Captan phytotoxicity and apple scab control
Be careful using Captan and some newer fungicides. Certain combinations could cause leaf burning or spotting.
Michigan State University Extension wants to make apple, stone fruit and strawberry growers aware of a potential to cause phytotoxicity (leaf burning or spotting) from a combination of fungicides. We have had a few inquiries from apple and strawberry growers over the last few weeks about leaf burning.
Last week (as of June 19, 2013), we received some information from Cornell University researchers alerting growers to this same issue. This information came in a newsletter called Scaffolds, produced by Cornell faculty and staff. The pertinent article starts on page 6 of the June 10, 2013, Volume 22, No. 12 issue.
The fungicide combination of Fontelis plus Captan may be causing this leaf damage. It appears that there is mineral oil in the Fontelis formulation that is interacting with Captan or sulfur under some environmental conditions. Most of us in the research and extension community were not aware that there is mineral oil in the Fontelis formulation.
It has long been known that using Captan in combination with pesticides that contain crop oil or using Captan just before or after one of the crop oil containing pesticides can occasionally cause phytotoxicity. It is thought that the oil is allowing more penetration of the leaf tissue by Captan or sulfur.
We have not seen symptoms that are as severe as some of the pictures in Scaffolds and so far have had no reports of any fruit injury, but we felt you would want to be aware of this potential phytotoxicity issue. The Cornell researchers are careful to point out that the vast majority of Fontelis plus Captan applications to apples have not caused damage. We encourage fruit growers and others in the industry to consult Scaffolds for more details.