Roses are traditionally renowned as royalty among flora and fauna lovers. They have become an icon of love, beauty and innocence. However, roses have also become synonymous with black spot, a fungal disease that can infect all rose varieties. If your roses have black or dark brown spots on the leaves, specifically after a rainy spring season, then you have black spot. Left unchecked, black spot can cause long-term problems such as less blooms and a weakened plant.
Black spots on roses are caused by a fungus called Diplocarpon rosae. According to the University of Illinois Extension Service, this fungus generally occurs during the winter when the fungus attaches itself to the rose canes and fallen leaves. "Primary infection occurs when spores are splashed to new leaves in the spring," UIE states. Secondary cycles may occur throughout spring and well into the summer months.
There are a variety of conditions that facilitate black spot. The University of Maine Cooperative Extension asserts that the fungus requires free water for infection to occur. "The spores must be wet for seven hours before they can germinate," the website states. In addition, humid conditions and warm temperatures promote black spot infection.
The most obvious symptom of black spot on roses is the spots. Spots are generally found on the upper leaf surface and can be 1/2 inch in diameter. The spots are black or dark brown and may have a fringed border. Commonly the leaf will turn yellow around the spot and spread to the tips of the leaf. Eventually the leaf will drop off the plant. According to UM, the fungus may also infect the canes of the plant, causing a dark purplish lesion that will later turn black.
Black spot of roses can cause an assortment of long-term complications. In addition to unsightly leaf blemishes, this fungus can cause overall disfiguration of the plant. UIE asserts that in time black spot will weaken the plant, making it more susceptible to contracting other disease as well as complete defoliation.
Diplocarpon rosae often infects plants through fallen leaves that are allowed to sit in wet conditions. For this reason, raking all fallen leaves prior to the winter or spring season is beneficial for preventing fungus infection. In addition, the University of Massachusetts Extension Service recommends watering rose plants below the leaves, to avoid water standing on the leaves for long periods or time. This can also be avoided by watering rose plants early in the day to allow the sun to dry the leaves. However, once infection has occurred a fungicide can be used to protect new leaf growth. During the rainy season fungicides need to be applied often.
Resistant Varieties and Cultivars
Although all rose plants are susceptible to black spots there are some varieties that have a higher resistance than others. UM asserts that rugosa, hybrid teas, floribunda, grandiflora, shrub and miniature roses have some resistance to block spot. Among these rose varieties, UM states that there are a number of highly resistant cultivars, including Bebe Lune, Carefree Beauty, Coronado, David Thompson, Ernest H. Morse, Fortyniner, Grand Opera, Lucy Cromphorn, Simplicity, Sphinx and Tiara.