For those who have seen a lot of this disease in their roses and other plants in the past 5 years or so, this article will not contain any surprises. Nor will it provide any comfort. We may be caught in the middle of a particularly virulent mutation event in the public life of downy mildew–or conditions have just been extremely favorable for its development.
Your right Peter that isnâ€™t too surprising, plants and their diseases are in a constant battle. Downy Mildew is just getting the upper hand for now on Impatiens anyway. Do we know if the strain attacking Impatiens is also the strain that attacks roses? DM hasnâ€™t really been a problem on my roses but Iâ€™ve noticed it quite a bit on my seedlings this year, mostly but not limited to the ones with Rugosa in them. I took some cuttings last fall and they are growing under lights alongside my seedlings, so they must have brought it with them even though I didnâ€™t notice it on the plants outside.
Over the past weekend, I attended a gardening seminar and one of the presenters was a plant disease expert from the University of Wisconsin Madison. The topic of the Downy Mildew attacking impatiens was brought up and it was apparently a major issue in Wisconsin last year. While I am a bit doubtful that the exact same organism is attacking roses, if weather conditions favor the development of Downy mildew in one Genus, then it stands to reason that the conditions would also favor its development in another, even if the organism causing the disease is different. I do know that DM is a water mold and I have certainly seen an increase in its presence on my roses over the last few years. It has become a major disease to screen out in my seedling populations in the late summer/early fall when conditions are right, but for the first time ever I am seeing it on mature specimens, although nothing that has been extremely detrimental, at least yet.
I have seen (read) many articles which explain the almost singular species attack per species of downy mildew. One example is the downy mildew that infects roses (Peronospora sparsa) only infects roses. But the downy mildew that infects mint (Peronospora lamii) can infect several members of the mint family, including dead nettles and salvia, and possibly basil and coleus. Plasmopara halstedii is a different downy mildew pathogen that infects members of the daisy family. Among roses the unique reactions of the different lines of roses is one of the few differentiations. I am just presuming that among the mint families, the relationship may be close enough to be inclusive, much like the peronospora sparsa attacks most (but not all) species of roses, but shows differing symptoms.
Fara, I don’t understand or like this usage for pesticide, but it’s accepted in public writing about fungicides, insecticides, herbicides–even (amazingly enough) disinfectants and rat baits. Take a look at this:
[size=small]A pesticide is any substance used to kill, repel, or control certain forms of plant or animal life that are considered to be pests. Pesticides include herbicides for destroying weeds and other unwanted vegetation, insecticides for controlling a wide variety of insects, fungicides used to prevent the growth of molds and mildew, disinfectants for preventing the spread of bacteria, and compounds used to control mice and rats. [/size]
More and more it seems that people who didn’t study are in charge.