In the spring of 2006, symptoms similar to those of fire blight were observed on pear (Pyrus communis), apple (Malus pumila), and quince (Cydonia oblonga) trees at the flowering and early fruit set stages in an orchard in the Meknès Region, 140 km east of Rabat. Symptoms consisted of i) water-soaked flowers that became wilted, shrivelled, and then turned brown to black; ii) “shepherd's crook” of the shoots; iii) dark green pedicles that became brown to black; iv) wilted and shrivelled leaves that turned brown but remained attached; and v) water-soaked fruits that became brown to black with droplets of exudates on the surface. In an effort to eradicate the disease, 42 ha of pears were dug up and burned in October 2006. In the spring of 2007, fire blight reappeared in the same orchard and was encountered in five other orchards with disease incidences from 1 to 60%. Three hectares of pears were removed and burned. Samples of diseased young shoots and fruits were collected. Bacteria were isolated either from washed tissues or directly from bacterial ooze on the host with King's B (KB) and semiselective CCT media (1,4). Colonies with morphology similar to that of Erwinia amylovora were purified through repetitive plating on KB medium. The isolates were first characterized based on colony morphology and biochemical and physiological tests (1). All isolates showed typical colony morphology on both media as compared with the reference strains of E. amylovora. They produced white colonies on KB medium, were gram negative, did not produce fluorescent pigment on KB, did not grow at 36°C, were levan positive, facultative anaerobes, and induced a hypersensitive reaction when infiltrated on tobacco leaves (cv. Xanthi). The isolates reacted negatively for oxidase, urease, indol, H2S production, and potato rotting tests, but they reacted positively for catalase and citrate tests. More than 87% of the isolates induced gelatine liquefaction, did not reduce nitrate, and produced acid from sorbitol but not from inositol, raffinose, and salicin. Thirty-three isolates were identified as E. amylovora by immunofluorescence microscopy with a polyclonal antibody (IF), double-antibody sandwich indirect-ELISA (3), and PCR (2). Pathogenicity was performed with a detached-leaf test tube assay (1). Each isolate was inoculated by wounding three young leaves of pears (cv. William's) along the main vein with a scalpel dipped in a 109 CFU/ml bacterial suspension. The inoculated detached leaves were kept at 25°C and 80% relative humidity in tubes with sterile 1% agar. Positive controls consisted of reference strains of E. amylovora. For negative controls, detached leaves were wounded with a scalpel dipped in sterile phosphate-buffered saline and the leaves were kept separate from the inoculated ones. Within a week, symptoms similar to those observed on leaves inoculated with the reference strains of E. amylovora, including discoloration, browning, and production of bacterial ooze along inoculated vein leaf, appeared. No symptoms developed on negative controls. Reisolation of bacteria produced colonies with characteristics like E. amylovora. Representative colonies were confirmed by IF. The occurrence of fire blight in Morocco creates a serious threat to the pome fruit industry.
References: (1) Anonymous. OEPP/EPPO Bull. 34:159, 2004. (2) S. Bereswill et al. Appl. Environ. Microbiol. 58:3522, 1992. (3) M. T. Gorris et al. Acta Hortic. 411:41, 1996. (4) C. Ishimaru and E. J. Klos. Phytopathology 74:1342, 1984.
