Infectivity titration of different strains of Xanthomonas axonopodis in Key Lime plants

Citrus canker is caused by pathovars of Xanthomonas axonopodis (Hasse) Vaut. This disease is currently spreading in the most important citrus growing state of Brazil and in Florida. The probability of the disease to spread to other countries of America is high. There are different studies of the relationship between inoculum level and disease intensity in bacterial diseases. This work had the objective to determine the curve of infectivity titration of known concentrations of A, B and C strains of Xanthomonas that infect citrus, in plants of Key Lime kept in greenhouses with temperature and light control. The half of three leafs were inoculated by infiltration-injection. Leaves were 14-21 days old. The concentrations inoculated were 500 000; 50 000; 5 000; 500 and 50 cells per milliliter. The assays were done in three different times. Infectivity titration curves were obtained. The values of infectivity (lesions per sq cm) obtained and the number of cells per milliliter inoculated were log transformed and linearized. The regression slopes of A and C were similar whereas the B type gave the lowest numbers for all doses. The regression equations allowed to obtain a prediction that can be used in different experiments to quantify the number of X. axonopodis cells in suspensions of different origins.