The most common fungal rose diseases are:
Blackspot — Sometimes called "Leafspot," this disease is the most common to all rosarians worldwide. Serious infestations can greatly weaken plants, producing stem die back and unproductive growth. A weakened plant will most likely experience winter damage in colder climates.
Damage usually begins as a black or brown splotch. Shortly after, the leaf tissues surrounding this spot turn yellow. This is because fungi feed only on dead tissue. In this case, the fungus exudes a chemical called ethylene to kill the leaf tissue as it advances; the result of this is the destruction of chlorophyll in the cell living tissue and the creation of a golden "halo" or yellow surface area indicating the dead tissue.
The spore germinates in as little as nine hours on moist leaf tissues where temperatures remain between 70 F to 80 F (21 to 27 C). Under most conditions, the leaves must remain wet a minimum of seven hours before infection can occur. Within 15 to 37 days, visible colonies can be observed from a single spore's infection.
The pathogen is susceptible to fungicides within the first 90 hours but may not be noticed. For this reason, preventive fungicide applications are best made during susceptible periods of the year if these are your preferred control means. For other controls, see Rose Diseases and Their Control.
Some roses are more susceptible to Blackspot than others. According to the American Rose Society, the highly susceptible genes of Rosa foetida were transmitted through R. foetida persiana and "Harison's Yellow" to the Pernetianas, the parents of our yellow roses. For this reason, most yellow roses are highly susceptible to Blackspot.
Powdery Mildew — Arguably the most pervasive and destructive of all the fungal diseases, Powdery Mildew will attack all rose species in a wide variety of climates. Severe, untreated infestations will cause distorted new growth, often killing growth tips and buds.
Powdery Mildew's advanced stages are easily identified by white or gray "flocking" on leaves, stems, buds and thorns. Early symptoms include slightly raised blisters on the upper leaf surfaces.
Spores optimally germinate in as little as three hours when temperatures are 71 F (22 C), but can reproduce when they are as low as 42 F (6 C). The spores are most viable when relative humidity is about 80 percent. When temperatures exceed 70 F (21 C) for more than 48 hours or 90 F (32 C) for more than 24 hours, the spores are killed. Although high humidity increases spore production, water films actually suppress their growth. In short, Powdery Mildew finds a comfortable home where roses are subjected to warm, cloudy days and cool, humid nights.
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