Epidemics of take‐all during 16 years of continuous spring barley

Abstract

SUMMARYIn 1969 spring barley was the third consecutive cereal crop following a mixed rotation, which included cereals, on a sandy‐loam at Woburn, Beds. The incidence (42%) of roots with take‐all was approximately eight times greater than in any of the next 11 crops of an ensuing spring barley monoculture. Until 1981 it was assumed that the peak of take‐all in 1969 represented the terminal phase of the epidemic that preceded take‐all decline and that the subsequent low level of take‐all was a consequence of take‐all decline. In 1981 a secondary outbreak of take‐all infected 12% of roots and in the next two years 16% and 27.5% of roots were infected, but in 1984 the disease resumed a very low incidence (0.6% of roots infected) which characterised most of the 1970s.The most probable number of infectious fragments per 150 cm3 of soil was determined by bioassay on 115 occasions. The maximum recorded was 36.5 in June 1969 and the most recorded in the secondary outbreak was 29.7 in July 1983. From early 1971 to mid‐1981 the numbers were always under five and mostly less than one per 150 cm3. Soil collected in mid‐August 1969 after harvest was most infectious (50 roots infected per 190 cm3 of soil) in bioassays and thereafter until mid‐1981 there were never more than 6.5 infected roots per 190 cm3 of bioassay soil. Soil infectivity in the second epidemic never matched that of the first, peaking at 23 roots infected in 190 cm3 of soil in early July, 1983.Take‐all on a silty clay loam, 13 km to the east at Silsoe, was prevalent in a third spring barley in 1973. Its decline to very little in a seventh crop in 1977, was interrupted in 1975 by a slight, temporary recovery also recorded at Woburn. Although changes in cultivars, nitrogen rates and sowing dates may have accounted for some of the changes in disease it is suggested that the polyetic epidemic that began in spring barley monoculture at Woburn in the late 1970s and entered its terminal phase in 1983 was weather‐determined and reflected a general, long‐term, trend in the UK and Ireland.