Populations of Xanthomonas axonopodis pv. citri in endemic areas

Citrus canker was introduced in Argentina 30 years ago and became endemic in the Northeast. Successful methods of integrated management were developed to decrease the damage of the disease. Current requirements to export fresh fruits include the certification of production in plots free of symptoms. Populations of the causal agent, Xa. pv. citri, were investigated on lesion-free leaves and fruits of grapefruit, lemon, orange and tangerine sampled from groves with high and low intensity of the disease, without and with canker control and on unselected harvested fruits. Methods of detection were by washing of leaves and fruits followed by plating on semi-selective medium (lima bean agar, kasugamycin, yeast extract and cycloheximide) and infiltration of susceptible leaves of grapefruit seedlings kept in growth chambers. Populations detected ranged from undetected level on lesionless leaves and fruits of orange and lemon from sprayed low disease plots and 0 to 10(6) cells of Xac per leaf or fruit (mean: <10) from highly infected unsprayed plots of grapefruit, lemon and orange. Harvested lesionless fruits sampled from boxes containing mixed diseased and healthy fruits carried populations from 0 to 10(4) Xac cells per fruit (mean: 10(2)) of all citrus before and after processing by waxing with no desinfestant. Both methods of detection were appropiate; the inoculation of leaves could detect the pathogenic Xac cells and allowed the elimination of other yellowish bacteria present on plates. The results demonstrate that populations of Xac are generally low even from highly infected plots in lesionless leaves and fruits and almost always undetectable in low disease intensity groves.