UC Riverside International Organization of Citrus Virologists Conference Proceedings ( 1957-2010 ) Title Possible Resistance to Citrus tristeza virus in Red Shaddock Permalink

Resistance to systemic infection by CTV occurs in trifoliate orange and in some hybrids of trifoliate orange and Citrus spp. A previous study indicated that pummelo and hybrids of grapefruit and trifoliate orange express resistance against some isolates of CTV, while other isolates can systemically infect these hosts. Since the isolates of CTV unable to replicate in these hosts were genetically similar to one another, seedlings of Red Shaddock pummelo were inoculated with single or mixed infections of isolates of CTV characterized as similar to the genetically distinct T30, T36, and VT isolates. Serological and molecular analysis showed that CTV with the T30 genotype systemically infected three of 17 inoculated seedlings, and CTV with the VT genotype systemically infected seven of 14 inoculated seedlings. In contrast, CTV with the T36 genotype systemically infected 17 of 17 inoculated seedlings. For both VT and T30 genotypes, systemic infection appeared delayed relative to inoculations to sweet orange control plants and to T36. The replication of the T36 genotype in all inoculated seedlings indicates a lack of resistance to the T36 genotype. The apparent lack of systemic infection of the T30 and VT genotypes in some inoculated seedlings indicated that resistance is expressed in some ‘Red Shaddock’ seedlings, but that there is variability in the expression of resistance among these seedlings. Citrus tristeza virus (CTV) is a globally distributed pathogen of citrus. Infection with certain strains of the virus is associated with economically important diseases such as decline, an incompatibility occurring with various scions grafted on sour orange rootstock, and stem-pitting, a disease characterized by the development of pits in the wood of twigs, small and large lateral branches and also the main trunk, as well as reduction of the growth of the tree and a decline in fruit yield, fruit size and quality in severe cases (15). Resistance to CTV infection has been demonstrated for Citrus relatives such as Severinia buxifolia and Swinglea glutinosa and trifoliate orange (8). Trifoliate orange expresses a general resistance to systemic infection by CTV via both graft inoculation with infected budwood and by inoculation with aphids. Recent work demonstrated that CTV replicates in trifoliate orange protoplasts, indicating that whole plant resistance might be due to restricted cellto-cell or long distance virus movement, or possibly to an induced resistance mechanism (2). The trifoliate orange resistance gene has been introduced into species in the genus Citrus by sexual hybridization, creating hybrids such as citrumelos and citranges that are used extensively as rootstocks in many citrus growing areas. The trifoliate orange resistance gene also has been introduced into advanced scion breeding lines as a means of introducing this gene into economically important citrus lines including scion cultivars (3). Progress also has been made in the identification and isolation of the trifoliate orange resistance gene(s), providing a pathway of introduction of only the resistance gene directly into important citrus varieties by plant transformation and regeneration methodologies (4, 19). Recent work showed that the intergeneric hybrid, Swingle citrumelo, and also varieties of pummelo express resistance effective against some isolates of CTV, but are susceptible to infection by others (5, 10). The studies undertaken by Fang and Roose (5) with “Chandler” pummelo used the California CTV strain T-514 as challenge inoculum, whereas the studies by Garnsey et Sixteenth IOCV Conference, 2005—Citrus Tristeza Virus 45 al. (10) tested seven isolates of CTV (B2, B3, B28 B31, B51, B52 and B280) from the USDA Exotic Citrus Pathogen Collection at Beltsville, MD (9) for their ability to systemically infect six pummelo varieties as well as Swingle citrumelo. Recent analyses of the sequences of several isolates of CTV have shown that CTV isolates are genetically distinct (1, 7, 12, 17, 18) and can be distinguished by molecular genetic marker-based methods which assign a “genotype” to an unknown isolate by comparison of the marker profile to those of designated standard isolates (6, 7). The aforementioned studies on resistance to CTV used isolates without knowledge of the isolate genotype. Marker analysis of isolates used in studies with pummelo and Swingle citrumelo (10) indicated these isolates had a T30 genotype (isolates B2, B51, B52), T36 genotype (B3), T3 genotype (B31) and non-standard genotypes (B280, B28). Studies on Chandler pummelo (5) used a T30 genotype, isolate T514 (Hilf, unpublished data). To assess the prevalence of selective resistance in pummelo and to determine if isolates with the same genotype are affected similarly by selective resistance, we selected nine independent Florida CTV isolates based on their assessed genotypes and inoculated these into small populations of seedlings of the pummelo cultivar Red Shaddock. We postulated that isolates which had the same genotype would behave similarly in the Red Shaddock seedlings. We report here the results of preliminary experiments to predict isolate behavior in pummelo hosts based upon viral genotype. MATERIALS AND METHODS Virus inoculum and plant inoculations. In planta cultures of CTV isolates FL-191, T-26, FL-188, T-30, FL-120, T-36, FL-207, FL-205 and FL-201 were maintained in seedlings of the sweet orange cultivar Madam Vinous under glasshouse conditions. Table 1 summarizes pertinent information on CTV isolates used in this study. Seeds of Red Shaddock pummelo were obtained from the Bureau of Citrus Budwood Registration, Dept. of Plant Industry, Florida Division of Consumer Services, Winter Haven, Florida, and planted in soil mix in a greenhouse for germination and growth under an appropriate temperature regime. Inoculation of individual seedlings with CTV was by budding with a mixture of three budeyes or blind-buds per plant. Grafts TABLE 1 ORIGIN, MCA13 STATUS AND GENOTYPE OF CTV ISOLATES INOCULATED INTO RED SHADDOCK SEEDLINGS Isolate Origin MCA13 Status Genotype 4 T26 Valencia [-] T30 T30 Mexican lime [-] T30