Downy Mildew
So, if you live in an area of high humidity and medium temperatures (less than 85F) you’re going to see DM.
We’re talking geographic areas such as the west coast and mainly the Pacific NorthWest. The South is also prone to this problem. Here in the North, we rarely see downy mildew.
What Downy Mildew Looks Like
DM has purplish red to dark brown spots (irregularly shaped) on leaves. Some times it can resemble black spot or chemical overspray. One clue is that DM does not cross leaf veins so the patches have straight sides and appear angular.
Black spot does cross over the veins as does powdery mildew so this is a major clue; these infections will be irregular shaped and cross over veins. DM may even create a gray layer resembling botrytis on the underside of leaves but this isn’t present in all infestations.
Easily Confused
It can be easy to confuse it with black spot because they both have yellowing rings around the fungus and both will lead to defoliation under uncontrolled conditions.
Defoliation and a weakened plant will lend itself to other problems and possibly overwintering death.
Cause of Downy Mildew
Downy mildew is caused by the fungus, Peronospora sparsa, and this is interesting in that this fungal problem can be systemic; that is it can be found in all parts of the plant.
Control then is very difficult if the fungus is inside the plant. Contact sprays will not penetrate the leaf.
As soon as the temperatures rise above 80F and the relative humidity below 85%, downy mildew is killed off. The spores live on in lesions in the rose canes for reinfection when the environmental conditions are, once again, present.
Controlling DM
The first level of control is by planting the rose in areas where ventilation is good. Do not crowd the rose with other plants. Yes, I know roses look best when surrounded by perennials but if you have a history of downy mildew in an area where it is common, you’ll have to make some tough choices.
Increasing ventilation is the number one control method for both commercial and recreational rose growers.
Control is by usually by spraying to prevent the fungus from spreading on the leaves or by preventing it from establishing on the leaves. Copper based fungicides are often recommended.
One organic website claimed that hot water (raising the temperature on the cane over 80F) would kill off the DM. I didn’t see this recommended anywhere else so pass it along for your experiments. I have never had DM in my northeast garden so can’t comment on this technique. Hot does not mean boiling I note.
It is critical to remove infected plant debris from the garden as it will act as a site for more infection.
One of the university websites put it nicely that “well-tended rose will be less susceptible than a neglected one” so one of the major bits of advice is to read and follow the directions for feeding and growing roses. A healthy rose, even an infected one, is going to be less troubled than a poorly growing plant.
Do you have a question about Downy Mildew?
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