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Splash dispersal of Phyllosticta citricarpa conidia from infected citrus fruit


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Simulated rain splash experiments using a rain tower and wind tunnel determined the potential for dispersal of Phyllosticta citricarpa (synonym Guignardia citricarpa) conidia (pycnidiospores) from infected oranges. High Power microscopy demonstrated presence of conidia in splash droplets. In still air, the highest splash droplets were produced by the largest (5 mm) incident drops, reaching mean maximum height of 41.9 cm as opposed to 35.5 cm with 3.5 mm drops and 28.8 cm with 2.5 mm drops. The largest splashes (2-3 mm diameter) were recorded up to 20 cm high. Larger drops contain more spores (4-5.5 mm splashes averaged 308 spores), but get splashed <30 cm. Most (80-90%) splashes were <1 mm diameter but carry far fewer spores per droplet. The 0-0.99 and the 1-1.99 mm droplets which splash furthest in still air (up to 70 cm) contained an average of 1 and 21 spores respectively. In multiple splash experiments, splashes combined, rebounded and were forced higher, up to 72.2 cm (mean 64.3cm), compared to the single splash experiments. In experiments combining wind speed with rain-splash, progressively higher wind speeds carried an increasing proportion of splashes downwind and these splashes travelled increasingly further downwind – up to 8 metres in the case of the highest wind speed (7m/sec). At wind speeds up to 4m/s, all splash droplets described an arc that was skewed due to the wind but at 7m/s a small proportion of droplets (<1mm) were dispersed higher than originally splashed (up to 73.2cm) suggesting they remain aerosolised rather than behaving as ballistic droplets. These experiments showed thatspores were dispersed from the infected oranges when carried in splashes of water. Laboratory experiments showed that infected oranges misted to simulate light rainfall continued to exude spores for at least an hour. Infected oranges are a potential source of spores to be dispersed by rainfall events.

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