Observations on the production and dispersal of spores, and infection by Rhynchosporium secalis

SUMMARY The numbers of spores of Rhynchosporium secalis washed from samples of barley plants taken weekly, varied markedly. No consistent association with amount of previous rainfall or length of the preceding dry period was detected. On potted seedlings exposed within a crop most infection occurred during long rain periods or when rain fell late in the evening; fewest lesions usually developed on seedlings prevented from touching adjacent plants. Rotorod traps fitted with a 13 mm diameter disc at the apex of each arm were operated under 24 cm diameter covers. Spores were collected on circular cover slips fixed to each disc with glycerine jelly. At all stages of crop growth more spores were trapped at ground level than at other heights tested up to 1 m. The number of spores trapped was not related to the quantity of or duration of rainfall but was related to the mean rate of fall during brief showers only. Efficiency of droplet retention was assessed in a wind tunnel. It declined rapidly when more than c. 5 pl was presented to the disc surface and was less at a wind speed of 1.0 than 0.5 m s‐l. Spore distribution on discs indicated that spores were washed from the surface during long rain periods.