Properties and interactions among citrus pathogens for disease expression

Citrus trees are often infected with different viral-pathogens. Interactions among pathogens and intra-population interactions within a single pathogen may affect disease expression. Altered disease symptomatology may affect bioindexing of citrus viral-pathogens. The probability for symptom expression, the efficacy of bioindexing tests and the number of indicators required for pathogen detection in the presence of multiple infections of citrus bioindexing hosts was statistically evaluated. Multiple infections did not preclude symptom expression or reduce the diagnostic efficacy of the primary indexing hosts for Citrus tristeza virus (CTV), Citrus psorosis virus (CPsV) and citrus tatter leaf virus. Symptoms of citrus vein enation virus (CVEV) and the diagnostic efficacy of Mexican lime were suppressed specifically by T30 group CTV isolates. CPsV suppressed symptom expression and diagnostic efficiency of Dweet tangor and sweet orange for concave gum disease. Dweet tangor, sweet orange and Citrus excelsa, are not typically used for bioindexing of CVEV, however, Dweet tangor and C. excelsa detected CVEV in single infections whereas, in sweet orange, CVEV was detected only when CPsV, concave gum or citrus viroids were present.; The intra-population variability of Citrus exocortis viroid and citrus viroid II (CVd-II) variants, CVd-IIc and cachexia 909 (Ca-909) as affected by host and environmental conditions was investigated. Genealogical analysis was performed using coalescent theory principals. Host specific viroid populations consisted of a swarm of closely related sequences around a master sequence. The positive cachexia disease bioassay of Ca-909 cannot be attributed to minor population variants containing the cachexia disease specific nucleotide cluster. Population profiles were affected by host genotype and viroid genome size but not by the bottleneck inoculation of clone transcript or the environmental conditions tested. Common mutations among different hosts, master sequence shifts and fitness peaks were monitored for the citrus viroids tested. The conventional structural domains of the viroid genome were not supported from the biological properties of individual viroids since the "variable" domain was generally more conserved than the "central conserved". The unique clone recovered from the chrysanthemum CVd-IIc pathogenic population with 99% sequence similarity with the non-pathogenic CVd-IIa indicated the rapid and complex evolutionary patterns of viroid RNA.