(Written by Sean Mullahy, U of I Plant Clinic Student Worker)
Downy mildew of roses is generally not a problem in the home garden; it
mostly affects roses grown in the greenhouse environment. Most rose growers are
familiar with its similar (in symptoms) cousin powdery mildew, although the two
are very different and can be differentiated.
Downy mildew first appeared in
the 1860’s in England, and from that point on, it was reported all throughout
Europe. After making its way through Europe in the early 1900’s, it made its way
over to Scandinavia and the Soviet Union. Downy mildew made its way stateside
in 1880, first appearing in the Midwest. Since then, it has made its way all
around the United States, and to almost every corner of the globe.
Downy mildew can be identified through a number of symptoms. Watch out
for purplish red to dark brown spots developing on leaves (these will be more
angular in shape) and the yellowing of leaflets. These yellow leaflets may have
green tissue sections of up to 1cm2 in size. Check beneath lesions
for mycelia and conidia, especially under cool humid conditions. Abscission of
leaves may also be quite severe. Purple to black colored spots may also appear
on stems and peduncles. Downy mildew can be differentiated from powdery mildew
because the grayish spores will be produced on the underside of the leaf as
opposed to the top.
For greenhouse management,
lowering the humidity should be the first step, this can be done with
ventilation and aeration. Also increasing temperatures to 27C / 80F will help
in controlling it; as spores are killed at this temperature. In both cases,
keeping the plants aerated and dry is a great idea. Prune roses to keep them
open, and try to water them at the base. This will help prevent germination of
the downy mildew. For field or garden grown roses, a fungicide application is
recommended. This should be done in a preventative manner, when environmental
conditions are favorable for developments (cool, humid). If you already see the
symptoms, it’s probably too late! The
growing of a resistant cultivar is as always recommended. For example, the rugosa
family of roses is resistant to downy mildew.
To prevent spread and seasonal carry over, all suspected plant parts should be removed and all infected plant material should be
destroyed.
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