Effects of shading and powdery mildew infection on senescence of the root cortex and coleoptile of wheat and barley seedlings,and implications for root- and foot-rot fungi

Summary Nuclear and cytoplasmic staining methods were used to study natural senescence of the root cortex and coleoptile of wheat and barley seedlings grown in glasshouse conditions. Coleoptiles of barley senesced more slowly than those of wheat, paralleling the known difference in rates of root cortex senescence in these cereals. The coleoptiles and root cortices of both cereals senesced more slowly in shaded than in unshaded conditions, but infection of the shoots of barley byErysiphe graminis had little effect on root cortex senescence. The results are discussed in relation to infection by root- and foot-rot fungi. Previous reports on the effects of illumination on take-all infection (Gaeumannomyces graminis) are explained. It is suggested that natural senescence of the coleoptile might affect establishment of infection by the eyespot fungus,Pseudocercosporella herpotrichoides, either directly or through the activities of competing microorganisms.     

禾顶囊壳4个变种对禾本科牧草的致病性研究

采用人工接种法测定了禾顶囊壳小麦变种、燕麦变种、玉米变种和禾谷变种对4种禾本科牧草鸭茅、披碱草、苇状羊茅和无芒雀麦的致病性。结果表明,除禾顶囊壳玉米变种Ggm01菌株对披碱草没有致病性外,4个变种对供试的其它牧草都有致病性,且能产生全蚀病的典型症状。其中小麦变种和燕麦变种对供试牧草的致病性强于禾谷变种,玉米变种致病性最弱。各变种的菌株存在着致病性分化。小麦变种Ggt9813菌株对苇状羊茅和披碱草的致病性强于燕麦变种,发病严重度均达到50％以上。目前在我国尚未发现燕麦变种,小麦变种主要出现在北方地区,在牧草上的发生亦不广泛,因此二者具有检疫重要性。     

Susceptibility to take‐all of cereal and grass species,and their effects on pathogen inoculum

Two field trials were conducted to investigate different herbage grasses and cereals for their susceptibility to the disease take‐all, for their impact on concentrations of the pathogen, Gaeumannomyces graminis var. tritici (Ggt), in soil and for their effect on development of take‐all in a subsequent wheat crop. In the herbage grass trial, Bromus willdenowii was highly susceptible to Ggt, produced the greatest post‐senescence Ggt concentrations in soil and highest incidence of take‐all in following wheat crop. Lolium perenne, Lolium multiflorum and Festuca arundinacea supported low Ggt soil concentrations and fallow the least. The relationship between susceptibility to Ggt and post‐senescence concentrations in soil differed between pasture grasses and cereals. In a trial in which Ggt was added to half the plots and where wheat, barley, triticale, rye or fallow were compared, the susceptibility of the cereals to take‐all was not clearly linked to post‐harvest soil Ggt concentrations. In particular, triticale and rye had low and negligible take‐all infection respectively, but greater post‐harvest soil Ggt concentrations than barley or wheat. This indicates that low Ggt concentrations on roots may build up during crop senescence on some cereals. Soil Ggt concentrations were greater following harvest in inoculated plots sown to cereals, but in the second year there was more take‐all in the previously non‐inoculated than inoculated plots. Thus, the grass and cereal species differed in susceptibility to take‐all, in their impact on Ggt multiplication and in associated take‐all severity in following wheat crop.     

A functional analysis of mitochondrial respiratory chain cytochrome bc1 complex in Gaeumannomyces tritici by RNA silencing as a possible target of carabrone

Gaeumannomyces tritici, an ascomycete soilborne fungus, causes a devastating root disease in wheat. Carabrone, a botanical bicyclic sesquiterpenic lactone, is a promising fungicidal agent that can effectively control G. tritici. However, the mechanism of action of carabrone against G. tritici remains largely unclear. Here, we used immunogold for subcellular localization of carabrone and the results showed that carabrone is subcellularly localized in the mitochondria of G. tritici. We then explored the functional analysis of genes GtCytc₁, GtCytb, and GtIsp of the mitochondrial respiratory chain cytochrome bc₁ complex in G. tritici by RNA silencing as a possible target of carabrone. The results showed that the silenced mutant ∆GtIsp is less sensitive to carabrone compared to ∆GtCytc₁ and ∆GtCytb. Compared with the control, the activities of complex III in all the strains, except ∆GtIsp and carabrone-resistant isolate 24-HN-1, were significantly decreased following treatment with carabrone at EC₂₀ and EC₈₀ in vitro (40%–50% and 70%–80%, respectively). The activities of mitochondrial respiratory chain complex III and the mitochondrial respiration oxygen consumption rates in all the strains, except ∆GtIsp and 24-HN-1, were higher with respect to the control when treated with carabrone at EC₂₀ in vivo. The rates of mitochondrial respiration of all strains, except ∆GtIsp, were significantly inhibited following treatment with carabrone at EC₈₀ (ranging from 57% to 81%). This study reveals that the targeting of the iron–sulphur protein encoded by GtIsp is highly sensitive to carabrone and provides a direction for the research of carabrone's target.     

Genetic variation, pathogenicity and mycelial growth rate differentiation between Gaeumannomyces graminis var. tritici isolates derived from winter and spring wheat

The variability of Gaeumannomyces graminis var. tritici ( Ggt ) isolates was evaluated at an intravarietal level using non-molecular and molecular methods. Pathogenicity and linear growth rate of the pathogen were estimated. Very high pathogenicity was found in 44% of the isolates, medium in 20% and low only in 8%. Significant differences in mycelial growth rate were observed. The quickest linear growth rate of Ggt mycelium was observed at 25°C. Isolates derived from winter wheat grew faster than those obtained from spring wheat. The correlation between growth rates and pathogenicity was not significant. DNA polymorphism determined by random amplified polymorphic DNA (RAPD)–PCR was used to assess genetic variation among isolates. Thirty-two RAPD markers revealed DNA polymorphism suitable for assessing variability among isolates examined. Cluster analysis of RAPD data identified a few groups of isolates. RAPD markers associated with pathogenicity as well as mycelium growth rate were found.     

Evaluation of endophytic actinobacteria as antagonists of Gaeumannomyces graminis var. tritici in wheat

Endophytic actinobacteria isolated from healthy cereal plants were assessed for their ability to control fungal root pathogens of cereal crops both in vitro and in planta. Thirty eight strains belonging to the genera Streptomyces, Microbispora, Micromonospora, and Nocardioidies were assayed for their ability to produce antifungal compounds in vitro against Gaeumannomyces graminis var. tritici (Ggt), the causal agent of take-all disease in wheat, Rhizoctonia solani and Pythium spp. Spores of these strains were applied as coatings to wheat seed, with five replicates (25 plants), and assayed for the control of take-all disease in planta in steamed soil. The biocontrol activity of the 17 most active actinobacterial strains was tested further in a field soil naturally infested with take-all and Rhizoctonia. Sixty-four percent of this group of microorganisms exhibited antifungal activity in vitro, which is not unexpected as actinobacteria are recognized as prolific producers of bioactive secondary metabolites. Seventeen of the actinobacteria displayed statistically significant activity in planta against Ggt in the steamed soil bioassay. The active endophytes included a number of Streptomyces, as well as Microbispora and Nocardioides spp. and were also able to control the development of disease symptoms in treated plants exposed to Ggt and Rhizoctonia in the field soil. The results of this study indicate that endophytic actinobacteria may provide an advantage as biological control agents for use in the field, where others have failed, due to their ability to colonize the internal tissues of the host plant.     

Identification of antifungal substances secreted by Bacillus subtilis Z-14 that suppress Gaeumannomyces graminis var. tritici

Bacillus subtilis strain Z-14 has biological control activity against the take-all fungus Gaeumannomyces graminis var. tritici (Ggt). In Petri dishes, the crude extract from B. subtilis Z-14 culture filtrate reduced take-all severity in roots of wheat seedlings by 91.3% and potted plants by 69.8% compared to the Ggt-inoculated control. Treatment with the crude extract also significantly (P?<?.05) increased growth of roots’ average length, and fresh weight in comparison with those of the Ggt-inoculated control. B. subtilis Z-14 culture filtrate was relatively thermally stable with 88.2% of the antifungal activity being retained after being heated at 100°C for 30?min. Meanwhile, the antifungal activity remained almost unchanged (>95%) when the culture filtrate was exposed to a pH ranging from 3 to 8, but significantly reduced in basic conditions. This activity was not transferred to the organic solvent phase after treatment with organic extraction agents. B. subtilis Z-14 culture filtrate exhibited a broad spectrum of antifungal activities against various phytopathogenic fungi. Three homologs of iturin A (C14–16) were characterised by liquid chromatography-mass spectroscopy (LC-MS) and electrospray ionisation mass spectrometry/mass spectrometry collision-induced dissociation (ESI-MS/MS CID).     

Effects of sowing date and volunteers on the infectivity of soil infested with Gaeumannomyces graminis var. tritici and on take-all disease in successive crops of winter wheat

In a field experiment on winter wheat, take‐all on plants and the infectivity of the soil were studied in crop sequences with different combinations of sowing dates. Take‐all was negligible in the first wheat crop, but thereafter the mean disease intensity (measured using a take‐all rating, TAR, with a maximum of 300) was 108, 190, 118 and 251 in the second to fifth successive crops. In each growing season, the disease differed amongst sequences and built up more rapidly and was more intense on plants sown in mid‐September than on plants sown in mid‐October. In late‐sown plots, where volunteers had been present during the mid‐September to mid‐October period, take‐all reached an intensity intermediate between that in early‐sown plots and that in late‐sown plots that had been kept free of volunteers. Volunteers, therefore, partially offset the expected beneficial effect of decreased disease with later sowing. Differences in take‐all amongst sequences were most pronounced in the second wheat crop and early sowing of the previous wheat increased intensity of disease. In the following (third) crop, differences in disease intensity amongst sequences were smaller. Soil infectivity (measured by seedling bioassay after harvest) built up progressively from a low level after the first crop to peak after the third crop. In this build‐up phase, soil infectivity estimates were always numerically greater after harvest of early‐sown treatments than after later‐sown treatments, although never significant at P= 0.05. The greatest difference (P= 0.06) was recorded in October before sowing of the third crop, where the comparison was between soil after two previous early sowings and soil after two previous later sowings and control of volunteers. In the same autumn, presence of green cover (i.e. volunteers) was associated with a smaller loss of soil infectivity between harvest and later sowing than occurred in an absence of green cover. In 2^nd–4^th crops, where comparisons were available and mean TARs indicated moderate levels of take‐all, sowing later had no yield benefit, despite more take‐all and greater soil infectivity associated with early sowing. Important considerations for the management of crops at risk of take‐all are 1) choosing appropriate sowing dates to minimize take‐all or to encourage take‐all decline and 2) controlling volunteers and weed hosts where crops are sown late to minimise take‐all.     

Influence of Some Environmental Factors on Populations of Pratylenchus minyus in Wheat

The distribution and density of Pratylenchus minyus and possible relationships of several environmental components, including ammonium nitrate, were investigated in a wheat field in South Australia. Seasonal variation as measured every 2-4 weeks was eliminated from the observations by periodic regression. Correlation and regression analyses were then used to investigate the association of host plant, rainfall, temperature, and the fungus Gaeumannomyces graminis with P. minyus. Other than seasonal effects, soil moisture and G. graminis were the only components associated with P. minyus. Ammonium nitrate usually was correlated with fewer P. minyus in wheat roots. Much higher numbers of P. minyus were observed in seminal than in crown roots of wheat.