Rose care and diseases are interlinked due to the frequency with which roses become ill due to improper maintenance. Understanding the appropriate care requirements, such as preferred soil type and sun exposure, allows your rose plant to flourish and remain vigorous. Diseases such as fungal infections are less likely to infect your roses if they remain vigorous.
Sun and Water Requirements
When caring for roses, be sure they are planted in an area exposed to full sunlight. Roses need at least six hours of direct sunlight (preferably in the morning) on a daily basis in order to thrive and grow successfully. Since morning dew on rose plants provides the moist breeding ground for a variety of diseases, early sunlight causes moisture to dry, reducing the potential risk of infection. Water lovers as well as sun lovers, roses need deep watering so moisture makes it down to the root system; do not overwater roses as wet sites are cause for disease concern. Wait for the top 2 inches of soil to dry out between waterings, according to the University of Missouri Extension.
Mulch and Pruning
Since roots need to be kept moist, but wet sites lead to disease problems for roses, the addition of a 2- to 3-inch layer of mulch can assist in keeping soil moist without overwatering. Instead of mulching all the way up to the surface of the rose's stem, lay mulch within a radius of 6 inches around the rose to create a catch for water and to keep surrounding soil cool. Another important aspect of rose care is pruning. Where mulch allows roses to thrive, pruning allows roses to continue growing and to rebloom. For seasonal care, give a conservative pruning during the fall season and a more aggressive pruning during the spring season. Correct pruning includes: leave two leaves per stem, prune to shape the plant, remove dead or diseased plant parts, use clean/sharp pruning shears and remove distracting branches that interfere with the natural direction of other branches, as suggested by the University of Missouri Extension.
Black spot is an infectious rose disease caused by a fungus called Diplocarpon rosae. Look for round black spots with yellow borders that reach up to 1/2 inch in diameter on leaf surfaces. This rose disease causes early leaf drop and branch dieback and can result in extreme weakening of the rose plant. For control, remove and destroy affected plant parts, prune diseased branches and apply a preventive fungicide (copper and lime-sulfur fungicides are appropriate). Apply the first treatment during the summer season up to two times a week (if growth is fast or weather conditions are moist); spraying a fungicide every 10 days is effective during normal growth habits and drier conditions. Cease fungicidal spray at first frost, according to the University of Minnesota Extension.
Brown canker is a fungal disease of roses caused by the fungus Cryptosporella umbrina. Look for small purple or red spots on stems; spots become gray to white and brown canker growths may appear. This disease may result in death. For control, remove, prune and destroy all affected plant parts and clean pruning shears before making a new cut as to prevent further spread of disease. Apply a sulfur-based fungicide for further control.