Poster Session: Host Resistance

258-P

Resistance to Phytophthora in new rootstocks for almond and stone fruits.
L. S. SCHMIDT (1), R. G. Bhat (2), D. A. Kluepfel (1), G. T. Browne (1)
(1) USDA-ARS CPGRU, Davis, CA, U.S.A.; (2) University of California, Davis, CA, U.S.A.

Almonds and stone fruits suffer serious losses to several soilborne diseases, including Phytophthora crown and root rots. Restrictive soil-fumigation mandates are motivating the development of fruit and nut tree rootstocks with improved disease resistance. Accordingly, we tested 17 new almond/stone fruit rootstock genotypes and five widely-planted rootstocks for resistance to Phytophthora taxon ‘niederhauserii’ (Ptn), an aggressive pathogen of almond in the San Joaquin Valley of California. Rootstock clones were transplanted into potting mix that was non-inoculated (control) or inoculated with Ptn. All plants received bi-weekly 48-hour soil flooding periods. Resistance was assessed 8 weeks after transplanting. Susceptibility to Ptn was relatively high among rootstocks with: peach parentage (Empyrean #1; Harrow Blood x Okinawa clones 1, 10, 28, 32, and 50; Lovell; and Nemaguard); peach x almond parentage (Bright Hybrid clones 5 and 106; Garnem; and Hansen 536); and (peach x almond) x peach parentage (Rootpac 20 and Tempropac); the mean percentages of crown rot (CR) were 37-100%. In contrast, rootstocks that included some plum parentage (Controller 5; Krymsk clones 1, 2, 9, and 86; Marianna 2624; Myrobalan; and Replantpac) were less susceptible (CR 1-30%). Our results suggest that the inclusion of some plum parentage in a rootstock’s genetic background may increase resistance to Ptn.