Black spot of rose is an ascomyte fungal disease caused by the pathogen Marssonina rosae (Mr, the imperfect or asexual stage / Diplocarpon rosae (Dr) is the sexual stage). This pathogen is likely the most serious and common disease of roses in your garden. The fungus colonizes leaves of different species of the genus Rosa generally causing severe dis-ease characterized by dark circular necrotic spots on leaves, often surrounded by chlorotic spots and can result in defo-liation of the entire plant if left unchecked. It’s quite apparent on the upper surface of leaves where the spots can be up to 0.5” in diameter and typically have fringed borders. Chlorosis (yellowing of the leaf) begins surrounding the spots and the com-plete leaf may yellow and drop off (see images below, note the dy-ing stems).
Black spot will cause a general weakening of the plant such that fewer blooms result if the disease is not controlled and such weakened plants are then more likely to suffer winter (cold-related) injury. The fungus can also infect the canes (stems; note lesions on the cane in the image below).
Black spot survives the winter in infected leaves and canes. Rainy spring weather or overhead irrigation disperses spores (conidia) from fruiting structures (acervuli) and provides wet conditions for them to germinate on new leaves and canes. Seven hours of wetness at 65o to 75o F is sufficient for infection to take place. The black spot fungus penetrates the cuticle and grows between cells to infect the rose. New acervuli develop in the center of the lesions and release conidia that initiate new infections when condi-tions are wet long enough. This cycle repeats numerous times during the growing sea-son (hence cleanup of rose litter is important to limit inoculum for the following sea-son). The spores need to be wet for at least 7 hours in order to germinate and symp-toms can begin to appear in 3-16 days. Mature, spore-containing conidia can be pro-duced a few days later and lead to further spread of the disease (starting the cycle over again and increasing the coverage and spread of the disease and damage to the plant). As noted, spores germinate best at temperatures in the low to mid 60s (64°F is noted to be an optimal temperature) but germination will occur over a range of temperatures up to 81°F; this wide temperature range allows the disease to develop as long as moisture is adequate. In our humid sum-mers and with dew on leaves potentially prevalent overnight (in addition to any wetness caused by overhead irrigation and/or rain) disease spread in our region can be common. Hence limiting leaf wetness as much as possible is critically important.
Because the fungus occurs in various pathogenic races (i.e. strains of the fungus that are pathogenic to some cultivars and not others are termed a race), it is difficult to select for black spot resistance. However, there are rugosa, hybrid tea, floribunda, grandiflora, shrub and miniature roses that have some degree of resistance. For example, highly resistant cultivars include Bebe Lune, Carefree Beauty, Coronado, David Thompson, Ernest H. Morse, Fortyniner, Grand Opera, Lucy Cromphorn, Simplicity, Sphinx, and Tiara. Grow roses where good air circulation and sunlight penetration facili-tate rapid leaf drying. Irrigate to avoid wetting the leaves or water early in the day so the leaves dry quickly. Fungicides are useful during periods when the weather is wet to protect leaves and canes from infection. Begin in early spring and maintain the protection as needed throughout the exist.
Many fungicides are registered for control of black spot; IFAS publication PP268 (see 5a) has an extensive list, and notes that the fungicides should be regularly alternated to limit resistance buildup of fungal resistance to any one chemi-cal treatment. Research has not shown any “organic” methods to provide significant control. Planting of resistant culti-vars, appropriate irrigation management and sanitation of plant material offers the best approach to managing black spot on your roses. Consulting with local rose growers of the Augusta Rose Society in order to determine appropriate rose cultivars for our area, that require less intensive “chemical warfare”, can be extremely useful (see 6). Finally, the rose grower should be aware that there are a number of other fungi that can cause leaf spotting disease on roses including powdery mildew (Podosphaera pannosa), Cercospora leaf spot (Cercospora rosicola), Alternaria alternata, Colleto-trichum capsici, and Glomerella cingulata. However, Cercospora leaf spot is a disease often confused with black spot . Both diseases cause severe defoliation in heavily infected plants. The infection starts from the bottom of the canopy and progresses towards the tips where new growth is present. Of interest is a “List of pests and diseases of roses” on Wikipe-dia (see 7).
Tips for disease management:
1) Rake and discard all fallen leaves which are the main source of spores in the spring.
2) Prune and discard any obviously infected canes.
3) Avoid wetting the foliage especially during dark cloudy days.
4) Grow plants in an open sunny location to promote rapid drying of the foliage.
5) Do not plant in dense plantings and avoid windbreaks to allow good air circulation.
6) Use resistant varieties for low maintenance plantings.
7) Remove infected leaves during dry weather to help retard the rate of disease spread.
Annotated list of some resources:
1) A 2 page color pdf file from University of Tennessee listing a number of “no-spray” (i.e. disease-ressistant roses for the southeast) and suppliers.
2) A related pdf file was published by the Greater Gwinnett (GA) Rose Society. http://gwinnettrose.org/UT_Trial.html
3) HGIC 2106 – Rose Diseases: discusses a number of rose disease problems including blackspot, mentioning some resistant varieties. It also lists a number of fungicides for rose disease control. http://www.clemson.edu/extension/hgic/pests/pdf/hgic2106.pdf
4) Walter Reeves (“The Georgia Gardener”) has a nice web page on managing black spot disease in roses; it lists a great num ber of rose cultivars (this data is a from U Kentucky). http://www.walterreeves.com/landscaping/roses-black-spot-resistant/
5) IFAS (The University of Florida Institute for Food and Agricultural Sciences) has published a several informative reports on Rose culture and rose diseases:
A) http://edis.ifas.ufl.edu/pdffiles/PP/PP26800.pdf (Black Spot of Rose)
B) http://edis.ifas.ufl.edu/pdffiles/EP/EP33900.pdf (Growing Roses in Florida.)
C) http://edis.ifas.ufl.edu/pdffiles/EP/EP37100.pdf (Rose Pests and Diseases in Florida)
D) http://edis.ifas.ufl.edu/pdffiles/PP/PP26700.pdf (Cercospora Leaf Spot of Rose)
6) The Augusta Rose Society
7) Wikipedia (list of rose diseases and pests. http://en.wikipedia.org/wiki/List_of_pests_and_diseases_of_roses
Scientific References on Diplocarpon rosae:
8) Blechert, O. and T. Debener. 2005. Morphological characterization of the interaction between Diplocarpon rosae and vari ous rose species. Plant Pathology. 54:82-90. 9) Xue, A.G. and C.G. Davidson. 1998. Components of partial resistance to black spot disease (Diplocarpon rosae Wolf) in garden roses. HortScience. 33(1): 96-99.