Many gardeners consider roses to be among the finest flowers that can be grown. Roses have a universal appeal, says Dr. A. G. Smith Jr., Virginia Tech horticulturist emeritus, who experimented with them for many years. "The blooms attract the eye through a wide range of colors and forms. The texture of the petals appeals to our sense of touch, while the fragrance of the rose is admired throughout the world."
It is possible to have some nice roses around the home with very little effort, Smith says. "On the other hand, one may take his rose-growing so seriously that it becomes a burden mentally, physically and economically."
The main problems with roses are inadequate sunlight, soil compaction, too much fertilizer, too heavy pruning, over-watering, blackspot and powdery mildew diseases, and insects.
Some gardeners grow roses year after year without having to spray for blackspot or mildew. Others are obliged to spray every year, from the time of the last frost in the spring until the first frost of fall, in order to get good roses.
Perhaps in years to come there will be disease-resistant roses. A rose has been developed at USDA Science and Education Research Center, Beltsville, that is immune to the strains of the blackspot fungus native to the East, South and Midwest. Budwood has been released to commercial and amateur breeders, and immune roses may be available for purchase in three to five years.
In took 20 years of experimenting and cross-breeding to develop the rose that is immune to blackspot.
The two-celled spores of the blackspot fungus are spread over short distances in drops of water, by wind and on passing animals. The spores must be wetted before they will germinate. Water droplets accrue on rose plants during rainfall, by condensation and in watering. Spores touched by water may germinate if evaporation of the water is not too rapid or the temperature too high or too low.
They germinate best at 79 degrees f., and not at all as low as 55 or above 91. Because the spores are heavy and sink rapidly in water, infection occurs mainly through the upper epidermis of leaflets, rachis, petiole or stipule of the expanding leaf.
Infection of growing stems probably takes place in the axil of buds or leaves along the lower edge of droplets of water or where the mucilaginous spores have stuck. Petals and sepals may also be invaded.
The spores of the powdery mildew fungus germinate best when the relative humidity ranges between 95 and 99 percent. There is no germination below 75 relative humidity, and germination is poor between 75 and 95 percent, at 100 percent and in standing water.
The range of temperatures which are optimum for both germination of the spores and subsequent infection is 65 through 75 degrees. The light, shortlived, wind-disseminated spores infect through all surfaces of aerial portions of the plant.
In almost every instance a fungicide prevents a potential infection rather than cures an established one, and for absolute protection complete coverage of all aerial shoots at all times would be necessary.
Dr. Eldon W. Lyle, plant pathologist and director of the Texas Rose Research Foundation, recommends the use of benlate, or manzate or Dithane M-22, or Phaltlan for blackspot control.
For control of powdery mildew, Acitione PM or benlate is recommended.
In all cases, recommendations on the label for mix and application should be followed closely.