Citrus Stubborn Severity Is Associated with Spiroplasma citri Titer But Not with Bacterial Genotype

The impact of citrus stubborn disease, caused by Spiroplasma citri, on citrus production is associated with the symptom severity of infected trees but its association with bacterial levels and virulence are unknown. Fifty-eight S. citri isolates were cultivated from severely and mildly symptomatic trees and randomly amplified polymorphic DNA and short-sequence repeat fingerprinting differentiated four major S. citri genotypes among these isolates. Each genotype was present in both mildly and severely symptomatic trees, suggesting that readily detectable genetic differences in the S. citri populations did not account for differences in disease severity. No variation in the size of amplicons of the pathogenicity-related fructose operon was observed in isolates from trees having varying degrees of symptom expression. Quantitative polymerase chain reaction demonstrated that spiroplasma titer is over 6,000 times higher in fruit from severely symptomatic than from mildly symptomatic trees. The genotypic similarities among S. citri isolates from severely and mildly symptomatic trees, and the consistently higher bacterial titer in the former than in the latter, suggests that titer but not genotype is, at least in part, responsible for the greater symptom severity in some of the S. citri-affected trees in the orchard evaluated.