Wheattoast wrote:Powdery Mildew Seems to be an issue in Chicago every year. It is for me and others Ive spoken with anyway.
I can honestly say I've never had PM on an outdoor crop; not that I'm an expert or anything, just have had a garden for many years. Because I haven't had it, hearing that it's often an issue here made me curious. I primarily grow heirloom veggies, though not all and I've only had gardens in two locations - one out in the country south of the south suburbs and one on the rooftop here in Chicago.
I do recall reading about research related to controlling PM. The
research showed milk to be more effective than any of the chemicals used. I think it was done in fields in South America (the research). Can't remember the crop.
I also grow African Violets and they are prone to PM. It's almost always lack of air flow and dry air.
But, since it's linked to lack of air flow and dry air, I wonder if there is something similar that contributes to it outside? It is humid here, so that doesn't make sense.
A bit of research and I found the following:
"Powdery mildews are severe in warm, dry climates. This is because the fungus does not need the presence of water on the leaf surface for infection to occur. However, the relative humidity of the air does need to be high for spore germination. Therefore, the disease is common in crowded plantings where air circulation is poor and in damp, shaded areas. Incidence of infection increases as relative humidity rises to 90 percent, but it does not occur when leaf surfaces are wet (e.g., in a rain shower). Young, succulent growth usually is more susceptible than older plant tissues."
And:
"Once the disease becomes a problem:
* Avoid late-summer applications of nitrogen fertilizer to limit the production of succulent tissue, which is more susceptible to infection.
* Avoid overhead watering to help reduce the relative humidity.
* Remove and destroy all infected plant parts (leaves, etc.). For infected vegetables and other annuals, remove as much of the plant and its debris in the fall as possible. This decreases the ability of the fungus to survive the winter. Do not compost infected plant debris. Temperatures often are not hot enough to kill the fungus.
* Selectively prune overcrowded plant material to help increase air circulation. This helps reduce relative humidity and infection."
Ah, so it can overwinter. That may help explain why some people get it over and over again. And it needs humidity to get started - that we have a lot of.