Microscopic analysis of the effect of azoxystrobin treatments on Mycosphaerella graminicola infection using green fluorescent protein (GFP)‐expressing transformants

Green fluorescent protein (GFP)‐expressing transformants were used to investigate the effects of strobilurin fungicide azoxystrobin on Mycosphaerella graminicola infection. Azoxystrobin treatments (125 or 250 g AI ha^?1) were applied at various stages of the infection process under controlled conditions. GFP transformants showed conserved in vitro sensitivity to azoxystrobin and pathogenicity. Azoxystrobin controlled over 90% of M graminicola infections when applied before or during penetration of the pathogen (15% of the incubation phase). Azoxystrobin also impaired the growth of intercellular hyphae in M graminicola post‐penetration infection stages when applied at up to 50% of the incubation phase. Incubating infections observed in treated leaves were viable, but their growth was impaired and they did not induce necrosis under controlled conditions. Reduction by half of azoxystrobin dosage had little or no effect on azoxystrobin efficiency in controlling M graminicola. The contribution of post‐penetration fungistatic effect to azoxystrobin curative properties toward M graminicola in a field situation is discussed. © 2001 Society of Chemical Industry     

The transmission by nematodes of tobraviruses is not determined exclusively by the virus coat protein

The coat protein gene of the nematode non-transmissible, SP5 isolate of pea early-browning tobravius was replaced with that of the highly nematode transmissible, PPK20 isolate of tobacco rattle tobravirus. Plants were infected with the recombinant virus when mechanically inoculated and the virus invaded the plants systemically. However, although the PPK20 isolate of TRV was transmitted by nematodes from these plants, the recombinant virus was not transmitted. Therefore, the virus coat protein is not the exclusive determinant of nematode transmission.     

Sexual Recombination in Gibberella zeae

ABSTRACT We developed a method for inducing sexual outcrosses in the homothallic Ascomycete fungus Gibberella zeae (anamorph: Fusarium graminearum). Strains were marked with different nitrate nonutilizing (nit) mutations, and vegetative compatibility groups served as additional markers in some crosses. Strains with complementary nit mutations were cocultured on carrot agar plates. Ascospores from individual perithecia were plated on a minimal medium (MM) containing nitrate as the sole nitrogen source. Crosses between different nit mutants segregated in expected ratios (3:1 nit(-):nit(+)) from heterozygous perithecia. Analysis of vegetative compatibility groups of progeny of two crosses indicated two and three vegetative incompatibility (vic) genes segregating, respectively. For rapid testing of sexual recombination between nit mutants, perithecia were inverted over MM to deposit actively discharged ascospores. Development of proto-trophic wild-type colonies was taken as evidence of sexual recombination. Strains of G. zeae group 2 from Japan, Nepal, and South Africa, and from Indiana, Kansas, and Ohio in the United States were sexually interfertile. Four group 1 strains were not interfertile among themselves or with seven group 2 strains. Attempts to cross G. zeae with representatives of F. acuminatum, F. avenaceum, F. culmorum, F. crookwellense, F. oxysporum, and three mating populations of G. fujikuroi were not successful.     

Diversity of Epidemic Populations of Gibberella zeae from Small Quadrats in Kansas and North Dakota

ABSTRACT Gibberella zeae (anamorph Fusarium graminearum) causes Fusarium head blight (FHB) of wheat and barley and has been responsible for several billion dollars of losses in the United States since the early 1990s. We isolated G. zeae from the top, middle, and bottom positions of wheat spikes collected from 0.25-m(2) quadrats during severe FHB epidemics in a single Kansas (KS) field (1993) and in a single North Dakota (ND) field (1994). Three amplified fragment length polymorphism (AFLP) primer pairs were used to resolve 94 polymorphic loci from 253 isolates. Members of a subset of 26 isolates also were tested for vegetative compatibility groups (VCGs). Both methods indicated high levels of genotypic variability and identified the same sets of isolates as probable clones. The mean number of AFLP multilocus haplotypes per head was approximately 1.8 in each population, but this value probably underestimates the true mean due to the small number of samples taken from each head. Isolates with the same AFLP haplotype often were recovered from different positions in a single head, but only rarely were such apparently clonal isolates recovered from more than one head within a quadrat, a pattern that is consistent with a genetically diverse initial inoculum and limited secondary spread. The KS and ND samples had no common AFLP haplotypes. All G. zeae isolates had high AFLP fingerprint similarity (>70%, unweighted pair group method with arithmetic means similarity) to reference isolates of G. zeae lineage 7. The genetic identity between the KS and ND populations was >99% and the estimated effective migration rate was high (Nm approximately 70). Tests for linkage disequilibrium provide little evidence for nonrandom associations between loci. Our results suggest that these populations are parts of a single, panmictic population that experiences frequent recombination. Our results also suggest that a variety of population sampling designs may be satisfactory for assessing diversity in this fungus.     

An Allee Effect Reduces the Invasive Potential of Tilletia indica

ABSTRACT The Karnal bunt pathogen, Tilletia indica, is heterothallic and depends on encounters on wheat spikes between airborne secondary sporidia of different mating types for successful infection and reproduction. This life history characteristic results in reduced reproductive success for lower population densities. Such destabilizing density dependence at low population levels has been described for a range of animals and plants and is often termed an Allee effect. Our objective was to characterize how the Allee effect might reduce the invasive potential of this economically important pathogen. We developed a simple population model of T. indica that incorporates an Allee effect by calculating the probability of infection for different numbers of secondary sporidia in the infection court. An Allee effect is predicted to be important at the frontier of an invasion, for establishment of new foci by a small population of teliospores, and when the environment is nonconducive for the production of secondary sporidia. Using estimated model parameter values, we demonstrated a theoretical threshold population size below which populations of T. indica were predicted to decline rather than increase. This threshold will vary from season to season as a function of weather variables and their effect on the reproductive potential of T. indica. Deployment of partial resistance or use of fungicides may be more useful if they push population levels below this threshold.     

Quantitative PCR monitoring of the effect of azoxystrobin treatments on Mycosphaerella graminicola epidemics in the field

Quantitative PCR and visual monitoring of Mycosphaerella graminicola epidemics were performed to investigate the effect of curative and preventative applications of azoxystrobin in wheat field crops. A non-systemic protectant and a systemic curative fungicide, chlorothalonil and epoxiconazole, respectively, were used as references. PCR diagnosis detected leaf infection by M graminicola 3 weeks before symptom appearance, thereby allowing a clear distinction between curative and preventative treatments. When applied 1 week after the beginning of infection, azoxystrobin curative activity was intermediate between chlorothalonil (low effect) and epoxiconazole. When applied preventatively, none of the fungicides completely prevented leaf infection. There was some indication that azoxystrobin preventative treatments may delay fungal DNA increase more than epoxiconazole at the beginning of leaf infection. Both curative and preventative treatments increased the time lapse between the earliest PCR detection and the measurement of a 10% necrotic leaf area. Azoxystrobin only slightly decreased the speed of necrotic area increase compared with epoxiconazole. Hence, azoxystrobin activity toward M graminicola mainly resides in lengthening the time lapse between the earliest PCR detection and the measurement of a 10% necrotic leaf area. Information generated in this way is useful for optimal positioning of azoxystrobin treatments on M graminicola.     

Major outer membrane proteins of Brucella spp.: past,present and future

The major outer membrane proteins (OMPs) of Brucella spp. were initially identified in the early 1980s and characterised as potential immunogenic and protective antigens. They were classified according to their apparent molecular mass as 36–38 kDa OMPs or group 2 porin proteins and 31–34 and 25–27 kDa OMPs which belong to the group 3 proteins. The genes encoding the group 2 porin proteins were identified in the late 1980s and consist of two genes, omp2a and omp2b, which are closely linked in the Brucella genome, and which share a great degree of identity (>85%). In the 1990s, two genes were identified coding for the group 3 proteins and were named omp25 and omp31. The predicted amino acid sequences of omp25 and omp31 share 34% identity. The recent release of the genome sequence of B. melitensis 16 M has revealed the presence of five additional gene products homologous to Omp25 and Omp31. The use of recombinant protein technology and monoclonal antibodies (MAbs) has shown that the major OMPs appear to be of little relevance as antigens in smooth (S) B. abortus or B. melitensis infections i.e. low or no protective activity in the mouse model of infection and low or no immunogenicity during host infection. However, group 3 proteins, in particular Omp31, appear as immunodominant antigen in the course of rough (R) B. ovis infection in rams and as important protective antigen in the B. ovis mouse model of infection. The major OMP genes display diversity and specific markers have been identified for Brucella species, biovars, and strains, including the recent marine mammal Brucella isolates for which new species names have been proposed. Recently, Omp25 has been shown to be involved in virulence of B. melitensis, B. abortus and B. ovis. Mutants lacking Omp25 are indeed attenuated in animal models of infection, and moreover provide levels of protection similar or better than currently used attenuated vaccine strain B. melitensis Rev.1. Therefore, these mutant strains appear interesting vaccine candidates for the future. The other group 3 proteins identified in the genome merit also further investigation related to the development of new vaccines.     

世界银行倡导以农业促发展

当今世界的农业发展概况 当今世界的农业,广袤、多样且快速变化.如果地方层面、国家层面乃至全球层面都有健全的政策和支持性投入,今天的农业将为数以亿计的农村穷人摆脱贫困提供新的契机.农业向他们敞开脱贫道路包括家庭种植业和养殖业、高附加值的"新型农业"就业机会、新兴的农村非农产业创业和就业机会等等.