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And further:

1) what qualifies as an "oil-based spray" and at what amount?

2) does this warning still apply during the rainy season-- when the need for quick cycles of medicines is necessary?

(and yes, I've tried to ask manufacturers but to no avail).

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The reason is that sulphur reacts with oils to form phytotoxic chemicals - and "phytotoxic" means "stuff that kills plants".

If you apply a spray containing oils (e.g. neem oil, or horticultural oils which are basically petroleum products) in enough quantity to do anything useful in killing pests, that counts as "using it".

Oil and water don't mix, so it would be safest to assume that rain doesn't affect the 30-day period. If the oil (or the sulphur) was washed off quickly by the rain, there would be no point in using it at all.

Sulphur is basically a fungicide. Oils are basically insecticides. Use whichever is appropriate to fix the problem you have.

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  • 1) I pretty much guessed it was phytotoxic, I wanted a little more detail so I can understand the interactions and chemistry, as well as any possible exceptions as things can get pretty desperate during Indian monsoons. 2) Clearly I'm using neem oil wrong then, because it doesn't even seem to last 5 days. Though perhaps I can't tell that by touch. However, the pests are usually back by that time during the rainy season. 3) Fungus and pests very often occur concurrently. I don't know if that's true only of some plants and some regions of the world-- but it is in mine. – kladhest Oct 25 '19 at 22:29

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