Splash dispersal of Phyllosticta citricarpa conidia from infected citrus fruit

Abstract

Rain-splash dispersal of Phyllosticta citricarpa (syn. Guignardia citricarpa) conidia (pycnidiospores) from infected oranges was studied in still air and combined with wind. High power microscopy demonstrated the presence of conidia in splash droplets from diseased oranges, which exuded conidia for over one hour during repeated wetting. The largest (5 mm) incident drops produced the highest splashes (up to 41.0 cm). A linear-by-quadratic surface model predicted highest splashes to be 41.91 cm at a horizontal distance of 25.97 cm from the target orange. Large splash droplets contained most conidia (4-5.5 mm splashes averaged 308 conidia), but were splashed <30 cm horizontal distance. Most (80-90%) splashes were <1 mm diameter but carried only 0-4 conidia per droplet. In multiple splash experiments, splashes combined to reach higher maxima (up to 61.7 cm; linear-by-quadratic surface model prediction, 62.1 cm) than in the single splash experiments. In combination with wind, higher wind speeds carried an increasing proportion of splashes downwind travelling horizontally at least 8 m at the highest wind speed tested (7 m/s), due to a small proportion of droplets (<1 mm) being aerosolised. These experiments suggest that P. citricarpa conidia can be dispersed from infected oranges by splashes of water in rainfall events.

Figure 1

(a). Conidia (blue bold arrows) in a 4 mm splashed droplet (edge indicated by fine arrow) observed under high power microscopy; (b). Three conidia (blue bold arrows) in a 1 mm droplet (edge indicated by fine arrow); (c). Splash emanating from infected orange (long exposure to show splash trajectories); (d). Splash droplets mid-flight (flash, short-term exposure).

Figure 2. Maximum vertical splash height achieved in still air in Rothamsted rain-tower against horizontal distance from infected orange for single incident drops of three sizes: 2.5 (circle), 3.5 (square) and 5 (triangle) mm.

Symbols: individual observations (open), observed means (solid grey), predictions from linear-by-quadratic surface model at observed distances (solid black). Observations and observed means are shifted right by one and two units of distance, respectively, for clarity. Vertical bars represent ± SE.

Figure 3. Numbers of splash droplets of different sizes falling on vertical strips (V) of water-sensitive paper at increasing heights (0–10, 10–20, 20–30, 30–40, 40–50 cm high) with increasing horizontal distance (10, 20, 30, 40, 50 cm) from an infected orange and relative percentages of the total numbers of droplets (5 mm incident drop).

Data were also collected using 2.5 mm incident drops, producing a similar pattern (data not shown). Symbols: <1 mm (solid circles), 1–2 mm (open circles), 2–3 mm (solid triangles).

Figure 4. Maximum vertical splash height achieved in still air in Rothamsted rain-tower against horizontal distance from an infected orange for multiple drops of size 5 mm.

Symbols: individual observations (open), observed means (solid grey) and predictions from quadratic surface model at observed distances (solid black). Observations and observed means are shifted right by one and two units of distance, respectively, for clarity.

Figure 5

(a). Maximum mean height of splashes in wind speed experiments with distance upwind and downwind from the source and 7 m/s regression line (Max Ht = 22.15 × Dist ^0.2717, R² = 0.916), (b). Frequency of splashes at differing wind speeds with distance upwind and downwind from the source. Symbols: still air (open circles), 1 m/s (solid circles), 2 m/s (open triangles), 4 m/s (solid triangles), 7 m/s (open squares) and 7 m/s regression line (solid line no markers) (mean of two repeats, except for 7 m/s which has three repeats).

Figure 6

(a). Phyllostica citricarpa colony on a PDA agar plate; (b). P. citricarpa conidia in aqueous suspension (bar represents 10 µm).