Factors affecting diseases of winter wheat in England and Wales, 1989-98.
Samples from 360 to 450 randomly selected winter wheat crops in England and Wales were collected annually during the milky ripe development stages (GS 73-75) from 1989 to 1998. The number of samples from each region was proportional to the area of winter wheat grown. The percentage area affected by disease was assessed on the top two leaves and the ear, and the incidence and severity of stem base diseases were also recorded. An estimate of the percentage area of the crop affected by barley yellow dwarf virus (BYDV) and take-all (Gaeumannomyces graminis) was made in the field. Septoria leaf blotch (Septoria tritici), and the teleomorph Mycosphaerella graminicola, was the major foliar disease recorded, with an average maximum severity of 7.8% of the area of leaf 2 affected in 1998. Eyespot (Tapesia spp.) was the major stem base disease, with the highest incidence of stems falling into the damaging moderate plus severe categories (18.9%) in 1998. Levels of powdery mildew (Blumeria graminis [Erysiphe graminis]) showed a decline from 0.4% of the area of leaf 2 in 1989 to 0.1% in 1998. This fall was associated with a reduction in the proportion of disease-susceptible cultivars grown. There were significant regional differences in levels of septoria leaf blotch, brown rust (Puccinia recondita), eyespot, sharp eyespot (Rhizoctonia cerealis) and BYDV. The percentage of crops treated with a fungicide rose from 96% in 1989 to 98% in 1998, and the mean number of spray applications per crop rose during this period from 2.1 to 2.5. A higher proportion of crops was treated with fungicides between the end of tillering and fifth node detectable (GS 24-35) than around flag leaf emergence (GS 36-48) or ear emergence (GS 49-71). Prior to 1994, the majority of late fungicide sprays was applied at, or after, ear emergence, but from 1994, the majority was applied around flag leaf emergence. The value and socioeconomic implications of the results are discussed.