Integrated Control of Phytophthora Root Rot of Red Raspberry

Various components of an integrated program for controlling Phytophthora root rot of red raspberries were examined in a factorial experiment conducted for three growing seasons in a silty clay loam naturally infested with Phytophthora fragariae var. rubi. The factors were: (i) cultivar (highly susceptible Titan versus moderately resistant Newburgh); (ii) bed height (flat versus raised); (iii) straw mulch application (yes versus no); (iv) preplant treatment with the biological control fungus Trichoderma (=Gliocladium) virens (yes versus no); and (v) seasonal applications of the fungicide metalaxyl (yes versus no). Disease incidence was assessed visually, and severity was measured with respect to cane density, cane vigor, and yield. Cultivar was the most important factor, having a highly significant (P < 0.0001) main effect on nearly every measure of disease incidence and severity; it also interacted significantly with most other factors for at least some of these measures. In general, such interactions resulted from the positive response of Newburgh to treatments that had little effect on the consistently poor growth and yield of Titan, or from the variable response of Titan to treatments that had little effect on the consistently low visual disease ratings for Newburgh. Raised bed cultivation consistently reduced disease incidence and improved plant performance (particularly for Newburgh) and was the only factor other than cultivar that significantly improved yield, i.e., 5.6 versus 2.0 kg per Newburgh plot on raised versus flat beds, respectively. Although metalaxyl initially provided a substantial decrease in disease incidence on Titan, most of these plants eventually died. In contrast, metalaxyl provided modest yield benefits for Newburgh, and highest yields were obtained when metalaxyl was used in conjunction with raised beds, i.e., means of 2.2 versus 1.8 kg for the treated versus untreated plots on flat beds and 6.3 versus 5.1 kg, respectively, on raised beds. Straw mulch increased soil moisture levels and exacerbated disease incidence and severity. The biocontrol treatment had relatively little effect on disease, although there was some indication of marginally improved plant growth in treated plots.