Integrated control of Phytophthora root rot of red raspberry

Various components of an integrated program for controlling Phytophthora ro ot rot of red raspberries were examined in a factorial experiment conducted for three growing seasons in a silty clay loam naturally infested with Phy taphthora fragariae var. rubi. The factors were: (i) cultivar (highly susce ptible Titan versus moderately resistant Newburgh); (ii) bed height (flat v ersus raised); (iii) straw mulch application (yes versus no); (iv) preplant treatment with the biological control fungus Trichoderma (=Gliocladium) vi rens (yes versus no); and (v) seasonal applications of the fungicide metala xyl (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) mai n effect on nearly every measure of disease incidence and severity; it also interacted significantly with most other factors for at least some of thes e measures. In general, such interactions resulted from the positive respon se of Newburgh to treatments that had little effect on the consistently poo r growth and yield of Titan, or from the variable response of Titan to trea tments that had little effect on the consistently low visual disease rating s for Newburgh. Raised bed cultivation consistently reduced disease inciden ce and improved plant performance (particularly for Newburgh) and was the o nly 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 inc idence on Titan, most of these plants eventually died. In contrast, metalax yl provided modest yield benefits for Newburgh, and highest yields were obt ained 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 an d 6.3 versus 5.1 kg, respectively, on raised beds. Straw mulch increased so il moisture levels and exacerbated disease incidence and severity. The bioc ontrol treatment had relatively little effect on disease, although there wa s some indication of marginally improved plant growth in treated plots.