approaching the end of the season for using mancozeb fungicides if applying at the full label rate of 6 lbs per acre. Captan is another effective fungicide that many growers tend to switch to at this time of year.

As orchards in the warmer parts of the state are in full bloom, and insect activity has been very low statewide with little to no European apple sawfly observed on traps in multiple orchards, I do not recommend application of insecticides at this time. Warm weather is expected to return on Monday and with it we should finally see substantial pollinator activity. There is a lot of concern around the state right now that the weather conditions in the past week have been inadequate for appropriate pollination of fruit crops, but I expect that a warm sunny day on Monday will provide adequate pollination for most orchards that are in bloom.

When we think of a blooming orchard we also think about fire blight. Conditions have been so cool during and prior to bloom that the epiphytic infection potential of the bacteria that causes the disease has been very low to such a point that treating blossoms has not been recommended. At the far end of the NEWA fire blight model five days out some orchards are showing an EIP of over 100, which could cause infection if all other conditions are met. You will need to revisit this later in the week, and I recommend that all orchards have a treatment of streptomycin on hand to use should an infection period be predicted.

Speaking of bloom, we have observed in multiple orchards relatively low blossom density this year. The frost that affected most orchards in 2023 allowed trees to save resources they would have put into growing crops that year which produced a large bloom last year. That heavy bloom event and resulting large fruit crop taxed the resources on many trees especially on varieties that lend themselves toward biennialism like Honeycrisp, which resulted in a lower density of fruit buds this year. There is nothing you can do to put fruit buds on a tree that doesn’t have them for this season, the best you can do is thin aggressively in those heavy blooming years. I guess most orchards will have to think about that next year. That doesn’t mean that I don’t recommend thinning crops this year, we’ll discuss that in the next newsletter.

__

Terence Bradshaw (he/him)
Associate Professor, Specialty Crops

Chair, Dept of Agriculture, Landscape, and Environment
(formerly Plant and Soil Science)
College of Agriculture and Life Sciences

University of Vermont
117/210 – Jeffords Hall | 63 Carrigan Dr
Burlington, VT 05405

(802) 922-2591 | tbradsha
https://go.uvm.edu/pssbradshaw

UVM Commercial Horticulture | UVM Fruit Blog
Horticulture Research and Education Center
Message me on Teams

UVM’s Our Common Ground Values:
Respect | Integrity | Innovation | Openness | Justice | Responsibility

UVM is subject to the Vermont Public Records Act and communications to and from this email address, including attachments, are subject to disclosure unless exempted under the Act or otherwise applicable law.