Lime sulfur in vineyards question

By Terence Bradshaw

I have discussed the use of lime sulfur in a pre-bud break spray to reduce overwintering disease inoculum on grapes, and this looks like a good time to be thinking about that practice. Multiple people have asked me the same question which tells me that more have it as well, so I may as well answer to the group. The question is about the rainfastness of lime sulfur, and how long it needs to be on the vine before rains come for it to do its job.

Lime sulfur used in a high rate directed application to dormant tissue isn’t acting like your normal protective fungicide. That is, it is not applied to tissue prior to infection to prevent sporulation, like a protective barrier. Its use in this manner is as a postinfection material, meaning that it is working on already established fungal infections. Therefore, as long as it’s on the surface of tissues, and hopefully soaked right into the cracks and crevices where inoculum hide out, it will do its job in fairly short order. Lime sulfur is a caustic material that denatures proteins and degrades cells, it’s not a highly refined biochemical process that’s going on. With that said, I can’t say specifically how long it takes for it to do its job, but I would surmise a few hours, and likely much happens immediately. As I said to one grower who asked me this question, think of lime sulfur more as a sanitizing wash than a fungicide where you want to maintain residue for as long as possible (until you don’t at harvest time that is).

So, today and through the weekend, I would feel comfortable applying any time I have a four-hour dry window. Spraying in the rain will likely dilute the material and reduce efficacy, so I’d avoid that.

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