Root rot caused by the persistent soilborne oomycete Phytophthora fragariae var. rubi is a serious disease of red raspberry in the Pacific Northwest and worldwide. It can decrease the vigor and yield of raspberry plantings and require accelerated replanting cycles, especially for susceptible cultivars. The pathogen thrives in cool soils that regularly experience prolonged periods of high soil moisture (Wilcox, 1989). Field symptoms of diseased raspberry plants often include reduced primocane emergence, wilted and chlorotic canes, and necrotic primocane lesions that spread upward from infected roots (Kennedy and Duncan, 1991; Wilcox, 1989). Phytophthora fragariae var. rubi is considered the dominant disease agent of root rot in red raspberries (Wilcox et al., 1993), although several other soilborne pathogens may contribute to a disease complex affecting raspberries (Schilder, 2007; You et al., 2006).
Integrated management of root rot in raspberry plantings can include the use of raised beds, soil solarization, preplant incorporation of gypsum or calcitic lime, application of registered fungicides, and the planting of resistant cultivars (Heiberg, 1999; Maloney et al., 2005; Pinkerton et al., 2002; Wilcox et al., 1999b). Of these, resistant cultivars are critical for long-term control. In a study examining the effect of cultivar, bed height, biological control treatment, fungicide (metalaxyl), and straw mulch on raspberry growth and disease symptoms, cultivar susceptibility was the most important factor (Wilcox et al., 1999b). Both raised beds and metalaxyl application provided slight benefit to the resistant cultivar Newburgh but did not improve growth or yield outcomes for the susceptible cultivar Titan (Wilcox et al., 1999b).
A few cultivars are resistant to root rot such as ‘Newburgh’ and ‘Latham’, but most commercial cultivars are susceptible to root rot and eventually show disease symptoms (Pattison et al., 2004). These include ‘Meeker’, which accounts for more than 70% of raspberry plantings in the Pacific Northwest (Washington Red Raspberry Commission, 2004), and ‘Willamette’ (Barritt et al., 1981). Several new cultivars adapted to the Pacific Northwest have unknown responses to root rot. A major goal of raspberry breeding programs is to combine root rot resistance with superior horticultural traits in new cultivars; identifying genetically resistant material is advantageous for commercial production and breeding efforts.
Field evaluations of raspberry genotypes for resistance to P. fragariae var. rubi have the advantage of being similar to the conditions of commercial raspberry plantings. However, field evaluations are time-consuming, as symptoms may take several years to develop, and soil conditions and disease pressure may not be uniform. Greenhouse pot tests (Bristow et al., 1988; Kennedy and Duncan, 1991; Laun and Zinkernagel, 1993; Wilcox et al., 1999a) and hydroponic culture systems (Pattison et al., 2004) can provide greater control of environment and disease pressure, but typically use small plants that may not respond in the same manner as mature, well-established plants under field conditions. ‘Cowichan’ was moderately resistant to root rot in a greenhouse pot test but was shown to be susceptible in subsequent field plantings (Kempler et al., 2005). This raises concerns about the applicability of greenhouse results to field performance. The objective of this research was to evaluate the relative susceptibility of red raspberry genotypes to root rot caused by P. fragariae var. rubi in field and greenhouse conditions and to compare the responses.
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