Great info everyone. I also plan to try the fall Urea spray this year. I have a few nearby wild crab apples I believe are contributing to my challenges.
@alan - I corresponded briefly with the professor that supervised the study when I tried the treatment. I asked her if they ever observed copper damage like I had and she said they hadn’t. But she couldn’t recall for sure many specifics and they never followed up on the study.
There are a decent number of studies that have followed up on the mechanism of how either methionine-riboflavin or each alone work. In general, it seems like they induce oxidative stress in the plant and the plant responds by kicking its defense mechanisms into gear. That makes sense to me, as riboflavin and its derivatives are important in electron transport reactions (oxidation-reduction). There may also be some toxicity to the fungus due to reactive oxygen species being produced by the mixture.
The russetting scared me off, but I might try this again in the future with ferrous sulfate rather than copper sulfate and see if that works. Iron salts are widely used for their production of reactive oxygen species, but are likely much less toxic to plants. It would also be interesting to see if it works against scab, but sulfur works just fine for that, so it isn’t high on my radar to try something different for scab.
No idea why Cornell did not follow up. There are lots of “biorationals” out there. I think folks like Scott use some of the newer ones that are supposed to up plant defenses. I’d suspect researchers got onto the effectiveness of microbial strains like Serenade and Double Nickel and ran with it.
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