I don’t get too much botrytis at my home garden – our volcanic sandy soils drain all too well, and our desert climate is naturally dry. But I do get some of it sometimes, and I see it on certain seedlings at the nursery sometimes as well.
Really, the only conclusion I’ve been able to draw is that plants which grow in a particularly vigorous manner in spring are often the ones which are most susceptible. This seems to have to do with the rapid rate with which the cells expand on vigorous varieties, and varieties with particularly thick stems.
One example are seedlings I call “One Stem Wonders”. These are plants with large root systems, but which tend to throw up just one single enormous stem, along with a massive sized bud. I’ve gotten 3 or 4 of these over the years, but never been able to keep a single one of them past their first mature year. Once dug and divided and kept for further evaluation, every single one of them has died from stem botrytis in spring, before they ever had a chance to grow into a plant. And this is at the nursery, where carry out a well thought out and responsible spray program.
One hates to think that vigor is a negative quality, so perhaps it has something to do with the relative toughness or thickness of the cell walls which make up the exterior of the stems ? A measure which may have something to do with the speed of cell expansion ?
These are just antidotal observations of course, but I know that I tend to pay particular attention to vigorous varieties which really rocket up out of the ground in spring.