Authors
H. Scherm and
A. T. Savelle, Department of Plant Pathology, University of Georgia, Athens 30602;
R. T. Boozer, Alabama Cooperative Extension System, Chilton Research and Extension Center, Clanton 35045; and
W. G. Foshee, Department of Horticulture, Auburn University, Auburn, AL 36849
ABSTRACT
Conidia produced on overwintered lesions on 1-year-old twigs constitute the only source of primary inoculum for the peach scab fungus, Fusicladium carpophilum; however, there is little quantitative information about the dynamics of sporulation throughout the season. Starting in late winter and continuing until midsummer over a 4-year period, twig segments were sampled every 1 to 2 weeks from peach trees untreated with fungicide from a total of 18 trials (site--cultivar--year combinations) in Georgia, Alabama, and South Carolina. Twig samples were incubated in a moist chamber in the laboratory for 48 h and washed on a wrist-action shaker, and conidial production potential was determined by microscopic counts in aliquots of the wash water. When plotted against calendar date (day of the year), there was considerable variation among cultivars, sites, and years in the temporal pattern of conidial numbers of F. carpophilum. For example, conidia first were detected on samples collected between mid-February and late March, and the highest peak in conidial numbers was observed between late March and mid-May. In contrast, when conidial numbers were expressed as cumulative totals in relation to phenological time (either days after full bloom or days after calyx-split), temporal progress was very similar among trials. Conidial production summarized in this manner generally commenced before bloom and reached 25 and 90% of the seasonal total by calyx-split and 10 weeks after bloom, respectively. A two-parameter sigmoidal function described the relationship between cumulative conidial production and phenological time very well (R2 = 0.9727 and 0.9790 for days after full bloom and days after calyx-split, respectively; P < 0.0001, n = 260). Expression of time in degree-days did not improve the relationship between cumulative conidial numbers and phenological time. Thus, knowledge of host tree phenology may be sufficient to derive strategic estimates of disease risk based on the predictable seasonal pattern of conidial production potential; this seasonal, inoculum-based risk estimate may be used to adjust daily infection risk estimates based on models that consider microclimatic conditions affecting pathogen growth and infection.
