O Pruvost1, TR Gottwald2 and C Brocherieux1

1CIRAD FLHOR, Laboratoire de Phytopathologie, BP 180, 97455 Saint Pierre Cedex, Réunion Island, France; 2USDA, Agricultural Research Service, Orlando, FL 32803, USA

Background and objectives
Asiatic citrus canker is a potentially severe disease of several citrus species and cultivars in many tropical and subtropical areas. In such areas, infected nursery plants constitute an important source of primary inoculum for newly established citrus groves. Most previous epidemiology-related research on Asiatic citrus canker was performed in more temperate to subtropical areas of Japan, Florida and Argentina [1,2]. Thus, there is a need of for similar data from tropical areas. In the present study, the influence of overhead, drip, and mist irrigation systems on the development of Asiatic citrus canker was studied in simulated Mexican lime nurseries on Réunion Island.

Materials and methods
Simulated nursery plots consisted of 240 (8X30) bagged Mexican limes. Plots were established at CIRAD research station at Saint Pierre Bassin Plat (altitude 150 ;m; annual rainfall approximately 1000 ;mm). Visual inspection of all plants indicated the absence of canker lesions at the beginning of the experiment. Plots were watered using overhead, mist or drip irrigation. All plots received similar amounts of water. Inoculum source plants were prepared by wound-inoculating leaves (10 leaf lesions per plant) using strain C40S of Xanthomonas axonopodis pv. citri (Xac), a highly aggressive spontaneous streptomycin-resistant strain. Inoculated plants were placed in a growth chamber (30°C, 95% r.h.) for optimal disease development. Twelve inoculated plants were then randomly placed in each experimental nursery plot in June 1994. Disease assessments based on the number of lesions present on each plant were performed weekly until December 1994.

Results and conclusions
Overhead irrigation exacerbated the increase of the streptomycin resistant strain of Xac. The temporal development of Asiatic citrus canker for overhead-irrigated nursery plots was best described by a nonlinear exponential model because disease incidence in these plots did not reach an asymptote during the experimental period. This can be explained by the continuous production of new growth susceptible to Asiatic citrus canker and splash dispersal of Xac associated with overhead irrigation. Spatial autocorrelation and spatio-temporal semivariance analyses confirmed the occurrence of aggregated disease patterns irrespective of the irrigation system. In overhead-irrigated plots, the spread of Xac lacked directionality. Rains of short duration and high intensity were apparently associated with an increase in disease in drip-irrigated plots. Our results indicate the need to improve cultivation practices in Réunion Island citrus nurseries to minimize Asiatic citrus canker incidence in the nurseries and minimize introduction of Xac to new groves.

1. Goto M, 1992. In: Kumar J, Chaube HS, Sing US, ukhopadhyay AN, eds. Diseases of Fruit Crops, Vol. III. Englewood Cliffs: Prentice Hall, pp.170-208.
2. Gottwald TR, Reynolds KM, Campbell CL, Timmer LW, 1992. Phytopathology 82, 843-851.