Experiments on Splash Dispersal of Fungus Spores

SUMMARY: Splash dispersal (first studied by Faulwetter, 1917a, b) is characteristic of many bacterial plant pathogens and slime-spored fungi. The mechanism of splash has been studied in the laboratory under simplified conditions with water drops falling from known heights on to thin films of a suspension of conidia of Fusarium solani spread on horizontal glass surfaces. The resulting splash droplets were caught, counted and measured by the naphthol green B slide method (Liddell & Wootten, 1957). Both the total number of droplets produced and of those carrying spores increased as the film thickness decreased, and as the size and velocity of the incident drop increased.

One incident drop 5 mm. in diameter falling on a horizontal film of spore suspension 0·1 mm. thick on a glass surface produced over 5200 splash droplets of which over 2000 carried one or more spores. The sizes of splash droplets ranged from 5μ about 2400μ, and their size distribution was of the log probability type. The median diameter was 70μ for all droplets, and 140 μ for droplets carrying spores. The medium horizontal distances travelled by these droplets in still air was 10 and 20 cm. respectively. Increasing the film thickness to 0·5 and 1·0 mm. reduced the total resulting droplets to 3500 and 2100 (1600 and 500 carrying spores), respectively. Neither median diameter nor median distance of horizontal travel were much altered by varying the diameter of incident drop or film thickness. Droplets of diameters between 164 and 655μ tended to travel further than either smaller or larger droplets. Small spores such as those of Gloeosporium album, Colletotrichum lindemuthianum and Nectria cinnabarina were even more readily picked up from suspension than the macroconidia of Fusarium solani.

The number of droplets deposited per unit area on a horizontal plane decreased rapidly with increasing distance from point of impact, and in still air few droplets travelled beyond 70 cm.

Splash on a twig bearing conidial fructifications of Nectria cinnabarina produced 2000 spore-carrying droplets, and the sequence of events is shown by high-speed photography.

A splash droplet consists of an intimate mixture of liquid from both the incident drop and the surface film. The larger splash droplets contain spores if either the incident drop or the surface film is a spore suspension. Both raindrops falling at terminal velocity and drops falling more slowly from vegetation may operate the splash dispersal mechanism. Splash in rain or drip from trees may act as a complete dispersal mechanism in still air, or as a ‘take-off’ mechanism leading to dispersal by wind. Both functions are important in plant epidemiology.