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United States Department of Agriculture

Agricultural Research Service

Title: Screening Citrus Germplasm for Resistance to Xanthomonas Axonopodis Pv. Citri

Authors
item McCollum, Thomas
item Bowman, Kim
item Gottwald, Timothy

Submitted to: Meeting Abstract
Publication Type: Proceedings
Publication Acceptance Date: November 1, 2005
Publication Date: June 1, 2006
Citation: Mccollum, T.G., Bowman, K.D., Gottwald, T.R. 2006. Screening citrus germplasm for resistance to xanthomonas axonopodis pv. citri. Second International Research Workshop on Citrus Canker and Huanglongbing, November 2005. P18, p.3.

Interpretive Summary: Citrus canker, is a serious disease caused by the bacterial plant pathogen Xanthomonas axonopodis pv. citri (Xac) . There is currently no effective control of citrus canker. Breeding citrus varieties resistant to Xac is one method which could lead to disease control. Numerous studies have been conducted to identify sources of resistance to Xac among citrus and citrus relatives. Kumquats (Fortunella sp.) and Calamondins (Citrus mitis) have shown to be highly resistant and mandarins (C. reticulata) somewhat resistant. Our objective was to compare the resistance of selected Citrus species, citrus hybrids, with an emphasis on selections of C. reticulata and hybrids of Fortunella sp. and citrus relatives to Xac. Two separate experiments were conducted. The first experiment focused on determining if differences in resistance exist among 20 C. reticulata genotypes and included three other citrus species and citrus relatives Glycosmis pentaphylla and Clausena hardimandiana. Plants were inoculated with Xac by injection infiltration or needle-prick. The second experiment included 10 members of the genus Citrus , 10 hybrids of Fortunella sp., 5 additional citrus hybrids and the citrus relative Poncirus trifoliata representing a total of 31 different selections. Plants were needle-prick inoculated with both Xac strain A and Xac strain A Wellington. In both experiments there were highly significant differences among genotypes in response to inoculation with Xac. In the first experiment, regardless of inoculation method, G. pentaphylla and C. hardimandiana were found to be highly resistant to Xac whereas C. paradisi was least resistant. Using the needle prick method of inoculation and measuring lesion dia. there appeared to be a wider range of resistance among the C. reticulata selections than with the infiltration method and measuring the number of lesions per inoculation. In the second experiment for both Xac strain A and AW, Chinotto sour orange, Carrizo citrange, Eustis limequat, and P. trifoliata were the most resistant. The Thornless key lime x Meiwa kumquat hybrids showed a range of resistance from among the most susceptible to among the most resistant. Our results expand on previous studies on resistance of citrus and citrus relatives to Xac and indicate that there may be potential for increasing resistance by breeding using selected parents.

Technical Abstract: Numerous studies have been conducted to identify sources of resistance to Xanthomonas axonopodis pv. citri (Xac) among citrus and citrus relatives (Gottwald et al., 2002 and references therein). Kumquats (Fortunella sp.) and Calamondins (Citrus mitis) have shown to be highly resistant and mandarins (C. reticulata) somewhat resistant. Our objective was to compare the resistance of selected Citrus species, citrus hybrids, with an emphasis on selections of C. reticulata and hybrids of Fortunella sp. and citrus relatives to Xac. Two separate experiments were conducted. The first experiment focused on determining if differences in resistance exist among 20 C. reticulata genotypes and included three other citrus species and citrus relatives Glycosmis pentaphylla and Clausena hardimandiana. Plants were inoculated with Xac strain A either by injection infiltration (Gottwald and Graham, 1992) or needle-prick. For each genotype and inoculation method there were 3 single tree replications; 4 leaves on each tree were inoculated on 3 sites. The second experiment included 10 members of the genus Citrus (C. macrophylla, C. aurantium, C. excelsa, C. jambhiri, C. macroptera, C. medica, C. paradisi, C. reticulata, C. sinensis, C. aurantifolia ), 10 hybrids of Fortunella sp. ([Fortunella sp. X Citrus aurantifolia]], [Citrus aurantifolia x Fortunella], [(Poncirus trifoliata x Citrus sinensis) x Fotrunella sp.], [Citrange x (Fortunella x Citrus) c. calamondin] ) 5 additional citrus hybrids ([Citrus limon x C. aurantifolia], [(C. reticulata x C. paradisi) x C. paradisi], [C. limon x C. aurantifolia], [Poncirus trifoliata x C. paradisi], [Poncirus trifoliata x C. sinensis]) and Poncirus trifoliata representing a total of 31 different selections. Plants were needle-prick inoculated with both Xac strain A and Xac strain A Wellington. There were 3 single tree replicates of each genotype. Two leaves on each plant were inoculated at 5 sites with Xac strain A and at 5 sites with Xac strain AW (Wellington strain) (Sun et al., 2004). All inoculations were done using suspensions of Xac at a concentration of 104 cfu/mL. Trees inoculated by injection infiltration were examined 14 da, at which time the number of lesions per inoculation site were recorded. Trees inoculated by needle-prick were examined 7 and 40 da at which times lesion diameter at each inoculated site was recorded. The difference in lesion diameter between 7 da and 40 da was calculated. All data were subjected to analysis of variance. In both experiments there were highly significant differences among genotypes in response to inoculation with Xac. In the first experiment, regardless of inoculation method, G. pentaphylla and C. hardimandiana were found to be highly resistant to Xac whereas C. paradisi was least resistant. Using the needle prick method of inoculation and measuring lesion dia. there appeared to be a wider range of resistance among the C. reticulata selections than with the infiltration method and measuring the number of lesions per inoculation. In the second experiment for both Xac strain A and AW, Chinotto sour orange, Carrizo citrange, Eustis limequat, and P. trifoliata were the most resistant. The Thornless key lime x Meiwa kumquat hybrids showed a range of resistance from among the most susceptible to among the most resistant. Our results expand on previous studies on resistance of citrus and citrus relatives to Xac and indicate that there may be potential for increasing resistance by breeding using selected parents.

Last Modified: 9/21/2014
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