Submitted to: International Congress of Plant Pathology Abstracts and Proceedings
Publication Type: Abstract Only
Publication Acceptance Date: August 9, 1998
Publication Date: N/A
No significant differences were found in the population sizes of the DhrpZ mutant relative to B728a on emerging seedlings and leaf samples. The DhrpZ mutant was isolated from brown spot lesions as was the wild type. Hence, hrpZ in Pss B728a appears neither to be3re- required for growth/survival nor for disease causation. Growth of the hrpJo and hrcC mutants was similar to that of the wild type on germinating bean seeds. At 12 days after planting (DAP) relatively large numbers (>105 to 106 cfu/leaf) of both mutants were found on some of the developing primary leaves. At subsequent sampling times, leaf-associated population sizes of the mutants diminished even during periods when numbers of B728a increased. Hence, mutations in hrp genes that affect the Hrp secretion system substantially affect growth and survival of Pss in association with bean leaves but not with seedlings before emergence. Population sizes greater than roughly 105 to 106 cfu/leaf are sufficiently large to be predictive of disease for strains capable of causing brown spot disease. By 14 DAP, lesions were detected in plots inoculated with the hrpJo and hrcC mutants. The only occupants of the lesions examined in detail to date from these plots, were either the hrcC or hrpJo mutant. Isolates purified from these lesions were nonpathogenic when inoculated into growth chamber bean plants-the expected reaction for hrp mu-tants. Under some conditions, hrp mutants are able to grow to sufficiently large population sizes to cause disease in the field. Hence, hrp genes are not absolutely required for lesion formation.