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, 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) 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.

Strobilurin-resistant isolates of Blumeria ( Erysiphe ) graminis f.sp. tritici , the cause of wheat powdery mildew, were more than 10-fold less sensitive to azoxystrobin than sensitive isolates. In all resistant isolates, a mutation resulting in the replacement of a glycine by an alanine residue at codon 143 (G143A) in the mitochondrial cytochrome b gene was found. Allele-specific primers were designed to detect this point mutation in infected wheat leaves. Using quantitative fluorescent allele-specific real-time polymerase chain reaction (PCR) measurements, strobilurin-resistant A143 alleles could be detected amongst strobilurin-sensitive G143 alleles at a frequency of at least 1 in 10 000, depending on the amount of target and nontarget DNA. Most isolates tested were dominant homoplasmic for either the A143 or G143 allele, although mixed populations of alleles could be detected in some isolates. In some of these isolates, strobilurin resistance was not always stable when they were maintained for many generations in the absence of selection. The allele-specific real-time PCR assay was also used to follow the dynamics of A143 alleles in field populations of B . graminis f.sp. tritici before and after application of fungicides. As expected, the A143 allele frequency only increased under selection pressure from a strobilurin fungicide. After three sprays of azoxystrobin, a pronounced selection for the strobilurin-resistant allele, with an increase in average frequency from 2·2 to 58%, was measured. The use of quantitative real-time PCR diagnostics for early detection of fungicide resistance genes at low frequency, coupled with risk evaluation, will be invaluable for further resistance risk assessment and validation of antiresistance strategies.

Two fully replicated trials were conducted with glasshouse-grown tomatoes, under conditions similar to commercial production, to define the impact of Pepino mosaic virus (PepMV). PepMV was not found to reduce bulk yields in these trials, but the quality of tomato fruits harvested was reduced significantly. Compared with uninoculated, PepMV-free control plants, 6·5% of fruits of PepMV-affected cv. Espero were downgraded from class 1 in trial 1. In trial 2, an average 38% of class 1 fruits from PepMV-affected cvs Espero and Encore were lost as a result of downgrading. Loss of quality was mainly a result of blotchy ripening, gold marbling, gold spot, and symptoms directly attributed to PepMV infection. PepMV infection also affected fruit size. The results are discussed in relation to the demands of multiple retailers in the UK for class 1 tomatoes only.

Comparative analyses were undertaken to characterize Xanthomonas campestris pv. musacearum, the causal agent of a wilt of enset and banana, and to assess its relatedness to other xanthomonads by fatty acid methyl esters, genomic fingerprinting using rep-PCR and partial nucleotide sequencing of the gyrase B gene. The results from all three analyses indicated that strains of X. campestris pv. musacearum are homogeneous and very similar to X. vasicola strains isolated from sugarcane and maize from Africa. Pathogenicity studies indicated that strains of X. vasicola pv. holcicola and X. vasicola from sugarcane induced no symptoms on banana, whereas X. campestris pv . musacearum produced severe disease. These data will support a future proposed reclassification of X. campestris pv. musacearum as X. vasicola pv . musacearum when more data are available.

A technique for inoculation of Brassica seedlings with single resting spores of Plasmodiophora brassicae is described. Three isolates derived from single spores were produced from one population of P. brassicae. They gave different reactions with the European Clubroot Differential (ECD) series, and two gave different reactions from those of the spore suspensions from which they were derived. When two isolates were mixed together, spores of one isolate restricted infection by spores of the other isolate.