As to foliar sprays, there is research indicating the value of seaweed
sprays, compost teas, wettable sulfur, neem and numerous plant oils against
powdery mildews and foliar diseases. There is also a commercial biological
agent on the market for controlling powdery mildew. Even high pressure
water sprays have been used to wash off powdery mildew spores. There is
even research showing that the glasshouse covering can have an effect on
controlling disease if it is chosen to filter out light wavelengths needed
by the pathogenic fungi. I think this is a situation where it would be
worthwhile to get in a consulting plant pathologist or someone from
university extension to identify the pathogen species for you. Then look up
details of the pathogen life cycle, and search out weak points that can be
exploited for control.
With a combination of several methods, each knocking back the foliar
pathogens a little bit, it should be possible to get the foliar pathogens
down to tolerable levels. Give yourself at least 2 or 3 seasons, allowing
for some trial and error, to get down a system that works. Keep good
records, and learn and improve each season.
Good luck,
Joel Grossman
email(internet): 3216125@mcimail.com