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Title: Screening citrus rootstock genotypes for tolerance to the Phytophthora-Diaprepes Complex under field conditions

Author
item GRAHAM, JAMES - UNIVERSITY OF FLORIDA
item Bowman, Kim
item BRIGHT, DIANE - UNIVERSITY OF FLORIDA
item ADAIR, JR., ROBERT - FLARES, VERO BEACH, FL

Submitted to: Proceedings of Florida State Horticultural Society
Publication Type: Proceedings
Publication Acceptance Date: 11/1/2007
Publication Date: 12/1/2007
Citation: Graham, J.H., Bowman, K.D., Bright, D.B., Adair, Jr., R.C. 2007. Screening citrus rootstock genotypes for tolerance to the Phytophthora-Diaprepes Complex under field conditions. Proceedings of Florida State Horticultural Society. 120:97-103.

Interpretive Summary: The pest and disease problem known as the Phytophthora-Diaprepes Complex causes a severe damage or tree death in many Florida citrus production situations. Development of rootstocks resistant to this complex has been an important component of the USDA rootstock development program and screening of candidate rootstocks for field resistance has been a cooperative project with researchers at University of Florida and Florida Research Center for Agricultural Sustainability. This report details results from some of the cooperative field testing for resistance and highlights good performance from several new USDA rootstock selections.

Technical 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 in Vero Beach, Florida. The screening site is located on Winder and Manatee fine sand soils naturally infested with Diaprepes abbreviatus, and Phytophthora nicotianae and P. palmivora. Seedlings and cuttings previously grown in containers 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 January 2005 and 2006. Seedlings were harvested after 6, 7, 10 and 10 months, respectively. At harvest, rhizosphere soil samples were 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 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 rootstocks tolerant to the PD complex in the field.