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Getting The Black Out Of Roses

Ken Rudisill
Horticulture Extension Agent
Bay County

One of the biggest problems facing homeowners who grow roses is black spot. This fungal disease can reduce the quality and life expectancy of a rose plant. Roses can also have poor performance from other factors such as root stocks and scions that are not suitable for Florida, under watering and under fertilization.

Black spot will cause a general weakening of the plant so that progressively fewer and fewer blooms are formed if the disease is left unchecked. Photo Credits: Theresa Friday, Santa Rosa County

Symptoms of this disease appear as black spots on the upper surface of the leaf. Yellowing around the spots on infected leaves also occurs. Leaf defoliation can be severe on some cultivars of roses.

Spores from the previous season are spread by rain or overhead irrigation. The spores must be wet for several hours to infect plant tissues. Therefore, it is important to water early in the morning to allow the plant to dry off quickly as the sun rises. Drip irrigation is a good way to keep water off leaves and canes.

It takes three to 16 days for the symptoms to appear after infection. The optimum temperature for disease development is 64°F. Spore germination occurs from 59 to 81°F. With such a wide temperature range, this disease can continue to develop as long as the moisture is available during the season.

Cultural practices such as planting resistant cultivars, removing and disposing of fallen leaves, pruning of canes late in the winter before new shoots are produced and proper irrigation will reduce the chances of black spot.

When using chemical control, apply a protectant fungicide at bud break and bimonthly until leaves are completely expanded. During the summer, fungicide applications every 7-14 days may be necessary to manage the disease.

Fungicides labeled for the control of black spot of roses include: captan, chlorothalonil (Ferti-lome® Liquid fungicide, Bonide® Fungonil Multipurpose Fungicide, Ortho Garden Disease Control), copper sulfate, mancozeb, sulfur, neem oil, thiophanate-methyl (Ferti-lome® Halt Systemic Fungicide, Green Light® Systemic Fungicide) and triforine (Ortho® Rose Pride Rose & Shrub Disease Control). To reduce fungicide resistance, rotate among products with different modes of action.

Source: Jozer Mangandi, graduate student, Department of Environmental Horticulture; Natalia A. Peres, assistant professor, Department of Plant Pathology, Gulf Coast Research and Education Center (REC)--Balm; Florida Cooperative Extension Service, Institute of Food and Agricultural Sciences, Document PP268.