Wind Speed Effects on the Quantity of Xanthomonas citri subsp. citri Dispersed Downwind from Canopies of Grapefruit Trees Infected with Citrus Canker

Citrus canker (caused by Xanthomonas citri subsp. citri (ex Hasse) Gabriel et al.) is a disease that affects several citrus species and causes erumpent, necrotic lesions on leaves, shoots, branches, and fruit. Yield loss due to fruit and leaf drop can be substantial, and on fruit the unsightly blemishes leads to reduced marketability (14,15,34). The disease is widespread in citrus growing regions where humid-wet tropical and subtropical conditions conducive for spread prevail. Citrus canker is endemic in Florida and continues to spread among groves (18,19, 21,25,34). Lesions of citrus canker exude bacteria the moment water is present, and the quantity produced can be substantial. The bacteria are expressed from lesions for a prolonged period (>52 h) when conditions are suitable for their reproduction and dispersal (4,33). After the initial burst of production from the lesion, the quantity of bacteria slowly declines as the population is depleted (4,33). With time the quantity produced levels off, becoming more consistent after the initial population of X. citri subsp. citri in the lesion is depleted and regeneration of the population occurs. Throughout this time, these bacteria are available for dispersal from the vicinity of the lesion, although several other factors might influence the quantity produced, including lesion age (33,40) and ambient temperature (4,26,39). Wind and rain are dispersal agents, either singly or in combination, for many plant pathogens (1,11,13,24,27,30), including various bacterial diseases (5,9,11,13, 42,43). The initial pattern of citrus canker in groves suggests a combined effect of wind and splash in dispersing X. citri subsp. citri (7,20,21). Furthermore, powerful storms, including tropical storms and hurricanes, have been implicated in disease development up to 56.3 km distance (19,25). Even in regular thunderstorms that occur on a frequent basis in Florida during the summer, the rain can be intense for short periods with locally strong winds. The effect of wind speed in combination with splash as agents dispersing X. citri subsp. citri has not been characterized, but if it can be demonstrated that reducing wind speeds reduces inoculum dispersal, both windbreaks and planting strategies can be optimized to minimize the incident wind. Furthermore, although wind can disperse droplets of spray containing X. citri subsp. citri (4), the relationship between wind speed and dispersal of the pathogen has not been quantified or characterized. In one report, in a simulated rainfall experiment in calm conditions, splash dispersal of X. citri subsp. citri was demonstrated to be 60 cm (33). X. citri subsp. citri has reportedly been collected up to 32 m from infected plants after storms (6,39), and some earlier studies found that wind could disperse bacteria-laden droplets from infected plants (37). The concentration of X. citri subsp. citri was reported to increase with wind speed in these droplets from both upper and lower surfaces of the leaves (35). Using simulated wind and rain, bacteria were not only dispersed for long periods of time but were collected up to 12 m (the maximum distance sampled) from an inoculum source with winds of 19 m⋅s applied to an infected canopy (4), but very few X. citri subsp. citri were collected downwind from cull piles of mature harvested fruit (15). Not only is the relationship between wind speed and dispersal of X. citri subsp. citri uncharacterized, there is little information available on the relationship among wind speed, quantity of inoculum dispersed, and distance from the source of inoculum. Typically, raindrop diameters range from 0.2 to 5.0 mm (13,27). Larger droplets are more efficient for dispersing fungal pathogens, but bacteria are much smaller and could be dispersed in aerosol-sized droplets (42). Rainfall rates are highly ABSTRACT Bock, C. H., Graham, J. H., Gottwald, T. R., Cook, A. Z., and Parker, P. E. 2010. Wind speed effects on the quantity of Xanthomonas citri subsp. citri dispersed downwind from canopies of grapefruit trees infected with citrus canker. Plant Dis. 94:725-736.