sooby said:
Edited to add, a new plant that suddenly gets rust after a while may have had what's called a latent infection, one that was internal and not showing on the outside of the leaf yet until it produces spores some time later, which can be after several weeks. So time, environmental conditions and/or the stress may have just held the rust up for a while before it became visible.
Anyone who has cut rusty foliage back knows how this goes. You know you got all the visible rust spots, and you cut off a bit extra just in case. Maybe you spray with a contact fungicide to additionally kill any spores on the leaves. Then, a few days or weeks later, you see spots on what had looked like clean foliage. Sneaky!
In addition to all the foliage-trimming, if a grower uses a systemic fungicide dip as part of processing for daylily shipment, this may add to the delay in latent infection showing up. I'm not sure how widespread the practice is, but I know of at least one grower who dips 'n ships. So, in the case of an azoxystrobin dip, one might get a couple of months before the rust shows. Beyond a simple bleach dip, I have mixed feelings about the practice of last-minute rust management because it disguises the infection and extends the time period during which the otherwise rust-free buyer must monitor for rust. It serves the seller, as it disguises the infection until later in the growing season, at which time it may be harder to identify the cause of an outbreak. At least when the plant shows up rusty, one knows what one is dealing with and can manage the situation accordingly.
I apologize for specifically referencing azoxystrobin so often. It must sound like I'm in their sales department! I would love to just say "systemics" or reference a class of systemics, but that wouldn't be as precise as there are differences in how they are absorbed by the plant, and I've only seen research results regarding a few of them.
