Valued for their flashy and showy flowers, rose bushes are among the most common ornamental plants for home gardens. Yellow spots and leaf drop are symptoms of a variety of fungal diseases. Prevention and early identification of these diseases can help reduce the severity of infection. Early control of these diseases is vital for treating severe infections.
The fungus Diplocarpon rosae is the casual pathogen that causes black spot of roses. The disease favours warm temperatures between 23.9 and 29.4 degrees Celsius and humidity or free moisture on foliage for infection, reports the Iowa State University Plant and Insect Diagnostic Clinic. Black spot causes 1/2-inch black spots on leaves, leaf drop and yellow spots around the infected area. The disease can spread to branches, causing reduced vigour and overwintering. Plant black spot-resistant rose cultivators, reduce free moisture on leaf surfaces and prune to ensure good air circulation to prevent infection. However, fungicidal sprays are an effective method of control for severe infections.
Mosaic is a common disease of rose bush foliage, caused by the virus Prunus necrotic ringspot. The fungus affects the whole plant, but symptoms occur on foliage during periods of cool weather. Symptoms of infection consist of yellow spots or yellow streaking on leaves, reduced plant vigour, reduced aesthetics and premature leaf drop. However, symptoms do not always develop on infected plants. The disease spreads from plant to plant by mechanical methods, such as the use of infected pruning equipment. The Arizona State University Extension recommends cleaning pruning equipment with a 10 per cent bleach solution to kill the pathogen.
Powdery mildew is a fungal disease that affects nearly every plant, including a variety of rose cultivators. The fungus favours warm, dry climates with high levels of humidity for infection. However, unlike other fungal foliage diseases, the pathogen does not infect wet leaves. Symptoms of infection consist of white fungal powder build-up, distorted leaf growth, yellow leaf streaking or small spots and reduced vigour. Increase plant vigour by using balanced fertiliser applications, resistant rose cultivators and destroy infected leaves to prevent severe infections. However, fungicidal control is recommended in the case of a severe infection.
Cercospora Leaf Spot
Cercospora leaf spot is a fungal disease of roses that has similar symptoms of black spot. Symptoms of infection consist of purple to black leaf spots, yellowing of foliage and premature leaf drop. The fungus favours wet leaf surfaces for infection. Infection occurs on leaves closest to the base of the bush and infects higher standing leaves as the disease progresses. The Alabama Cooperative Extension System recommends fungicidal control for Cercospora leaf spot. However, the same fungicides that control powdery mildew and black spot also control Cercospora.
- Iowa State University Plant and Insect Diagnostic Clinic; Black Spot; July 2010
- Arizona State University Extension Plant Pathology; Rose Mosaic Virus; February 2010
- Colorado State University Extension; Powdery Mildews; B. Edmunds and L.P. Pottorff; May 2009
- The Alabama Cooperative Extension System; Chemical Control of Cercospora Leaf Spot; A. K. Hagan and J. R. Akridge; April 2005