What's this spot on my rose? Downy mildew showing up on rose this spring.

There’s a new disease in town.  Downy mildew of rose is showing up across the state.  It has been several years since downy mildew has been diagnosed on rose in KY, but we have already seen multiple cases this spring.  During the past weeks, there have been reports of an “incurable black spot” that is “unresponsive to fungicides.”

The downy mildew pathogen is a water mold, not a true fungus.  Thus, humid greenhouse conditions are ideal for disease spread.  So far, all reports originate from greenhouse-grown roses.  Additionally, retail centers who purchased finished roses indicated that roses arrived with early stages of the disease. 

Symptoms of downy mildew occur primarily on young apical leaves.  Initial symptoms begin as purplish-red irregular spots (photo below).  Rapid spread occurs with high moisture and reduced air circulation.  Within days, lesions coalesce, resulting in leaf yellowing and/or leaf drop.  The fuzzy sporulation of rose downy mildew occurs on the lower leaf surface, but it is typically very sparse and difficult to see.  This season, infection has been diagnosed on hybrid tea roses and some varieties of Knockout rose. 

Downy mildew on rose begins with irregularly-shaped lesions that are purple to reddish in color.

Roses are susceptible to a variety of leaf spots (photo below).  Black spot, the most common foliar disease of rose, begins as circular spots with feathery margins.  While hybrid tea roses are extremely susceptible to black spot, Knockout roses are mostly resistant.  Cercospora leaf spot, common on many shrub roses, produces circular lesions with purplish red edges and light gray or tan centers.  Both of these leaf spots can be confused with early symptoms of downy mildew.

Leaf spots of rose can appear similar.  Black spot lesions are circular with feathery edges (left, photo by J. Hartman).  Cercospora spots have dark halos with light centers (right, photo by A. Windham).  Downy mildew infections cause blotches, not spots (center, photo by D. Hull).

Management of downy mildew requires a combination of cultural and fungicidal controls.  This first step requires reduction of humidity.  Greenhouses should be well ventilated.  Nursery and greenhouse plants should be spaced for sufficient air circulation to promote leaf drying.  Drip irrigation can greatly reduce leaf wetness, as well.  Growers should practice good sanitation, removing fallen leaves and pruning away diseased plant parts, as the pathogen can survive for several weeks on debris.  Fungicides registered for use in commercial greenhouses and nurseries include Aliette, Banol, Segway, Stature, and Subdue MAXX.  Homeowners may apply copper + mancozeb.  Refer to labels for rates and application intervals.