Association of Pectolytic Strains of Xanthomonas campestris with Soft Rots of Fruits and Vegetables at Retail Markets

Liao, C. H., and Wells, J. M. 1987. Association of pectolytic strains of Xanthomonas campestris with soft rots of fruits and vegetables at retail markets. Phytopathology 77:-418-422. Five strains of pectolytic, yellow-pigmented bacteria were isolated from xanthomonads might represent an unusual cluster of X. campestris that rotted specimens of tomato, bell pepper, cucumber, and papaya collected at exclusively attack plant materials after harvest and cause only soft rot retail markets. The bacteria possessed cultural, morphological, and symptoms. The tomato isolate excreted pectate lyase, pectin esterase, and physiological properties conforming to those of Xanthomonas campestris. polygalacturonase in medium containing polygalacturonic acid or pectin Each of the five strains obtained was capable of causing soft rots of but not in a medium containing glucose. Two strains of X. campestris pv. detached plant parts of eight different crops (including potato tubers, campestris also produced extracellular enzymes in pectate media and were carrot roots, celery petioles, cauliflower and broccoli curds, and fruits of capable of causing mild rots of potato slices and other plant parts. This cucumber, bell pepper, and tomato). The pathogenicity of each strain to study provides the first experimental results indicating that some plants (tobacco, cucumber, bell pepper, cabbage, and tomato) grown in the postharvest rots of plant crops may be attributed to pectolytic greenhouse could not be conclusively determined. These soft-rotting xanthomonads. Pectolytic activity in phytopathogenic xanthomonads has been representing pathovars campestris, phaseoli, and vesicatoria were observed in a large number of strains examined by several workers included. (1,5,11,15,26,28,31). Although pectolytic activity is often implicated in the ability of a bacterium to rot plant tissues (23), MATERIALS AND METHODS such a correlation does not fully account for the pathogenicity of xanthomonads. Symptoms caused by xanthomonads in the field, Rotted plant specimens. Rotted fruits of tomato, bell pepper, unlike those induced by soft rot erwinias and pseudomonads, are cucumber, and papaya were collected from local food stores in the usually blights, cankers, and necrotic lesions (22). The reason summer (June-September) of 1984. The specimens were used for xanthomonads encoded with pectolytic capability fail to induce isolations on the same day that they were brought into the typical soft rots in the field is not yet known. It has been suggested laboratory. Rots caused by bacteria could be visually (4,7), however, that xanthomonads may be occasionally associated differentiated from those caused by fungi. Bacterial rots frequently with rots of fleshy vegetables after harvest, led to total disintegration of fruit tissues, whereas lesions infected After harvest, plants and their products gradually lose vigor and by fungal rots very often retained firmness and coherence. defense mechanisms against invading microorganisms. Detached Specimens were regularly examined under the light microscope plant parts ready for marketing are therefore vulnerable to the before isolations were conducted to confirm the presence of attack of an array of bacteria that normally do not cause diseases of bacteria. plants growing in the field. The storage organs of some plants, such Isolation media and methods. The crystal-violet polypectate as tubers of potatoes, are affected by several genera of pectolytic (CVP) medium of Cuppels and Kelman (9) was used as an isolation bacteria generally considered as saprophytes in soil, including medium, but the concentration of sodium polypectate (Sunkist Clostridium (19), Bacillus (10), and Flavobacterium (18). Previous Grower Co., Inc., Ontario, CA) was reduced to 9 g/ 500 ml of studies on bacteria causing postharvest rots of vegetables have medium as suggested (27). The CVP medium was occasionally been limited to two genera (Erwinia and Pseudomonas). Efforts enriched with 0.1% yeast extract and later used to isolate a have not been made to investigate if pectolytic bacteria of other pectolytic strain of Cytophaga sp. (17) and two pectolytic, yellowgenera may be involved. pigmented xanthomonads (PF 083 and CJ 093, Table 1). Primary With these considerations, we reasoned that a reexamination of isolations were performed according to the conventional streakpectolytic bacterial flora associated with decays of fruits and plate method. A loopful of rotted tissues was streaked onto an agar vegetables was needed. A survey was conducted during which more plate, and incubation was at 26 C for 2 days. Single colonies were than 127 soft-rotting isolates belonging to the genera of Erwinia, individually picked and successively cloned for purity on the same Pseudomonas, Bacillus, Cytophaga, and Xanthomonas were medium at least three times. For long-term preservation, a low-salt collected. Properties of the first four of these genera have been medium containing beef extract (1%), proteose peptone (1%), and reported in depth elsewhere (16,17). This paper details the cultural, agar (0.9%) was stab-inoculated with bacteria and subsequently physiological, and pathological properties of five strains of yellowoverlaid with a layer (2-3 cm) of sterile mineral oil. pigmented bacteria tentatively identified as X. campestris. For Bacterial strains. In addition to five strains of pectolytic, yellowcomparative purposes, four known strains of X. campestris pigmented bacteria (PYB) obtained in the study (Table 1), four known strains of X. campestris (pv. campestris 42 and 43, pv. phaseoli, and pv. vesicatoria) were included. These known strains The publication costs of this article were defrayed in part by page charge payment. This were provided by E. Civerolo (USDA/ ARS, Beltsville, MD). The article must therefore be hereby marked "advertisement" in accordance with 18 U.S.C. § 1734 solely to indicate this fact. orange-pigmented Cytophaga sp. (strain PF 062) and the yellowpigmented Pseudomonas sp. (strain SJ 074) were isolated from This article is in the public domain and not copyrightable. It may be freely rotted bell pepper (17) and zucchini squash (16), respectively. The reprinted with customary crediting of the source. The American orange-pigmented Flavobacterium pectinovorum (strain 19399) Phytopathological Society, 1987. was obtained from the ATCC (Rockville, MD). The orange-