I have been receiving quite a few questions about the application of chlorothalonil and oil. This dormant spray has been shown to be highly effective in reducing in season scab, as well as delaying the sporulation of the pathogen, making in-season fungicide sprays more effective.
Q. The work by Dr. Adaskaveg shows 6 pts, the product is only registered for four (winter, 2013)…is the 4pt rate effective?
A. Yes. The 4 pt rate has been used in previous studies and has been shown to be effective in reducing scab. The recent work by Dr. Adaskaveg looked at increased rates to determine if an increase in efficacy occurs – and it does. This information will be used to help re-label the product.
Q. Do I have to use 4 gallon of dormant oil per acre and 100 gallon ride?
A. Dr. Adaskaveg purposely used 4 gals/A in order to provide a general insect control with the dormant oil (the rate range is 4 -6 gals for insects). A 100 gallon ride should be considered the minimum to ensure coverage, and if more volume is applied (i.e. 200 gallons/acre) the rate of oil per acre is still 4 gallons. Coverage is very important and thus, higher gallonage would be better. After the application the water dries and the oil and fungicide is left behind with an affinity to spread out over the waxy stem.
Q. When is it too late to use chlorothalonil and oil?
A. The product is labeled for application up until bud swell. This is the point where the buds increase in size and a little bit of green can be seen at the tip of the bud. If the earlier timing has been missed, an application of copper and oil (5lbs of Kocide 3000+4 gallon oil) can be made to help for scab as it has also been shown to be very effective in reducing disease and will not generally harm the tree/buds.
Q. I am running the rig through the orchard, what else can be applied?
A. Boron has been shown to increase flower set when added in dormant sprays. A rate of 2-3 lbs of Solubor (or B equivalent)/100 gallon of water can be applied up to pink-bud. Applying boron after pink bud is not recommended as it may mis-direct pollen tube formation, reducing nut set. Zinc can also be considered at this time.