Cactus pear tissues have a relatively high water content and therefore infections, especially bacterial infections, rapidly turn into rot (Table 14.2). Saprophitic colonization occurs, which makes it difficult to isolate the pathogenic agent. Bacteria are prokaryotes. About a hundred species lead to plant and animal diseases (Bradbury 1970; Buchanan and Gibbons 1974). The most common pathogenic genera of bacteria are Agrobacterium, Corynebac-terium, Erwinia, Pseudonomas, and Xanthomonas (Skerman et al. 1980; Krieg 1984).
Bacterial agents do not possess penetrating mechanisms and can only access plants through wounds. Infection requires specific weather conditions characterized by low temperatures and elevated atmospheric humidity plus the presence of a film of water on the plant tissues. Consequently, bacterial diseases are more widespread in moist regions or during wetter times of the year. Bacteria-induced symptoms are caused by cell death (necrosis) and sometimes abnormal growth (tumors) of the infected organs due to hyperplasia (abnormal cell multiplication) or hypertrophy (abnormal cell size), and they generally result in soft rots in the Cactaceae (Lelliott and Stead 1987).
Bacterial spot has been reported from India, Italy, and certain other cactus-pear growing countries (Argentina, Chile, and Mexico). It is the most severe cactus pear dis-
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