Studies were made of isolates of Goeumannomyces graminis (Sacc.) Arx & Olivier var. tritici J. Walker from different length crop sequences of wheat and barley following permanent grassland. Studies were done on the influence of crop ping intensity of soils, natural and steam disinfested, on disease expression from added inoculum. Efforts were also made to detect interactions between isolates and soils from the same and different cropping sequences. Investigation of variability in the pathogen related to virulence and specificity as well as to saprophytic traits.In general, mean virulence of isolates increased up to the point where approximately peak disease levels occurred in the field, and diminished subsequently. Host specificity of isolates from a monospecies system was also pronounced at this point whereas it was not evident in the first 2 crops ; it did not increase further after the 5th or 6th cereal of a single species. There was evidence of loss of competitive saprophytic ability in isolates from long sequences as against those from lst, 2nd and 3rd crops after permanent pasture. A differential sensitivity response from Gliomastix murorum (Cda.) Hughes var. felina (Marsh) Hughes was observed with isolates from 1 to a 3 year sequence compared with isolates from sequences of 4 to 8 years. Isolates from a 1st to a 3rd crop restricted colony growth of G. murorum in culture whereas those from 4th to the 8th cereal were overgrown by this fungus. Isolates from the 1st cereal after grass darkened considerably with age, those from a 2nd less so and those from a 3rd successive crop remained almost totally hyaline ; those from longer sequences darkened considerably but were very variable for this trait.The influence of soil from more intensively cropped cereal sequences, which was destroyed b steam disinfestation, was to depress disease levels from applied inoculum. This soirinhibitory effect set in between the first and 3rd cereal. 1965 66 67 68 69 70 71 72 73 74 Take-all Ructuatlons in a I 0 year barley succession.
Symptoms of red leather leaf of oats were widespread and severe in spring 1988 on autumn‐sown oat crops in the south and east of Ireland. The disease was characterized by grey‐brown elliptical lesions, developing into light‐grey eyespots surrounded by dark borders. Plants were stunted and showed reddish‐brown foliar pigmentation extending beyond the lesions. Spermospora avenae was isolated and shown to be pathogenic to oats. Cultural characteristics of the pathogen on several agar media are described. The disease has previously been reported only from the USA, Turkey and Australia.
S U M M A R YIncidence of eyespot was assessed in different-length sequences of spring-sown wheat and barley in field experiments. Disease levels in the fourth crop were greater with single cereal systems than when the alternative crop was grown in the fourth year. The existence of host specificity as a possible cause of this phenomenon was examined in comparative pathogenicity studies of isolate collections obtained from differentially-cropped plots from the same field.In two experiments, isolates from the third, fourth or fifth consecutive barley crops produced more disease on barley than on wheat and isolates from wheat caused more disease on wheat than did isolates from barley. Isolates from 4-yr wheat and barley sequences showed significant specificity of isolates for their original hosts but in another experiment such specificity was not detected in isolates from 3and 7-yr sequences of wheat and barley. However, there was a significant reduction in virulence of isolates in the seventh compared with the third crop, when comparisons were done on the alternative rather than on the original host species. This suggests that extreme selection pressure for pathogen virulence on a particular host species may lead to impairment of pathogenic range.
Variation in host response of isolates of the eyespot pathogen from different sources was examined over a number of years. Pathogen types were found in intensively‐cropped couch‐infested cereal sites that were almost as virulent on Agropyron repens (couch) as on wheat or barley. The commonly occurring wheat (W) type isolates from couch‐free cereal crops were virulent on wheat and barley but avirulent on couch. Couch (C) types were isolated not only from couch but also from wheat, barley and oat crops with couch infestation. In pathogenicity tests on rye, C. types did not differ in virulence from the more commonly occurring W types. Aegilops ventricosa was equally resistant to both types.W type isolates from wheat and barley were examined to assess differential pathogenicity on wheat and barley. Sequential cropping with single cereal crops was used to separate out possible specific types. Isolates from fourth wheat and fourth barley crops were more pathogenic on the original than on the alternative host. When comparisons were made between isolates from third and fifth consecutive wheat and barley crops only those from barley showed a preference for the original host. An experiment comparing isolates from third and seventh consecutive wheat and barley crops showed a decline in virulence from the short to the longer sequences on the alternative but not on the original host.
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