Not necessary. Powdery mildew is plant specific. The mildew that infects roses isn't the mildew that affects zinnias. Separate animals although they look the same.

Mildews are largely airborne and if you have susceptible plants they will get it in bad years and not get it in good years without regard to what you do.

As far as the mildew spores overwintering in your soil this is easily mitigated with a mulch layer. In the spring after the rainy season lets up (assuming your springs are wet) add a 2-4" layer of mulch. I don't really care what you use. Straw, hay, grass, wood chips, rocks, whatever. This will prevent the spores in the soil from splashing up onto the plants from the hard rains or overly strong irrigation.

Beyond this if your plants again get mildew it is from the air. Consider low toxicity fungicides such as milk applied regularly as a preventative. Milk works against mildew as a curative in some cases, but from the testing results I have seen is best as a preventative. Corn meal applied around the plant seems to be university tested to attract trichoderma fungi which more or less eat other fungi.

If this isn't enough then I suggest not planting whatever still gets mildew even in the good years. It simply isn't cut out for your area.

If you can't bear to be without something then be prepared to use more potent fungicides like copper, sulphur and daconil. Even then be prepared to spray regularly as most fungal diseases aren't cured once they start, they are simply controlled. Controlled means they are knocked back enough with regular sprayings to have the plant survive.

when i grew roses ... in fall .. i used a blower to remove all fallen leaves from the beds ... and sent them to the dump .... i then did my pruning .... and then did a late spray for mildew .... then put the rose cones on ...

i resprayed in spring, before they leafed out... to kill any left over spores ...

i also blew out all the leaves from lilac, and monarda [beebalm] .... and crabapples [though that isnt technically a mildew problem] ..... but didn't waste spray on those .....

this significantly reduced future problems.. as you know.. prevention is a lot easier than cure ...

i used chemicals ... before kids and sanity struck ... i don't use them much anymore... i may be mistaken ... but i think baking soda and water is one cure... but you will have to research that ... or post another forum post ...

good luck.. ken

YES!!!!!!!!!!!!!!!!!!!!!!!!!!!!

Did I make myself clear? Or do I need to let you know what I really think?

One of the simplest, and most effective, methods of disease control is sanitation. (The simplest, most effective, and cheapest is resistance, but sometimes, you deal with realities, not theory.)

So, if you had a really bad year, remove the debris. The fungal spores will use the debris to overwinter and provide a plentiful source of inoculum to start the year off next year to a roaring start, assuming environmental conditions are optimum.