Peach Rusty Spot Epidemics: Management with Fungicide, Effect on Fruit Growth, and the Incidence-Lesion Density Relationship

Abstract

Different numbers of consecutive fungicide applications, beginning at petal fall and continuing into the summer, were examined for their effect on rusty spot epidemics. Disease progressions for each fungicide level were quantified by fitting either the logistic or monomolecular model. When the weighted absolute infection rate (ρ) and maximum disease level (Kmax) parameters were expressed as functions of the number of applications, the logistic decline model provided the best fit for five of six data sets. This model described a gradual decrease in ρ and Kmax in response to the initial fungicide application, a rapid decline in parameter values with the addition of one or two applications, and a diminished parameter response as fungicide applications continued toward the end of the epidemic. Based on examination of model behavior across all 3 years of the study, adequate management was achieved with a total of three to five fungicide applications. Additional analyses of area under the disease progress curve and final disease intensity at harvest supported these results and indicated that further reduction in fungicide usage may be possible. Unlike earlier findings, rusty spot did not significantly decrease fruit volume or weight at midseason or at harvest; as lesion density increased, fruit volume remained constant. The relationship between disease incidence and lesion density within any given year was best explained by the zero-intercept version of the exponential model. However, comparison of model parameters across years revealed significant seasonal variation. Nevertheless, the incidence-lesion density relationships were fairly uniform across years at incidence values below 0.5, where lesion density increased gradually and in a near-linear fashion.

Peach Rusty Spot Is Caused by the Apple Powdery Mildew Fungus, Podosphaera leucotricha

“…E.S. Salmon) was generally thought to be responsible for the symptoms (8,12,13,19,21,26,27) but additional, unidentified causal agents were also suspected to play a role in this disease (12,13,31). P. leucotricha was considered to be the rusty spot pathogen because the disease occurred mainly on peach trees planted close to apple orchards infected by P. leucotricha (8,26,27) and powdery mildew tnyceliutn was sometimes found on symptomatic peach fruit (7,26).…”

Susceptibility of Peach Cultivars to Rusty Spot and Characterization of Susceptibility Groups

“…The quantitative relationship between disease incidence and lesion density, derived from observations on all 33 cultivars, was successfully modeled using the zero intercept version of the exponential function. This same model, based on data from a single cultivar, was previously shown to provide the best overall fit for this relationship (Furman et al, 2003b). Predictions of lesion density from the current and prior models were nearly identical in the low to moderate disease incidence range of 0 # i # 0.5, whereas considerable divergence occurred at higher disease incidences.…”

“…Additional data are needed to verify the integration of cultivar resistance, biorational fungicides, and conventional fungicides for effective management of peach rusty spot. (Furman et al, 2003b). Each data point represents the incidence and severity calculated for a single tree (40 fruit/tree).…”

“…The model was fit using nonlinear regression analysis with the marquardt iterative method (NLIN procedure; SAS Version 9.3). This model was previously determined to provide the best overall fit for this relationship on 'Jerseyqueen' (Furman et al, 2003b). In this study, data consisted of incidence and lesion density estimates from each replicate tree of each cultivar.…”

“…As fruit growth slows and approaches initiation of pit hardening toward the end of this stage, the rate of disease increase slows and approaches zero. Given this pattern of disease and fruit growth, the optimum timing for fungicidal control consists of three to five applications beginning at 90% to 100% petal fall (Furman et al, 2003b). Subsequent efficacy studies have demonstrated that four fungicide sprays, applied at petal fall, shuck-split, first cover, and second cover, provide adequate control in most years (Lalancette et al, 2006;Lalancette and Foster, 2001).…”