Kira L. Bowen, Bridget K. Behe, and Elizabeth A. Guertal

Roses are among the most valuable and familiar plants in residential and commercial landscapes. In particular, hybrid tea roses, with their large blooms and pleasant fragrance, are favorites among rose enthusiasts. Hybrid tea roses, however, can be devastated by blackspot disease, caused by a fungus called Diplocarpon rosae. AAES research is helping identify ways to protect these popular flowers.

Blackspot disease manifests itself as circular black spots on plant foliage, which greatly reduce the beauty and performance of roses in the landscape. Blackspot is easily distinguished from other diseases by the darker color and fringed borders of the spots that can occur on either side of the leaf. Spots often are surrounded by a yellow halo, and infected leaves fall prematurely. This disease may cause severe defoliation, resulting in a weakened plant and reduced flower production.

In Alabama and the Southeast, high humidity and frequent rainfalls promote fungal pathogen development from March through November. Nighttime temperatures between 59° and 80° F and heavy dews or frequent showers are ideal conditions that allow the fungus to thrive and continuously reinfect plants.

The fungicide chlorothalonil (Daconil) is effective in controlling blackspot by killing the fungal spores that spread the disease. However, optimal disease control with chlorothalonil requires frequent applications to protect newly developing leaves and to replace fungicide washed off by rain. Control of blackspot on roses in Alabama, therefore, may require more than 15 fungicide applications, at 7-10 day intervals, during the growing season.

Recent concerns about the safety and environmental impact of frequent fungicide use have caused rose growers to consider alternatives for control. Baking soda in solution with horticultural oil (a light petroleum oil labeled for control of insects), for example, has been shown to reduce diseases on roses in New York state. In addition, many people believe that fertilizing roses with epsom salts (MgSO₄) produces more vigorous plants.

Research underway at the E.V. Smith Research Center is evaluating these and other blacks pot control methods. Initial research showed that using horticultural oil may damage rose foliage due to the higher temperatures that prevail in Alabama. More recently, during the summer of 1995, applications of the horticultural oil solutions (oil alone and in suspension with baking soda) were alternated with chlorothalonil fungicide applications to avoid the phytotoxic effects previously observed. Oil solutions were applied weekly but were substituted with the fungicide when rainfall between sprays was less than 0.25 inch. Rain removed some of the oil and reduced phytotoxic effects.

Levels of blackspot disease, averaged over the entire season when roses were treated with alternating applications of horticultural oil and chlorothalonil, were statistically similar to disease levels on plants treated weekly with the fungicide and about 20% lower than on nontreated plants (Figure 1).

Figure 1. Treatments were applied weekly until run-off. “Fungicide” was Daconil 2787 at two teaspoons per gallon of water; one tablespoon of baking soda and two and a half tablespoons of oil were mixed with one gallon of water. “Baking soda-plus-oil” and “oil-alone” treatments were substituted with a fungicide application if no rain occurred between spray dates.

Numbers of flowers produced on rose plants treated with oil alternated with the fungicide were not statistically different from plants treated weekly with the fungicide. Still, more flowers were produced on plants sprayed alternately with oil and the fungicide than on nontreated or baking soda-pIus-oil treated plants.

In addition, alternating oil and fungicide according to rain events resulted in eight fungicide applications out of 22 total foliar applications.

Blackspot disease reduction and increased flower production on plants sprayed with the oil solution alternated with the fungicide, compared to nontreated plants, indicate that rose lovers can reduce the number of fungicide sprays using this alternate strategy. Although these spray schedules involve a weekly regime, the number of applications of potentially hazardous fungicides is reduced.

Nutrients such as calcium (Ca) and magnesium (Mg) have been associated with disease control and improved flower production on roses. This has caused some people to use epsom salts to treat roses. Indeed, one
manufacturer of epsom salts even includes a recommendation for rose use on its label of one cup espsom salts per plant per month.

To test this idea, a study was initiated in 1995 to evaluate different fertilization practices for roses and how these practices affect disease and plant growth. Treatments consisted of monthly applications of epsom salts (245 grams, or one cup), calcium chloride (CaCl₂, 133 grams), calcium sulfate [gypsum (CaSO₄, 163 grams)] plus sulfur (S, 27 grams), or no treatment. These nutrient applications were made in addition to regular N-P-K applications and were adjusted for equivalent amounts of Mg and Ca. Fertilizers were applied as granular formulations to the base of plants.

Preliminary data showed that epsom salt application (in addition to N-P-K), on a monthly schedule, did not reduce the disease incidence but did reduce defoliation compared to plants receiving no secondary nutrients. However, the plants treated with epsom salts or CaCl₂ were less vigorous and produced fewer flowers than nontreated plants or plants treated with calcium sulfate (Figure 2).

Figure 2. Disease, plant vigor, and flower numbers on rose plants treated with differential additional fertilizers, 1995.

These results suggest that specific nutrient treatments will not effectively control blacks pot disease in roses, but they may provide other benefits and lessen the need for application of fungicides.

Complete control of blackspot disease may never be attained without the use of fungicides, but proper management of rose plants will reduce the amount of fungicide needed. Proper rose management includes annual replacement of ground cover, proper pruning and fertilization, and removal of fallen leaves. Pruning and removal of debris is important because the fungus readily survives in fallen leaves, buds, or infected canes. Proper fertility will keep a plant in optimal health, which makes it less susceptible to disease.

Bowen is an Associate Professor of Plant Pathology, Behe is an Associate Professor of Horticulture, and Guertal is an Assistant Professor of Agronomy and Soils.