Rusty spot is an important disease of peach (Prunus persica L. Batsch) that occurs in many fruit-growing regions of the world (Dolovac et al., 2009; Grove, 1995b; Jankovics et al., 2011). Although the disease was first described in 1941, early attempts to identify the causal agent were thwarted as a result of sparse pathogen growth on fruit (Blodgett, 1941). Eventually, results from an inoculation study provided evidence that the causal agent may be Podosphaera leucotricha (Ellis & Everh.) E. S. Salmon, the apple powdery mildew pathogen, but results were not definitive (Manji, 1972). Subsequent field studies examining disease gradients in peach orchards adjacent to apple orchards provided further support for P. leucotricha as the causal agent (Daines and Trout, 1977; Ries and Royse, 1978). Recent molecular-based research examining the rDNA internal transcribed spacer sequences of fungal thallus from rusty spot lesions has confirmed the pathogen to be P. leucotricha (Jankovics et al., 2011).
Unlike peach powdery mildew, caused by Sphaerotheca pannosa, rusty spot symptoms only occur on fruit and not on leaves or shoots (Grove, 1995a, 1995b). Symptoms on young fruit consist of small, mostly round, orange–brown lesions that typically increase in size as the fruit grows. In disease-favorable seasons, lesions can cover large areas of the fruit surface and multiple lesions often coalesce. As the lesions age, the epidermis becomes necrotic and the trichomes detach. The resulting lesion surface becomes smooth and russetted, often rendering the fruit unmarketable. On highly susceptible cultivars, estimates of yield loss have been as high as $504 per hectare in disease-favorable growing seasons (Polk et al., 1997).
Temporal analyses of rusty spot epidemics revealed that disease increases in a logistic fashion from the shuck-off stage of fruit development until approximately 60 d after full bloom, a period ranging from 17 to 30 d (Furman et al., 2003a). This period coincides with the first stage of fruit development, which is characterized by rapid cell division (Flore, 1994; Tukey, 1933). 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). These studies also showed that demethylation inhibitor fungicides were much more effective than sulfur and that some quinone outside inhibitors also exhibited good efficacy. A recent evaluation of biorational materials known to be effective against powdery mildew pathogens revealed that some of these materials can consistently reduce rusty spot development (Lalancette et al., 2013). However, although these products significantly reduced disease, their level of control was low relative to conventional fungicides.
As outlined previously, much progress in understanding the etiology, epidemiology, and control of peach rusty spot has been made over the last decade. However, little information is available on the relative susceptibility of peach cultivars to rusty spot. Differences in disease levels among cultivars are often observed in any given year, but quantitative comparisons have not been performed. Consequently, the major objective of this study was to determine the relative susceptibility of commercial cultivars and advanced selections by comparing their disease levels over two growing seasons. The resulting susceptibility ratings will not only aid commercial growers in cultivar selection, but also provide important additional information for implementation of integrated disease management programs. A secondary objective was to characterize susceptible cultivars by identifying those horticultural traits that are associated with disease development. Finally, because a wide variation in disease levels was observed across the cultivars, the quantitative relationship between disease incidence and severity was investigated.
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