Raised Beds and Metalaxyl for Controlling Phytophthora Root Rot of Raspberry

Titan' red raspberry (Rubis idaeus L.), highly susceptible to root rot caused by Phytophthora fragariae Hickman var. rubi Wilcox & Duncan (syns. P. erythroseptica Pethyb., "highly pathogenic" P. megasperma Drechs.), was planted in June 1990 in a silt loam naturally infested with the pathogen. Raked beds (0.36 m high) dramatically reduced disease incidence and severity relative to flatbed treatments. In contrast, metalaxyl at 372 mg·m -1 of row provided little benefit when applied to flat beds and provided consistently moderate but statistically insignificant effects when applied to raised beds. Relative to the flat bed system, primocane vigor was increased in 1992 by 16%, 190%, and 224% in the flat bed plus metalaxyl, raised bed, and raised bed plus metalaxyl treatments, respectively; total yields were increased by 7%, 231%, and 272% with these same respective treatment. The results indicate that raised-bed planting systems can provide substantial control of phytophthora root rot of red raspberries even when highly susceptible varieties are grown on otherwise marginal sites. Metalaxyl appears more effective as a supplement rather than substitute for raised beds under such conditions. Chemical name used: N- (2,6- dimethylphenyl) -N- (methoxyacetyl)alanine methyl ester (metalaxyl). During the 1980s, phytophthora root rot became recognized as a major cause of declin- ing red raspberry plantings in commercial production areas worldwide. A general pro- gram has been proposed for controlling the disease (Wilcox, 1991)—one that integrates pathogen exclusion, site modifications to pro- mote water drainage (e.g., raised-bed planting systems), use of partially resistant cultivars, and treatment with specific fungicides. How- ever, this program's individual components are not equally practical or desirable. For instance, many raspberry fields are now con- taminated with pathogenic Phytophthora spp., precluding the possibility of pathogen exclu- sion when such sites are replanted. Further- more, some cultivars, which are otherwise among the most profitable and horticulturally desirable, are also highly susceptible to the disease (Barritt et al., 1981; Wilcox and Nevill, 1992). Therefore, the purpose of this experi- ment was to examine the efficacies of a raised- bed planting system and registered fungicide treatment, both singly and in combination, when replanting a highly susceptible cultivar in a pathogen-infested site. The experiment site, located near Geneva, N.Y., on Lima silt loam soil, was nearly level. A previous raspberry planting was removed from the site in 1989, following a prolonged