7/2/2013 - By Harrell's

Downy Mildew or Pernospora Spp. is a fungal family of diseases that is closely related to Phytophtera and Pythium. The pathogen is an obligate parasite that attacks the vascular system of the plant and needs to absorb their nutrition from the plant tissue.

Downy Mildew first can be observed on the underside of the leaves of a wide range of host plants, however most of the time maginification or a hand lens is needed. As it grows, Downy mildew starts to appear on the tops of the leaves as yellow to reddish to brown spots as the pathogen progresses and depletes the foliage of its nutrition. Since the disease attacks the vascular system it is typically limited by the larger veins of the plant leaves. In severe infestations, the pathogen can moves its way down into the trunks or canes of the host.

Downy Mildew needs a very specific environment to thrive in. Temperatures around 65 F are optimal for the disease to reproduce, while temperatures below 39 F or above 80 F will stop the disease from spreading. The reproductive structures of this disease “ripen” during the night hours and air moves them around in the morning as the leaf tissue dries, so sprays are best in the morning before the leaf tissue completely dries.

Use Alliette, Protect D/F, Insignia, Cygnus, Stature, Pageant,  Segway,  Heritage,  Compass or Dithane to combat this disease.

By Matt Shultz, Horticultural Tech Support Specialist and Sales Representative, Harrell’s LLC and Marshall Horsman, Technical Services Manager, Harrell’s LLC.