Root Traits Associated with Phytophthora Root Rot Resistance in Red Raspberry

Phytophthora root rot is a serious problem for commercial production of red raspberry (Rubus idaeus L.). Developing new cultivars with high resistance or tolerance to Phytophthora is therefore critical to sustaining profitable production and is a major focus of numerous raspberry breeding programs. Current breeding efforts to identify resistant genotypes screen large numbers of plants in the greenhouse and field, and select those demonstrating high tolerance to the disease. Little is known, however, why certain genotypes exhibit a better response than others under the presence of Phytophthora spp. In August 2009, we started collecting root images from minirhizotron tubes installed in a field trial of seven cultivars with varying degrees of resistance to root rot. The objective was to identify prominent root traits associated with little or no Phytophthora infection in raspberry so that the traits can be selected and incorporated into breeding material. The trial is located at the Washington State University Puyallup Research Center at a site with a long history of problems with root rot. The cultivars include 'Summit', which is highly resistant to root rot, 'Cascade Bounty' and 'Cascade Delight', also found to have high resistance, 'Meeker', the industry standard in the region with moderate resistance to root rot, 'Tulameen', which is similar to 'Meeker' in resistance, and 'Malahat' and 'Saanich', two cultivars highly susceptible to root rot. Our first year results indicate that cultivars with the highest resistance to root rot produced the most roots and the deepest root system. Resistant cultivars also had considerably less infection, according to qPCR analysis, by P. rubi, which is the species most often associated with root rot in raspberry. We began to examine the roots for evidence of suberin deposition, a physiological trait in some plants that inhibits hyphal penetration by soil pathogens such as Phytophthora. Preliminary histochemical observations on first-and second-order roots revealed that the most resistant cultivar, 'Summit', had less suberin in the root exodermis than blackberry, considered completely resistant to Phytophthora, but significantly more suberin than the least resistant cultivar, 'Malahat'. The work is ongoing, but so far, it appears that root rot resistance may be related to both rapid root production and deposition of suberin in the exodermal layer of the root cortex. If this is the case, the traits could be selected during breeding to increase raspberry resistance to root rot.