Pathogens Associated with Necrosis of Cactus Pear Cladodes in South Africa.

The commercial cultivation of spineless cactus (Opuntia ficus-indica (L.) Miller) for its fruit is a relatively recent undertaking in South Africa but has been shown to possess huge export potential. To date, only one fungal pathogen, Didymosphaeria opulenta (De Not.) Sacc., has been officially reported on the genus Opuntia in South Africa, but the report is from O. stricta Haw. and not O. ficus-indica (1). The need for research on diseases of O. ficus-indica in South Africa has recently become important since local growers are increasingly reporting disease-related yield losses. Surveys conducted over a period of 3 years indicated that stems or cladodes are particularly prone to various forms of tissue necrosis, caused primarily by three fungi, which can ultimately lead to death of entire cladodes. Alternaria tenuissima was isolated from a dry superficial necrosis of the cuticle and underlying tissue as much as 3 mm deep. Symptoms include small chlorotic spots on the cuticle, which coalesce to form raised gray scabs. Fusarium sporotrichoides was isolated more commonly from dry necrotic lesions that were darker, larger, and less superficial, sometimes extending through the tissue to the opposite side of the cladode. Lasiodiplodia theobromae (teleomorph Botryosphaeria rhodina) was isolated from roundish black cankers (15 to 50 mm diameter) on cladodes and characterized by black gum exudation from the perimeter of the canker. Pycnidia were often evident on the surface of the canker. The fulfillment of Koch's postulates demonstrated that an isolate of each respective species was very aggressive in colonizing cladodes following artificial inoculations in the glasshouse. Mean lesion diameters measuring 15, 27, and 44 mm for A. tenuissima, F. sporotrichoides, and L. theobromae, respectively, were recorded 14 days after inserting wooden toothpick tips that had been colonized by the three pathogens into each of five cladodes of 18-month-old potted plants of O. ficus-indica (cv. Morado). Alternaria sp. and B. rhodina have been reported on Opuntia sp. in the United States (2), but no records of the above three fungi occurring on O. ficus-indica were found. References: (1) P. W. Crous et al. Phytopathogenic Fungi from South Africa. University of Stellenbosch, Department of Plant Pathology Press, Stellenbosch, South Africa, 2000. (2) D. F. Farr et al. Fungi on Plants and Plant Products in the United States. The American Phytopathological Society, St. Paul, MN, 1989.