Incidence of conidia of Erysiphe graminis in the air over a mildew-infected barley field

Abstract

Air over circular plots of mildew-infected, spring-sown barley was sampled with a Hirst automatic volumetric spore trap in the summers of 1955 and 1956 at the Imperial College Field Station, Ascot, Berks. Air-borne conidia of Erysiphe graminis DC. displayed a regular pattern of diurnal periodicity with the daily peaks at about noon. Simultaneous observations were made in 1956 on the growth phases of the barley plants, the incidence of mildew in the plot, and the number of E. graminis conidia in the air. A sudden increase in conidia was observed in mid-June, with the outbreak of the disease in the plot, when the barley plants were in the stem extension phase. In the second half of June, when the crop was passing through the heading phase, the seasonal maximum occurred. In July, when the flowering and ripening phases occurred, very few conidia were caught. They disappeared completely from the air by the end of July when the crop was ripe and the straw dead. Rain reduced the number of conidia in the air to a very low level. In dry weather following rainy spells there was slow recovery; it took about 3–5 days for the counts to reach pre-rain levels. The diurnal periodicity pattern was not affected by rain or by variation in wind speed, but both affected catches quantitatively to some extent. The catches were greatest when strong winds and low relative humidities occurred simultaneously in the afternoon; the highest hourly estimate recorded was 18,300 conidia per m.3 of air.