Doctoral Degrees (Plant Pathology)

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Browsing Doctoral Degrees (Plant Pathology) by browse.metadata.advisor "Crous, P. W."

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    The biology of Endophyllum osteospermi, and its use for the biological control of Chrysanthemoides monilifera ssp. monilifera
    (Stellenbosch : Stellenbosch University, 2004-12) Wood, A. R. (Alan Robert); Crous, P. W.; Lennox, C. L.; Stellenbosch University. Faculty of AgriSciences. Dept. of Plant Pathology.
    ENGLISH ABSTRACT: Chrysanthemoides monilifera ssp. monilifera is a shrub indigenous to South Africa, which has become a serious weed of native vegetation in Australia. Endophyllum osteospermi is a microcyclic, autoecious, rust fungus that induces witches' brooms on C. monilifera ssp. monilifera. This rust is considered as a candidate biocontrol agent for use against C. monilifera ssp. monilifera in Australia. The vegetative growth and reproductive output of healthy branches on bushes with different levels of E. osteospermi infections were measured at three sites. The growth of healthy branches on infected bushes was 26- 81% less than that of healthy branches on uninfected bushes. The number of buds, flowering capitulae, fruiting capitulae, and cypselas on healthy branches of infected bushes was 35-75%, 45-90%, 15-99%, and 15-90% less, respectively, than those on uninfected bushes. At five sites, the infection levels and number of witches' brooms were determined every two months. The increase in number of witches' brooms per bush ranged between o and 282 within one year, with an average increase per bush of28 (SE ± 4.8) and 39 (SE ± 9.2) during two years. The average simple interest rate (rs) increase of infection levels for all bushes was 0.015 month-I (s.e. ± 0.0041, n = 72) and 0.0098 month" (s.e. ± 0.0073, n = 43) during two years. Aecidioid teliospores germinated between 10 and 20oe, with 15°e as optimum. Light, and particularly near-uv light, stimulated germination. A period of 6 to 8 hours of light was needed to obtain optimum germination levels. The temperature requirements for basidiospore development differed from that of aecidioid teliospore germination. Optimum was at 15°e, but a rapid decrease in basidiospore production occurred at higher temperatures, few developed at 19°e. Two nuclear divisions occurred within 12 hours of germination to produce a metabasidium with three or four nuclei. A third nuclear division occurred in the basidiospores between 24 and 48 hours. Plants inoculated under controlled conditions took 5 to 24 months before witches' brooms began to develop. A Geographic Information System (GIS) approach was used to model the potential distribution of E. osteospermi in South Africa, based on monthly average climate surfaces with parameters derived from the above experiments. The same model was applied to Australia to suggest a potential distribution of the rust if released in Australia. This potential distribution was similar to one generated using the climate matching computer programme CLIMEX©, but gave greater spatial accuracy. Both approaches indicate that E. osteospermi should establish in temperate Australia. Chrysanthemoides species, as well as other South African asteraceaus plants, were monitored for E. osteospermi between 1992 and 2003. Endophyllum osteospermi was recorded on C. monilifera ssp. monilifera, C. monilifera ssp. pisifera, C. monilifera ssp. rotundata, C. monilifera ssp. canescens, C. monilifera ssp. subcanescens, C. incana, an undescribed taxon of Chrysanthemoides, Osteospermum ciliatum, 0. polygaloides and 0. potbergense. Endophyllum dimorphothecae sp. nov. is described on Dimorphotheca cuneata. Aecidium elytropappi, which was recorded on Elytropappus rhinocerostis and Stoebe plumose, is transferred to Endophyllum as E. elytropappi comb. nov. Germination of aecidioid teliospores and penetration by basidiospores were observed on the surface of excised leaves of 32 plant species at 4 days after inoculation. Germinating aecidioid teliospores aborted on 14 plant species, whilst no penetration was attempted on a further 12. Penetration only occurred on 9. Therefore only these 9 plant species need to undergo traditional host specificity testing. Pending these results, E. osteospermi could be safely released in Australia for the biological control of C. monilifera ssp. monilifera.
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    Botryosphaeria diseases of proteaceae
    (Stellenbosch : Stellenbosch University, 2002-03) Denman, Sandra; Crous, P. W.; Wingfield, M. J.; Stellenbosch University. Faculty of AgriSciences. Dept. of Plant Pathology.
    ENGLISH ABSTRACT: Fungi belonging to the genus Botryosphaeria are heterotrophic micromycetes that can be pathogens on woody plants. They cause serious, and in some cases devastating losses to crops through leaf necrosis, stem cankers and plant death. The Proteaceae cut-flower industry in South Africa accounts for 70% of the national cut-flower enterprise. Botryosphaeria diseases are a major impediment to production and trade of Proteaceae and there is an urgent need to investigate the etiology, epidemiology and control of these diseases. Losses of one of the most important proteas, P. magnifica, amount to 50% or more, locally. The main aims of this study were therefore to establish the etiology and aspects of epidemiology of Botryosphaeria stem cankers on P. magnifica and other Proteaceae, and to investigate methods of disease control. Although there is a vast body of information pertaining to this fungus, which was reviewed in Chapter 1, there is relatively little information available on Botryosphaeria on Proteaceae. The taxonomy of Botryosphaeria requires thorough review, and molecular techniques need to be employed to resolve species identities. In Chapter 2, it was found that Phyllachora proteae, a leaf pathogen of proteas, produced a Fusicoccum anamorph, which is described as F. proteae. A sphaeropsis-like synanamorph was associated with F. proteae and a new combination for P. proteae is proposed in Botryosphaeria, as B. proteae. The taxonomy of Botryosphaeria is in disarray at both the generic and the specific level. In Chapter 3 the taxonomic history of Botryosphaeria is reviewed, and the genus circumscribed and distinguished from other morphologically similar genera. Although several anamorph genera have been linked to Botryosphaeria, based on morphological observations and phylogenetic analysis of lTS rDNA sequence data, two anamorph genera are now recognised, those with pigmented conidia (Diplodia), and those with hyaline conidia (Fusicoccum). Botryosphaeria proteae should thus be excluded from Botryosphaeria. Several pathogenic Botryosphaeria spp. have an endophytic phase within their hosts. They are therefore imported unwittingly into other countries where they may pose a risk to agriculture and indigenous vegetation. The current global distribution of Botryosphaeria spp. associated with Proteaceae is clarified and a key to these taxa associated with Proteaceae is provided in Chapter 4. Five Botryosphaeria spp. are associated with cut-flower Proteaceae worldwide viz. B. lute a, B. obtusa, B. protearum, B. proteae and B. rib is. B. protearum is described as a new species. A thorough understanding of disease epidemiology is essential to effect a reduction of losses. In Chapter 5, I show that on P. magnifica, lesions caused by Botryosphaeria protearum, which lead to the formation of stem cankers, are initiated in the mid-rib vein or margin of leaves. Koch's postulates were satisfied and it was found that the number of lesions that developed from artificial inoculations correlated with starch levels present in leaves at the time of inoculation. In Chapter 6 it is shown that B. protearum exists as an endophyte in leaves of P. magnifica in naturally occurring as well as cultivated plants. In natural stands of proteas stem cankers are rare, but in cultivated plantations the incidence is high. Nutritional analyses indicate that higher levels of nitrogen occur in leaves of cultivated plants in spring, which could enhance disease development. High levels of sodium in the leaves of wild plants may restrict disease development. The severe economic losses caused by B. protearum make the search for improved methods of disease control essential. Fungicide applications form an important component of an integrated approach to disease management. In Chapter 7, in vitro tests demonstrate that tebuconazole, benomyl, prochloraz me, iprodione and fenarimol reduce the mycelial growth of B. protearum effectively. In the field there was a 25-85% reduction in the occurrence of stem cankers by applying fungicides or sanitation pruning. The best control was achieved by using benomyl, bitertanol, fenarimol, iprodione, prochloraz manganese chloride alternated with mancozeb and tebuconazole prophylactically. If sanitation pruning is combined with regular applications of fungicides, disease can be combated.
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    Genetics of pathogenicity in Pyrenophora leaf diseases of barley
    (Stellenbosch : Stellenbosch University, 2001-12) Campbell, Graham F. (Graham Findlay); Crous, P. W.; Lucas, J. A.; Stellenbosch University. Faculty of AgriSciences. Dept. of Plant Pathology.
    ENGLISH ABSTRACT: Net blotch of barley, caused by Pyrenophora teres, is one of the most important diseases of this cereal in the south Western Cape Province of South Africa. This fungus exists as two different types (forms), namely a nettype and a spot-type that are distinguished by differential symptom expression on barley leaves. Based on this specific plant pathological difference a series of studies of agricultural importance were executed to investigate the effects of sexual recombination between these two types. In addition, studies were done to determine the difference between local net- and spot-type populations with regards to population structure and fungicide sensitivity. This dissertation therefore, consists of a collection of separate publications and as a result a certain degree of redundancy has been unavoidable. Recombination is one of the most important evolutionary forces involved with sexual reproduction. In plant-fungal agricultural ecosystems this may result in pathogenic fungal populations adapting more rapidly to control programs such as fungicide applications. The first section of the review in part 1 of this dissertation covers different aspects of sexual reproduction in ascomycetes, specifically focussing on mating-type genes, vegetative incompatibility and recombination. The major part of the review is then dedicated to various plant pathological aspects of P.teres, specifically addressing the differences between the two types, and in various cases highlighting the significance of sexual recombination within and between the net- and spot-type. Using morphological criteria for identification purposes there have been many conflicting reports concerning the identity of leaf spot isolates in the Western Cape Province of South Africa. In part 2, the correct identity was eventually achieved employing mating studies and molecular markers .: This was accomplished after single ascospores were obtained from pseudothecia after in vitro mating had occurred between a verified P. teres net-blotch isolate from Denmark and a representative Pyrenophora leaf spot isolate from South Africa. Using amplified fragment length polymorphism (AFLP) and RAPD markers, recombination was demonstrated in the progeny that had DNA banding patterns different from the two parental isolates. Pathogenicity trials also confirmed that recombination had taken place during mating. Inoculations were conducted on the differential cultivars susceptible to the net-blotch and leaf spot forms. The two parents induced typical net-blotch or leaf spot symptoms whereas the progeny mostly induced a jagged spot symptom on each cultivar. Fungicide sensitivity tests using the ergosterol biosynthesis inhibitors showed that, due to recombination, some progeny could have increased resistance to these fungicides. Due to mating and subsequent recombination between a net blotch isolate of P. teres and a representative leaf spot isolate, it was concluded that the latter was P. teres f. maculata. Fifteen of the net-spot hybrid progeny (F1) produced from the mating study in Part 2 were screened in Part 3 to assess their viability and genetic stability. Hybrid progeny (F1) inoculated onto barley seedlings consisting of the cultivars Stirling (differentially susceptible to net-type isolates), B87/14 and Clipper (both differentially susceptible to spot-type isolates) produced intermediate symptoms on all cultivars. Axenic cultures (F1-1) isolated from foliar lesions, followed by repeated inoculation and isolation (F1-2) onto a healthy set of seedlings produced similar intermediate symptoms. RAPDs conducted with two 1Q-mer primers on all isolates of F1-1and F1-2progeny revealed profiles similar to those obtained for F1 isolates. RAPD molecular data, therefore, indicated that hybrid progeny of this net x spot mating were genetically stable after having been subjected to two repetitive inoculation and reisolation cycles. Phylogenetic analysis of DNA sequences of the internal transcribed spacers (ITS1 and ITS2) flanking the 5.8S nuclear ribosomal RNA gene and the 5' end partial histone-3 gene confirmed the genetic stability of the hybrid progeny. These results also indicated that the hybrid progeny produced consistent symptoms throughout the series of experiments, and maintained their virulence to the differential cultivars screened. Both types of P. teres are prevalent in the south Western Cape Province of South Africa, found on susceptible cultivars often grown within close proximity of each other. In Part 4, a net- and spot-type population were characterised in terms of their population structure using RAPD markers. Samples were collected from infected barley leaves from two separate quadrants in each field, the two quadrants positioned in corners of the fields, diagonal to one another. A total of 65 loci were produced of which 54 were polymorphic. Total gene diversities determined for all loci resulted in mean indices of 0.063 and 0.082 being obtained respectively for the net- and spottype populations. A coefficient of genetic differentiation (Gs) of 0.0149 was obtained between sites within populations while a coefficient (GT) of 0.63 was obtained between the two populations. Genotypic variation revealed 13 distinct multilocus genotypes (haplotypes) in the net-type population while there were 12 in the spot-type population. UPGMA cluster analysis done on the two populations together with six progeny from the mating between a netand spot-type isolate resulted in three main clusters being produced, one for each population and one for the progeny. One isolate collected from the nettype population also contained a unique spot-type RAPD fragment. This suggested that sexual recombination may be taking place between isolates of the net- and spot-type under field conditions. Fungicide application is the most important method used in the control of net blotch in South Africa. In Part 5 the fungicide sensitivities (ICsD values) of 89 monoconidial isolates (46 net-type and 43 spot-type) of P. teres to sterol demethylation inhibiting fungicides were determined, based on the inhibitory effect on radial mycelial growth. The fungicides evaluated were triadimenol, bromuconazole, flusilazole, propiconazole and tebuconazole. Both net- and spot-type isolates revealed strong resistance to triadimenol while flusilazole was shown to be the strongest inhibitor of fungal growth. Spot-type isolates showed a higher resistance than net-type isolates to all five fungicides screened. The ICsD values indicated significant differences between four of the fungicides (triadimenol, tebuconazole, flusilazole and propiconazole). The ICsD values between propiconazole and bromuconazole were not significant. This study suggested that spot-type isolates showed a higher degree of resistance to commercially used fungicides than net-type isolates. The overall conclusion of this study is that the spot-type of P. teres is the pathogen associated with leaf spots of barley in the south western Cape province of South Africa and not P. japonica as earlier reported. Together with the net-type, both types exist as genetically variable populations in this barley production region. Mating between the two types results in sexual progeny that are genetically stable. This implies that barley fields adjacent to one another in which either net- or spot-type susceptible cultivars are being cultivated may lead to sexual progeny being produced. This in turn may lead to an increased rate at which fungal populations may become resistant to commercially used fungicides. It is furthermore suggested that an alternative fungicide seed treatment is used instead of triadimenol due to high resistance of P. teres to this fungicide.
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    Phylogenetic relationships and population dynamics of Calonectria
    (Stellenbosch : Stellenbosch University, 1999-12) Schoch, Conrad Lamoraal; Crous, P. W.; Wingfield, M. J.; Wingfield, B. D.; Stellenbosch University. Faculty of AgriSciences. Dept. of Plant pathology.
    ENGLISH SUMMARY: This dissertation is presented as a collection of separate publications and an amount of redundancy has thus been unavoidable. Although several species are newly described they are not effectively published and will thus be formally published in scientific journals. There were two main objectives: I. To investigate the variability and mating compatibility of species and populations, in order to contribute to the systematics of Calonectria. II To identify loci that would be useful for DNA sequence comparisons in this genus and to present a reliable phylogeny of Calonectria and other closely related hypocrealean taxa. In the introductory review a synopsis of the current knowledge regarding the taxonomy and life cycle of Calonectria and Cylindrocladium spp. is presented. The importance of these pathogens are noted, as well as the problems related to identifying them. Aspects regarding specific species complexes and topics are discussed in more detail in the following chapters. The morphological and phylogenetic variation was investigated for the Cy. candelabrum species complex in Part 2. DNA sequence comparisons of the ribosomal 5.8S gene and flanking ITS1 and ITS2 spacers were employed in order to determine whether mating incompatibility and general morphology was supported by molecular evidence. Although only small differences were found these proved to be consistent and resulted in the recognition of Galonectria scoparia (anamorph Cylindrocladium candelabrum), and the description of three new species, namely Calonectria pallciramosa (anamorph Cyfindrocladium pauciramosum), Calonectria insularis (anamorph Cylindrocladium insulare) and Calonectria mexicana (anamorph Cylindrocladium mexicanum). The Cyfindrocladium scoparium cultures studied in Part 3 were isolated from several hosts in the U.S.A. Isolates were mated in all combinations, and one successful mating was selected to establish whether recombination occurred. RAPD and mating type data of parental isolates and progeny confirmed Cy. scoparium to have a heterothallic mating system. Furthermore, to determine the phylogeny of Cy. scoparium with several morphologically similar Cylindrocladium spp., DNA sequences of the ribosomal 5.8S gene and the flanking internal transcribed spacers (ITS), as well as part of the high mobility group (HMG) box (forming part of the MA T- 2 mating type gene) and the β-tubulin gene, were analysed. Maximum parsimony yielded concordant trees for all three data sets. These data supported the morphological and biological species concepts proposed for Cy. scoparium and other, similar, small-spored Cylindrocladium spp. Part 4 represented an investigation into the mating compatibility and mating type distribution of populations of Cy. pauciramosum. This enabled the determination of the effective population for the different areas studied. A sample collected over a period of six years, reflecting a number of locations in South Africa were found have 1: 1 mating type ratio, as expected in a random mating population. However, the mating type ratio was found to be significantly different in single nursery populations. In the South African nursery, the MAT-1 mating type was dominant, while the MAT-2 was more common in other samples obtained from nurseries in Italy and the U.S.A.. This was consistent with one or more founder effects. The high percentage of hermaphrodites also suggested that recent introductions had occurred in nurseries in Italy and the U.S.A. In addition to this, DNA sequence comparisons of the β-tubulin gene was used to investigate variation below species level in Cy. pauciramosum. All isolates from South Africa, Australia, U.S.A. and a group from Italy had identical sequences. A second group with identical sequences were found in the Italian sample. In addition to this, variation was found between all isolates from Brazil, Colombia and Mexico. Some of these base pairs were shared between the South and Central American isolates as well as isolates of Cy. candelabrum. This points towards a speciation event in South or Central America. After investigating variation below species level, this study was also expanded to generic level. In Part 5 information obtained in the preceding chapters culminated in a phylogeny of all known species in Calonectria and Cylindrocladium based on DNA sequence comparisons of the β-tubulin gene. Many clades, containing small numbers of isolates were strongly supported by bootstrap. However, relationships between these clades were often ambiguous. A number of phylogenetic placements based on DNA data did not always agree with preconceived morphological relationships. Two large groupings were evident and both contained small-spored, one-septate species. The only morphological character that correlated with DNA based phylogenies was vesicle shape of the anamorph. Finally, in Part 6, the generic phylogeny was investigated. In order to obtain a generic phylogeny a subset of Calonectria species was selected, as well as isolates from other genera, closely related to Calonectria. All of these genera were originally described under the broad concept of Nectria sensu lato. A gene tree phylogeny, based on β-tubulin was presented for selected nectriaceous genera with anamorphs bearing cylindrical macroconidia. Based on molecular data and the distinct anamorph genera, new teleomorph genera were proposed for Gylindrocladiella (Nectricladiella), Gliocladiopsis (Glionectria) and Xenocylindrocladium (Xenocalonectria). Ca/onectria was also found to form a monophyletic lineage. Eight species of Cylindrocladiella were recognised, with two having teleomorphs in Nectricladiella, namely N. camelliae (Ce. microcylindrica) and N. infestans (Ce. infestans). This study concluded that the current morphological species concepts in Cylindrocladium and its Calonectria teleomorphs can comprise several biological as well as phylogenetic species. The use of mating testers in this study was shown to provide a powerful tool to separate morphologically similar, but genetically isolated species. The biological and morphological species also agreed with the phylogenetic concepts used, but only vesicle shape were found to define phylogenetic clades. However, phylogenetic species concepts based on DNA sequences data obtained from genomic regions such as the β-tubulin and MA T-2 genes and additional areas will become increasingly important for further taxonomic studies in Calonectria and related genera.
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    Virulence spectrum, molecular characterisation and fungicide sensitivity of the South African Rhynchosporium secalis population
    (Stellenbosch : Stellenbosch University, 2000-12) Robbertse, Barbara; Crous, P. W.; Lennox, C. L.; Stellenbosch University. Faculty of AgriSciences. Dept. of Plant Pathology.
    ENGLISH ABSTRACT: Barley leaf scald, caused by Rhynchosporium secalis, is the most important disease of barley (Hordeum vulgare) in the Western Cape province of South Africa. The disease was first reported from South Africa in 1937. The present study is the first attempt to characterise the South African R. secalis population. Topics such as pathogenesisrelated proteins, virulence spectra, variability of pathotypes, sources of variation, host resistance, breeding strategies, molecular characterisation and fungicide sensitivity are summarised in Part 1 of this dissertation. In succeeding Parts the focus is on the characteristics of the local R. secalis population regarding virulence spectrum, DNA polymorphisms, in vitro as well as in vivo fungicide sensitivity. These aspects are treated as separate entities, leading to some duplication which is unavoidable. In Part 2 the virulence spectra of 50 R. secalis isolates from a population in the. Western Cape province were determined. Twenty-one races were detected using 17 differential barley cultivars. The two most prevalent races, namely races 4 and 7 had three and four virulence genes respectively. Both race 4 and 7 were virulent on the most susceptible cultivars, namely West China, Steudelli, C.I.8618 and C.I.2226. Considering the resistance genes reported for cultivars Atlas 46, Turk, and C.I.3515 which showed no susceptible cultivar-pathogen interaction, it would appear that the Rh- Rh3-Rh4 complex is primarily involved in conferring resistance to the local R. secalis isolates. A total of 20 races (47 isolates) characterised in Part 2 were selected for further characterisation by means of DNA fingerprinting. In Part 3 an anonymous multilocus DNA probe was used to characterise the genotypic structure of these isolates by means of RFLP analysis. No correlation between any particular fingerprint pattern, race, district, field or lesion was found. The two most prevalent races, 4 and 7, did not share the same genotypes, even when isolated from the same field or lesion. The genotypic diversity of the isolates studied was 46.5% of the theoretical maximum diversity. The high level of genotypic variation observed in the South African R. secalis population resembled the genotypic diversity observed in other cereal pathogens with known sexual structures. Although no teleomorph has yet been observed, these data suggest that sexual recombination may operate within the local population of R. secalis. In South Africa barley scald is primarily controlled by means of fungicides. The continued use of fungicides on cereal crops results in the build-up of fungicide resistance in the population, which could lower the efficacy of these compounds. These aspects were investigated in Part 4, where isolates (collected during 1993 to 1995) were evaluated in vitro for sensitivity to triadimenol, tebuconazole, flusilazole and propiconazole. The sensitivity fluctuated but in 1995 isolates were significantly less sensitive towards triadimenol than in the previous two years. In a second experiment, isolates collected from two fields with a 5-6 year-history of triadimenol seed treatments and tebuconazole applications, were evaluated for their fungicide sensitivity. A significant positive correlation was observed between tebuconazole and triadimenol sensitivity among,R. secalis populations from these fields. However, such a correlation was not found within the R. secalis population collected during 1993-1995 where shorter crop rotation patterns and a range of fungicides was applied. In a third experiment, the fungicide sensitivity of local R. secalis isolates was evaluated towards two new triazole fungicides, namely bromuconazole and triticonazole. Correlation coefficients observed between these new triazoles and those previously applied in South Africa were not significantly positive. The lack of significant cross-resistance has important practical implications regarding the management of fungicide resistance. In Part 5, isolates with different minimum inhibitory concentration (MIC) towards tebuconazole in vitro (1, 3 and 10 ug/ml) were compared in vivo. The aim of this study was to determine how MIC values would influence virulence (leaf area affected) and sporulation. Results indicated that all isolates were equally fit to induce lesions and sporulate in the absence of tebuconazole. Thus no fitness cost was associated with the degree of tebuconazole sensitivity in the present study. All R. secalis isolates were able to induce lesions on tebuconazole treated leaves, but differed significantly with respect to the percentage leaf area affected. Isolates, least sensitive (MIC = 10 ug/rnl) towards tebuconazole were more adapted on tebuconazole treated leaves, being able to repeatedly cause larger lesions than sensitive R. secalis isolates (MIC = 1 ug/rnl), Sporulation was not significantly different between isolates on lesions of untreated or tebuconazole treated leaves. Larger leaf areas affected and adequate sporulation suggest that a less sensitive population would result in more disease in tebuconazole treated fields. In conclusion, this study revealed the variability associated with the South African R. secalis population regarding virulence spectrum and genotypic structure. The data in this study suggest that it is likely that the local population will easily adapt to newly introduced, single gene resistance. For more durable resistance, higher levels of quantitative resistance should be introduced. This type of resistance is, however, more difficult to identify and incorporate than single gene resistance. Consequently, barley scald control will remain dependent on the efficacy of fungicide applications. Furthermore, the lack of cross-resistance and low frequency of resistant isolates indicates a low risk for the development of fungicide resistance in the local R. secalis population. Other factors such as current crop rotation practices and the range of fungicides being ~pplied also contribute to this low risk level. However, the status of these factors can change over time. The in vivo tebuconazole sensitivity study has indicated that a resistant field population of R. secalis may be able to build-up. It is, therefore, necessary to monitor the fungicide sensitivity of R. secalis isolates at timely intervals with view to successful barley cultivation in the future.