Sandandsun, what I’ve always called the foliage type you’re referring to is “geriatric”. I guess you can also refer to it as “senile”. Perhaps the foliage tested needs to be mature but not “senile”? Kim
Decay of fallen leaves and parts is from non hurting saprophytes that are part of natural microorganism plant cortege. Host specialized or not. Senescent parts susceptibility to saprophytes is not rare. Saprophytes microorganisms compete at settling earlier. When able to damage useful parts microorganisms fall in parasitic category be it through saprophytes earlier settling and or unfavorable for the host environment, more virulence or host plant plant defenses weaknesses.
Spraying allows to grow susceptible plants in environments that do not fit them.
Pierre,
Senescence is not what I mean. I observe selective discarding. The most interesting example I’ve seen is when new leaves shade other leaves and the shaded leaves are “cut off” even if they were produced less than a month earlier. I was also thinking of cases in which a plant under stress sacrifices parts for the survival of the whole. Neither of these is due to age.
The blackspotting of these types of leaves occurs prior to leaf drop but not until after the plant has decided to shed them. And as I said previously, the rest of the foliage remains uneffected. To my mind, these roses are demonstrating resistance.
Chris
However, it would apply to senescent leaves as well.