3.7.41
ECOLOGY OF ENDEMIC CITRUS CANKER: SEASONAL FLUCTUATIONS OF DISEASE INTENSITY

BI CANTEROS

Instituto Nacional de Tecnologia Agropecuaria, Estaci6n Experimental Agropecuaria INTA Bella Vista, CCS, 3432 Bella Vista, Corrientes, Argentina

Background and objectives
Citrus canker, caused by Xanthomonas axonopodis pv. citri became widespead in northeastern Argentina after being introduced in 1975 1 1}. Management of the disease is achieved using integrated measures, among them protection with copper-containing bactericides and use of windbreaks. Observations made since the begining of the epidemic have shown that there are great variations in the response to the disease of the cultivated varieties and that the intensity also fluctuates according to the seasons and the years. Most epidemiological data that exist on citrus canker {2} relate to the spread in time and space of the new epidemics and little information exists on the disease after it has become established in an area. The purpose of the research presented here was to investigate the dynamics of canker intensity throughout the seasons in some common citrus cultivars differening in their levels of resistance in the field. The data obtained will help in establishing plots to produce disease-free fruits for export to cankerfree areas.

Materials and methods
An experimental grove was planted in 1991, at seven meter spacing, with the following citrus cultivars: Red Blush grapefruit (C. paradisi Macf), Murcott tangor (C. reticulata Blanco x C. sinensis). Valencia orange (C. sinensis (L.) Osbeek), Eureka 22 lemon (C. lhnon (L.) Burmf.), and Okitsu satsuma tangerine (C. unshiu Marc.). A randomized block design was used with three replications (five plants per plot). Blocks were arranged with the windbreak located to intercept the predominant rain-bearing winds. The grove was not sprayed with any bactericide. Disease intensity in each tree was monitored every 1-2 weeks on a scale 0-100%. Severity of disease on fruits was assessed at midseason and at harvest using a three grade scale. Rainfall, predominant winds, relative humidity, temperatures, and other weather variables were recorded.

Results and conclusions
The data obtained during the first seven years were subjected to regression analysis and ANOVA. The results indicate that the disease intensity in each tree and severity of disease on fruits varied markedly with the seasons and were lowest in those years with low rainfall in the spring. These effects were similar for all cultivars, regardless of their resistance. Regression analysis between distance to the windbreak and disease intensity gave a highly positive correlation throughout the study for all varieties. There was an inverse relationship between the disease intensity in midseason and the severity on fruits at harvest, and a direct relationship between the proportion of healthy fruits in midseason and the proportion at harvest. In those years with low infection only grapefruit differed statistically from the other cultivars in terms of the severity of infection on fruits. Satsuma fruits never showed canker symptoms even in those plants located very near heavily infected grapefruit trees. It was determined that canker intensity will vary markedly according to predisposing factors even in plants without chemical sprays. It is possible to obtain canker-free fruits through the use of all the recommended measures, and through the intensive use of these practices in those years of low disease severity. Pruning of affected tissue will facilitate the decrease of the disease in selected plots, and increase the production of disease-free fruits suitable for export during all seasons. Production of healthy fruits in areas in which canker is established is possible when there is enough knowledge of the dynamics of the disease.

References
1. Stall RE, Civerolo EL, 1991. Annual Review of Phytopathology 29, 399-420.
2. Stall RE,Gottwald TR, Koizwni Nt Schaad NC, 1993. In: Swing JG, Civerolo EL, eds. Xanthomonas, Chapman & Hall, London, UK: Chapman & Hall, 265-99.