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(NUV) and white light.  Yields of conidia were higher for isolates of both fungal varieties (P < 0.01) and conidial suspensions were produced free of the mycelial fragments present in suspensions from PDA only. The conidial inoculum produced on cellophane had lower pathogenicity to wheat cv. Equinox in a detached leaf assay showing significantly longer latent periods (P < 0.01) than conidia produced on PDA alone. However the decline in pathogenicity of conidial suspensions produced on cellophane compared to PDA alone was too small to adversely affect the new method of inoculum production. The use of the cellophane technique to produce conidial spore suspensions of M. nivale var. majus and var. nivale has a number of advantages, namely the shorter period to the onset of sporulation and the increase in the yield of mycelium-free conidial inoculum produced in culture, particularly for isolates originally showing poor sporulation.

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Potential mechanisms of resistance to Fusarium head blight detected using a detached leaf assay of wheat, barley and oats

R.A. Browne and B.M. Cooke

Department of Environmental Resource Management, University College Dublin, Belfield, Dublin 4, Ireland (E-mail roy.browne@ucd.ie)

Wheat, barley and oats in common with other cereals are susceptible to Fusarium head blight (FHB). However the relative susceptibility to FHB among the three crops and resistance mechanisms and potential susceptibility factors are poorly understood. The detached leaf assay was successful in identification of an important component of the resistances to FHB in European wheat germplasm with significant correlations between whole plant resistance and components of partial disease resistance (PDR) detected in a detached leaf assay [Browne and Cooke 2004, European Journal of Plant Pathology 110: 91-102; Diamond and Cooke 1999, European Journal of Plant Pathology 105: 363-372]. The aims of this research were to comparatively assess the PDR components detected in detached leaf assays of a range of commercial cultivars of wheat, barley and oats thereby providing a starting point to investigate the utility of these techniques within each cereal crop. Barley cultivars showed incubation periods comparable to, and latent periods longer than the most resistant wheat cultivars evaluated. Incubation periods were longer for oats than for barley or wheat cultivars. However, oat cultivars differed from both wheat and barley in that mycelial growth was observed before obvious tissue damage was detected under macroscopic examination, indicating tolerance of infection rather than inhibition of pathogen development. Latent periods were longest in oat cultivars with morphology of sporodochia appearing less well developed.  Latent periods in barley were shorter than oats but longer than any wheat cultivars. The longer latent periods in barley and oat cultivars than wheat in the detached leaf assay suggests that these PDR components are likely to play a role in overall FHB resistance if the resistances are under the same genetic control as PDR components expressed in the head.  The results for barley and oat cultivars evaluated in the leaf assay contrast with previously published reports for wheat cultivars where incubation and latent periods were shorter and more variable between cultivars.  The significance of the various combinations of PDR components detected in the detached leaf assay as components of FHB resistance in each crop requires further investigation, particularly with regard to the apparent tolerance of infection in oats and necrosis in barley associated with retardation of mycelial growth and sporulation.

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Some aspects of tan spot epidemiology in Latvia, 1999-2003

Biruta Bankina, Ilze Priekule

Department of Plant Biology and Protection, Latvia University of Agriculture faizkat@cs.llu.lv

Tan spot, caused by Pyrenophora tritici repentis (Died.) Drechs., anamorph Drechslera tritici-repentis is one of the most important wheat disease in the world, especially in the regions of intensive wheat growing. In Latvia the disease was noticed and identified in the first half of the 1990s. Spreading of tan spot began rapidly with increase of wheat density in crop rotation. At the present time it is the most wide-spread and harmful disease of wheat in Latvia. Field observations were carried out in different locations during 1999-2003, where development of the disease was investigated in untreated plots (25-28 m2 with four replicates) in trials applying fungicides at different time and in different dosages. Several wheat varieties also were included in trials, total number of plots – 23. Assessments of the disease were made on three upper leaves every week from the beginning of stem elongation to complete ripening. The incidence and severity of the disease was determined. The apparent infection rate (r) was calculated during a definite period and during all growing seasons. Meteorological conditions were different at the time of observations in each year and severity of tan spot fluctuated from 0,4 % up to 99,0 % depending on year, locality and variety. However characteristic tendencies of the disease development were observed. The first symptoms of tan spot emerged at the time of stem elongation until milk ripening. Appearance of first symptoms was not dependent on meteorological conditions or varieties. Rapidity of appearance of first symptoms also did not influence further disease development and yield losses. A break in development of tan spot was observed in some cases after very early first symptoms. The severity of tan spot achieved 1 % only in 87 % of observed plots. Mostly it happened only at the time of flowering or later (65 % of observed plots). In general, tan spot developed slowly, average rate of infection reached only 0,1 during the vegetation season. The rate of infection changed during disease development, r value reached 0,4-0,5 at the time of flowering and especially at the time of milk ripening. The rate of infection mostly depends on wheat growth stages. The crucial period of tan spot development appears at the time of wheat flowering or later. Increase of disease development was determined by decrease of resistance of older leaves, peculiarities of spore maturation and distribution. Therefore tan spot is most harmful for late varieties with long period of ripening. Further studies on maturation time and liberation of ascospores in Latvian climate conditions are necessary.

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