SUSCEPTIBILITY OF CITRUS-FRUIT TO BACTERIAL SPOT AND CITRUS CANKER

A pressurized spray (1 g/mm2) that water-soaked the rind of citrus fruit was used to obtain infection by Xanthomonas campestris pv. citri, X. c. citrumelo, and other X. campestris pathovars capable of infecting leaves of the citrus hybrid Swingle citrumelo (Poncirus trifoliata X Citrus paradisi). An aggressive strain of X. c. citrumelo readily infected fruit 20-40 mm in diameter, but fruit of smaller and larger diameters were not as susceptible. Marsh White and Marsh Red grapefruit cultivars developed larger lesions over a wider range of fruit sizes compared with Hamlin and Valencia sweet orange and Orlando tangelo. After 28 days, lesions caused by X. c. citrumelo strains did not expand further into rind tissue. Resistance of fruit to several strains of X. c. citrumelo and other pathovars of X. campestris, both of which produced small, discrete lesions, was confirmed by the inability of these strains to multiply in the rind tissue of Marsh White grapefruit. Nearly all strains of X. c. citrumelo were also incapable of sustaining growth and lesion expansion in leaf tissue of Ruby Red grapefruit and Swingle citrumelo; exceptions were aggressive strains, which produced expanding lesions on Swingle citrumelo. The relationship between fruit size and infection of citrus fruit cultivars by an Asiatic strain of X. c. citri was more susceptible to Asiatic citrus canker than Hamlin sweet orange, whereas Capurro mandarin was resistant. Unlike lesions produced by X. c. citrumelo, canker lesions continued to expand up to 106 days after inoculation of fruit 20-40 mm in diameter. Lesions did not expand on fruit > 60 mm in diameter.