Barry M. Cunfer

Identity and conservation of mating type genes in geographically diverse isolates of Phaeosphaeria nodorum.

Mating type idiomorphs (MAT1-1 and MAT1-2) were identified from the heterothallic loculoascomycete Phaeosphaeria nodorum (wheat biotype) using DNA from a pair of isolates from Poland and Georgia, USA that are known to mate. MAT predicted proteins of P. nodorum are similar in sequence and in phylogenetic relationship to those described for other loculoascomycetes such as Cochliobolus spp., Alternaria alternata, and Didymella zeae-maydis. The organization of the MAT locus of the P. nodorum differs from these species in that its idiomorph begins within an adjacent upstream conserved ORF of unknown function. MAT-specific primers were used to identify isolates of both mating types in field populations, demonstrating that an absence of either mating type is not the reason that the teleomorph has not been found in New York. Portions of MAT1-1 and MAT1-2 were sequenced from geographically diverse isolates, including those from regions where the teleomorph has been reported. MAT was highly conserved and no significant differences in sequence were found.

Stagonospora and Septoria diseases of barley, oat, and rye

This paper presents a comprehensive review of the foliar diseases caused by Stagonospora and Septoria fungi on barley, oat, and rye. The relationship of the pathogens to related species that cause disease on wheat is discussed. The diseases are more serious during periods of cool wet weather and are more prevalent in northern Europe and North America. They are frequently associated with the planting of highly susceptible cultivars. The biotype of Stagonospora nodorum that infects barley differs significantly from the biotype that attacks wheat and is occasionally important in the southeastern U.S.A. and northern Europe. Stagonospora avenae f. sp. triticea infects barley, rye, and wheat. It is usually a minor pathogen of barley and rye, but these hosts may serve as an inoculum reservoir for infection of wheat. Stagonospora avenae f. sp. avenaria causes stagonospora leaf blotch and black stem on oat. The black stem phase of the disease causes significant yield reduction and is frequently important in eastern Canada. Morphological characteristics and host range are similar among the Stagonospora species. Teleomorphs of these fungi are in Phaeosphaeria, and the epidemiological importance of this state varies with each pathogen. Septoria passerinii causes speckled leaf blotch on barley. It has become increasingly important in recent years in north central North America and may be associated with reduced tillage. Septoria tritici f. avenae occurs infrequently on oat, and Septoria seculis causes a minor disease on rye. Teleomophic states of these Septoria species have not been identified. Resistance to Stagonospora nodorum has been identified in barley and to Stagonospora avenae f. sp. avenaria and Septoria passerinii in the cultivated hosts and wild relatives.. Race-specific resistance has not been identified and resistance is primarily partial resistance. Recent use of molecular genetics is providing a clearer understanding of the relationships among these and related pathogens on wheat and grasses.

Relative Contribution of Seed-Transmitted Inoculum to Foliar Populations of Phaeosphaeria nodorum.

ABSTRACT A marked-isolate, release-recapture experiment was conducted to assess the relative contributions of seed-transmitted (released isolates) versus all other inocula to foliar and grain populations of Phaeosphaeria nodorum in winter wheat rotated with nonsusceptible crops in New York and Georgia, United States. Seed infected with two distinct groups of marked isolates of P. nodorum containing rare alleles (identified by amplified fragment length polymorphisms [AFLPs]) and balanced for mating type were planted in experimental field plots in two locations in each state. Recapture was done by isolating P. nodorum from leaves showing necrotic lesions at spring tillering and flowering stages, and mature grains from spikes showing glume blotch. Isolates from these samples were genotyped by AFLPs and categorized as released or nonreleased to infer sources of inoculum. Both infected seed and other sources of the pathogen contributed significant primary inocula to populations recovered from leaves and harvested grain. Seed-transmitted genotypes accounted for a total of 57% of all isolates recovered from inoculated plots, with a range of 15 to 90% of the populations of P. nodorum collected over the season in individual, inoculated plots at the four locations. Plants in the noninoculated control plots also became diseased and 95% or more of the isolates recovered from these plots were nonreleased genotypes. Although other potential sources of P. nodorum within and adjacent to experimental plots were not ruled out, nonreleased genotypes likely were derived from immigrant ascospores potentially from sources at a considerable distance from the plots. Our results suggest that, although reduction of seedborne inoculum of P. nodorum may delay foliar epidemics, this strategy by itself is unlikely to result in high levels of control in eastern North America because of the additional contribution from alternative sources of inoculum.

Sequence diversity of mating-type genes in Phaeosphaeria avenaria

Abstract. Phaeosphaeria avenaria, one of the causal agents of stagonospora leaf blotch diseases in cereals, is composed of two subspecies, P. avenaria f. sp. triticea (Pat) and P. avenaria f. sp. avenaria (Paa). The Pat subspecies was grouped into Pat1–Pat3, based on restriction fragment length polymorphism (RFLP) and ribosomal DNA (rDNA) internal transcribed spacer (ITS) sequences in previous studies. Mating-type genes and their potential use in phylogeny and molecular classification were studied by DNA hybridization and PCR amplification. The majority of Pat1 isolates reported to be homothallic and producing sexual reproduction structures on cultural media had only the MAT1-1 gene. Minor sequence variations were found in the conserved region of MAT1-1 gene in Pat1 isolates. However, both mating-type genes, MAT1-1 and MAT1-2, were identified in P. avenaria isolates represented by ATCC12277 from oats (Paa) and the Pat2 isolates from foxtail barley (Hordeum jubatum L.). Cluster analyses based on mating-type gene conserved regions revealed that cereal Phaeosphaeria is not phylogenetically closely related to other ascomycetes, including Mycosphaerella graminicola (anamorph Septoria tritici). The sequence diversity of mating-type genes in Pat and Paa supports our previous phylogenetic relationship and molecular classification based on RFLP fingerprinting and rDNA ITS sequences.

Components of partial resistance to Leptosphaeria nodorum among seven soft red winter wheats

SummarySeven soft red winter wheat cultivars were evaluated for partial resistance to Leptosphaeria nodorum under field conditions. The results demonstrate that resistance is available among cultivars that are adapted to the southeastern U.S. and that resistance is long lasting. Resistant cultivars had longer incubation and latent periods, slower rates of lesion development, and reduced the level of sporulation of L. nodorum. Seedlings of susceptible cultivars sustained severe disease with dew periods as short as 48 hr in greenhouse tests. One resistant cultivar sustained little damage with dew period as long as 144 hr.Latent period was shortest on the second leaf below the flag leaf (F-2 leaf) of all cultivars and longest on the flag leaf which reflected the effect of microclimate and leaf age. The range in length of latent period on the flag, F-1, and F-2 leaves of resistant cultivars was less than that for susceptible cultivars. Differences between cultivars were greatest (up to 6.8 days) for the F-2 leaf. A delay in production of inoculum on the F-2 and lower leaves of resistant cultivars should delay infection of the flag leaf and spike.The greatest differentiation among cultivars for sporulation of L. nodorum was on upper leaves at Feekes growth stage 11.2. ‘Oasis’ consistently had less sporulation than other cultivars at all sampling dates and leaf positions. The rate of disease progress up the plant (disease severity) and area under the disease progress curve were also least on ‘Oasis’.There were significant correlations among components of resistance and associated components suggesting that a single or interrelated mechanisms control expression of resistance. Overall, ‘Oasis’ was the most resistant cultivar followed by ‘Coker 762’ and ‘Coker 747’. There cultivars have remained resistant for ten or more years. ‘Stacy’ was intermediate in resistance and was the most variable of the seven cultivars for the components tested. ‘Holley’, ‘Omega 78’, and ‘Florida 301’ were highly susceptible by all criteria tested. Plant height was not a factor in resistance. The resistant cultivars are mostly later in maturity than the susceptible cultivars. In this study, crop maturity was similar under existing environmental conditions during the two seasons when most data were collected.

Relationship of glume blotch symptoms on wheat heads to seed infection by Septoria nodorum.

Glume blotch on wheat heads was assessed visually in field plots. There was no correlation between head symptoms and seed infection caused by Septoria nodorum . Secondly, individual seeds were assigned to several severity classes based on the area of their lemmas that exhibited glume blotch symptoms. Seed infection was related to lemma symptoms only when the area exhibiting symptoms was > 0% and S. nodorum was more superficial than colonization associated with greater lemma symptom severities. High possession (i.e. extensive colonization) of infected seeds was directly related to the severity of lemma symptoms. The extent to which S. nodorum colonizes wheat seeds may be more important than the percentage of infected seeds. Thousand kernel weight was inversely related to the severity of lemma symptoms; maximum kernel weight reduction was reached when the area of blotching exceeded 50%.

Survival of Claviceps purpurea and C. paspali sclerotia.

Claviceps purpurea sclerotia did not survive longer than 6 mo regardless of the time of year sclerotia were placed outdoors in soil in Georgia. Sclerotia were rapidly deteriorated by soil microorga...

Inheritance of resistance to Xanthomonas campestris pv. translucens in triticale

SummaryThree triticale lines, Siskiyou, M2A-Beagle, and OK 77842 have been reported to possess resistance to bacterial leaf streak caused by Xanthomonas campestris, pv. translucens (Xct.). The three resistant lines were crossed to susceptible lines and crossed with each other. F2, BC1-F1, BC2-F1 plants were inoculated with a mixture of two Xct strains. The segregation data indicate the presence of a single dominant gene in each of the three resistant lines to bacterial leaf streak. These three genes are either the same or closely linked herein designated as Xct1.

Survival of Septoria nodorum in wheat seed.

Septoria nodorum , detected in wheat seed by isolating on ox gall agar, increased 1–25% during storage up to 24 months after harvest. S. nodorum detection increased more rapidly in seed lots stored at 25 °C than at 5°. This increase was generally related to a decrease in other seed fungi, especially Alternaria and Epicoccum . These fungi were competitive with but not antagonistic to S. nodorum on ox gall agar. The ox gall agar assay was preferred because of its accuracy and savings of time and labour. The differences of these results from those of some other workers were not related to assay procedures or the initial percentage colonization of the seeds by S. nodorum .

Germination Requirements of Claviceps Paspali Sclerotia

Germination of Claviceps paspali sclerotia occurred over a wide range of storage and incubation temperatures. Cold, moist storage conditions greatly enhanced germination but were not obligatory. Highest percentage germination and most rapid germination occurred after storage in moist sand at 5 and 10 C for 2 mo followed by incubation at 20 C for 41-44 da. Preconditioning periods at 20 and 25 C, incubation at 30 C, and dry storage of sclerotia were not conducive to germination.