AVOIDING AND CONTROLLING ROSE DISEASES
By Dr. Leonard Perry, Extension Professor
University of Vermont
That said, keep in mind that the health and vigor of your roses depend on two things: the weather and the cultural practices you follow. The first you can't control, but the second one you can. Let's start with what can be controlled.
Roses prefer a planting site with good drainage and ventilation. Avoid shady spots and dense plantings. Good air circulation helps the leaf surfaces dry faster, which helps prevent disease. Roses also need to be fertilized on a regular basis. Apply fertilizer specific to roses, which is available at full-service garden centers.
What gardeners can't control is the weather. Wet, humid conditions increase disease problems in roses, especially weaker ones. However, one of the more common rose diseases--powdery mildew--can occur in dry as well as humid weather.
If you see a white, talcum powder-like film on the top and bottom of the leaves and stems, your roses are infested with powdery mildew. Some varieties are more susceptible than others. Plants also may exhibit signs of stunted and curled leaves. As a rule, this disease develops when daytime temperatures are about 80 degrees F with a humidity of 40 to 70 percent, and nighttime temperatures are 55 to 60 degrees F.
Old-fashioned rose varieties often fall victim to black spot disease, especially when weather is wet with temperatures around 65 to 75 degrees F. This disease is not a problem with the newer, hybrid tea roses, which have been bred with resistance to this disease.
How do you know if your roses have black spot? Look for circular black spots with fringed edges on the leaves. The spots may be tiny or more than one-half inch in diameter. If infestation is severe, defoliation may occur.
Another disease that infests roses is crown gall. It is caused by a soil and rootstock-borne bacterium that attacks a number of different crops and ornamentals. It is more of a problem for peaches, cherries, and other stem fruits, but can be a problem on woody ornamentals, including roses.
Crown gall bacterium is sometimes introduced on transplant stock. Once established, the bacteria are moved in soil and water, as well as during cultivation practices like weeding, grafting, and pruning. They can survive in the soil, but will enter plants only through wounds.
To avoid crown gall disease, buy disease-free plants or choose resistant varieties. Avoid wounding plants during transplanting.
Many rose diseases, including crown gall disease, can be controlled or prevented by following these practices:
* Select disease-resistant varieties if available.
* If plants are diseased, remove and destroy infected leaves and canes during the season. Never compost diseased material.
* Avoid overhead watering.
* Apply protective fungicide sprays as soon as the growing season begins. Continue applying the sprays every seven to 12 days until fall. During wet periods, apply once a week. During dry weather, once every 10 to 12 days should be adequate.
* Remember, all fungicides are not alike. Be sure to chose the right product for the job. When properly applied, they will penetrate the foliage and kill the fungus in the leaf and stems. However, they cannot restore the normal green color of a plant already spotted or yellowed from disease.