Abstract
Rootstock germplasm from the USDA Horticultural Research Lab breeding program was evaluated in each of four growing seasons at the Florida Research Center for Agricultural Sustainability (FLARES) in Vero Beach. The screening site is located on Winder and Manatee fine sand soils naturally infested with Diaprepes abbreviatus, and Phytophthora nicotianae and P. palmivora. Seedlings previously grown in conetainers were field planted into a mixture of rhizosphere soil with fibrous roots from beneath 'Sunburst' trees on Swingle rootstock adjacent to the test block supporting both Phytophthora spp. Adjacent trees also served as a source of egg-laying adults of D. abbreviatus. Seedlings were planted in May 2002 and 2003 and in Jan. 2005 and 2006. Seedlings were harvested after 6, 7, 10, and 10 months, respectively. At harvest, rhizosphere soil was taken from beneath each tree for enumeration and identification of Phytophthora spp. Root systems were visually rated for root rot by the fungi and feeding damage by the weevil on a scale from 1 to 5 (1 = no damage, 5 = severe root damage). When 2002 and 2003 data were combined, there was a significant positive correlation between whole-root system damage and total Phytophthora spp. populations. Among the genotypes, mandarins and pummelo hybrids showed greater tolerance to the Phytophthora-Diaprepes (PD) complex than trifoliate orange and some of its hybrids. In 2005 and 2006, screening focused on hybrids of pummelo and mandarins. In these two seasons, phytophthora populations were lower overall (<20 propagules/cm3), and no relationship between populations and root damage was detected for these genotypes. Tolerance of genotypes tested in the third and fourth seasons was greater than for genotypes tested in the first two seasons. Findings confirm the promise of certain pummelos and mandarins as parents for hybrids with requisite Phytophthora resistance to develop rootstock tolerance to PD complex in the field.
