Aerial dissemination of barley loose smut ( Ustilago nuda)

Abstract

Changes in concentration of air-borne chlamydospores of barley loose smut ( Ustilago nuda (Jens.) Rostr.) were estimated with a Hirst spore trap, operated continuously at the centre of circular plots of infected spring-sown barley in the summers of 1955 and 1956. An analysis of the variation in number of spores forming a clump (‘dispersion unit’), from the counts of 6 normal days, showed that clumping in Cladosporium was greatest at midday, but in Ustilago it was unrelated to time of day or weather conditions. The daily mean concentrations from 28 June to 26 July in the summer of 1955, and from 15 June to 13 July in 1956, and the mean diurnal periodicity curves, expressed as percentage of the peak mean concentration for spores and spore clumps, have been given to indicate the seasonal and diurnal changes during the period of smut dissemination. Two-hourly changes in concentration, together with the changes in weather conditions, have also been given for the periods of maximum smut dissemination in both the years to show the influence of weather. Inoculum persisted in the air over the field, mostly during the day time, for about 3–4 weeks in June–July. On many days the maximum amount of air-borne inoculum was caught at the time at which the highest gust of wind was recorded. The distribution of wind and its extreme velocities, when examined in relation to changes in the amount of air-borne inoculum on different days, indicated that the prevailing wind velocity, especially its gustiness, influences the rate of smut dissemination. Observations of the growth phase of barley in 1956 showed that flowering coincides with the maximum dispersal of the pathogen, which occurred over a period of 10–20 days, and was influenced by weather. Occurrence of rainfall and/or high wind velocity during this period results in loss (more rapid if associated with a thunderstorm) of spores from the infected ears, thus reducing the period of smut dissemination.