Z. A. Mior Ahmad, and
G. Vadamalai, Department of Plant Protection, Faculty of Agriculture, Universiti Putra Malaysia, 43400UPM, Serdang, Selangor, Malaysia
Cabbage (Brassica oleracea L. var. capitata L.) is one of the most important vegetables cultivated in Pahang and Kelantan, Malaysia. Pectobacterium carotovorum can cause soft rot on a wide range of crops worldwide, especially in countries with warm and humid climates such as Malaysia. Cabbage with symptoms of soft rot from commercial fields were sampled and brought to the laboratory during the winter of 2010. Disease symptoms were a gray to pale brown discoloration and expanding water-soaked lesions on leaves. Several cabbage fields producing white cultivars were investigated and 27 samples were collected. Small pieces of leaf samples were immersed in 5 ml of saline solution (0.80% NaCl) for 20 min to disperse the bacterial cells. Fifty microliters of the resulting suspension was spread on nutrient agar (NA) and King's B medium and incubated at 30°C for 48 h. Purification of cultures was repeated twice on these media. Biochemical and phenotypical tests gave these results: gram negative, rod shaped, ability to grow under liquid paraffin (facultative anaerobe); oxidase negative; phosphatase negative; positive degradation of pectate; sensitive to erythromycin; negative to Keto-methyl glucoside utilization, indole production and reduction sugars from sucrose were negative; acid production from sorbitol and arabitol was negative and from melibiose, citrate, and raffinose was positive. Hypersensitivity reaction on tobacco leaf with the injection of 106 CFU/ml of bacterial suspension for all strains was positive. Four representative strains were able to cause soft rot using cabbage slices (three replications) inoculated with a bacterial suspension at 106 CFU/ml. Inoculated cabbage slices were incubated in a moist chamber at 80% relative humidity and disease symptoms occurred after 24 h. Cabbage slices inoculated with water as a control remained healthy. The bacteria reisolated from rotted cabbage slices on NA had P. carotovorum cultural characteristics and could cause soft rot in subsequent tests. PCR amplification with Y1 and Y2 primers (1), which are specific for P. carotovorum, produced a 434-bp band with 15 strains. PCR amplification of the 16S-23S rRNA intergenic transcribed spacer region (ITS) using G1 and L1 primers gave two main bands approximately 535 and 580 bp and one faint band approximately 740 bp when electrophoresed through a 1.5% agarose gel. The ITS-PCR products were digested with RsaI restriction enzyme. According to biochemical and physiological characterictics (2), PCR-based pel gene (1), and analysis by ITS-PCR and ITS-restriction fragment length polymorphism (3), all isolates were identified as P. carotovorum subsp. carotovorum. This pathogen has been reported from Thailand, Indonesia, and Singapore with whom Malaysia shares its boundaries. To our knowledge, this is the first report of P. carotovorum subsp. carotovorum in cabbage from Malaysia.
References: (1) A. Darraas et al. Appl. Environ. Microbiol. 60:1437, 1994. (2) N. W. Schaad et al. Laboratory Guide for the Identification of Plant Pathogenic Bacteria. 3rd ed. The American Phytopathological Society, St. Paul, 2001. (3) I. K. Toth et al. Appl. Environ. Microbiol. 67:4070, 2001.