Leaves, fruits and seeds of melon (Cucumis melo) type Yellow were inoculated with Acidovorax avenae subsp. citrulli (Aac1) in order to study the penetration and colonization of this pathogen using scanning electron microscopy. Leaves sampled at 24, 48, 72, 96 and 120 h after inoculation showed clusters of bacterial cells interconnected by fibrils, and very rarely isolated. Bacterial cells were located mainly on the cuticular flange of the abaxial epidermis, on the base of tector trichomes, near the diacytic stomata, and inside the stomat pores. Twenty-four hours after inoculation bacteria were observed mainly on the surface of 37 day-old fruits concentrated around stomata and lenticels; after 48 h bacteria were detected in the fruit pulp as well. In the other samples (72 and 96 h) few or no bacteria were found on fruit surfaces. Fruits 51 days old showed expressive colonization of the surface, stomata and lenticels until 72 h after inoculation. The pathogen was only found in the pulp of the 96-hour samples. These results suggest that A. avenae subsp. citrulli penetrated the fruits through the stomata and lenticels. In seeds, the bacteria colonized the internal and external teguments, embryo and endosperm.
scanning electron microscopy; fruit blotch; Cucumis melo; ecology
