The “dry air” thing has done my head in… moving on now
dry air = low humidity
So how come greenhouses get lots of powdery mildewed seedlings in an environment of wet air = high humidity?
“a common disease, especially when cool, damp evenings follow warm days. These fungi are unusual compared to most plant disease fungi because they do not cause infection when the leaves are wet; they just require high humidity during infection. However, they produce more spores when the humidity is low”
Robert - Thank you for your answers. It appears then that PM requires both high and then low humidity for initiation and progression of the infection.
Your welcome George. It is confusing. There’s lots of additional information on the web regarding Powdery Mildew.
It’s a frustrating disease in that few cultivars and species seem totally immune.
No rose is perfect. You could try working with ‘Lamarque’. There are already a great many descendants but you might be able to take it in a different direction.
About two years ago, one of my Fortuneana cuttings destined for budding later that spring developed PM in the spring. It affected the tips and flower buds only. Admittedly this specimen was also rather thin and weak .
Anyway, after budding it later that spring, the scion grew with terrific vigour, and showed no disease. However in the late autumn of the same year, the PM showed up on the scion growth. In the following years the same plant has never showed PM ever again. Improved immunity to PM may be one explanation for this observation.
Fortuneana sometimes mildews here, as can laevigata.
You’ll note PM often gains a foothold when specimens exhibit drought stress.
I imagine, like in humans, anything that upsets the balance between host immunity versus pathogen numbers will lead to infection. Stress in the plant (eg. dehydration, or some nutritional deficiency) would be expected to upset this balance, and infection could result if there is an adequate dose of available pathogen.
Also George, you will probably find the mildew that appears in greenhouses is not actually powdery mildew but is instead something like Botrytis (aka grey mould), and other fungi that cause damping off. Grey mould can look like a grey beard over everything and spreads lightning quick in humid still air conditions such as is found in a greenhouse. It was the number one cause of mortality in my cuttings last year.
Hi Simon. Your experience in your greenhouse is totally believable…sad to hear about it.
I am happy to hear about your other cuttings however - good job!
George