Sulphate and sulphurous acid alter the relative susceptibility of wheat to Phaeosphaeria nodorum and Mycosphaerella graminicola

The relative abundances of DNA of Mycosphaerella graminicola and Phaeosphaeria nodorum in archived wheat samples are closely correlated with UK anthropogenic emissions of oxidized sulphur over the last 160 years. To test whether this could be a causal relationship, possible modes of action of sulphur on the two fungi were examined. Mycelial growth of the two fungi in solutions of sulphurous acid was similar. Sulphurous acid at pH 4 reduced percentage germination of P. nodorum conidia more strongly than M. graminicola conidia. In spray inoculations of wheat cv. Squarehead's Master, Cappelle Desprez and Riband with water or sulphurous acid (pH 4), the ratio of leaves infected by P. nodorum to leaves infected by M. graminicola was increased by factors of 2·5, 2·1 and 0·6, respectively at pH 4. The same three cultivars of wheat were grown in sand and vermiculite and fertilized with nutrient solution containing 2·5 or 0·5 mm sulphate. Both pathogens infected less frequently at 2·5 mm sulphate, by a factor of about 2. The severity of infection by M. graminicola was reduced on all three cultivars by a factor of about 4·5 at 2·5 mm sulphate, but severity of P. nodorum was reduced only by a factor of about 2. Both elevated free sulphate concentrations in soil and sulphite in rainwater could therefore increase the prevalence of P. nodorum relative to M. graminicola, which is consistent with the historical changes in abundance.     

Temperature Dependent Seed Transmission of Stagonospora nodorum in Wheat

The transmission of Stagonospora nodorum from four naturally infected winter wheat seedlots was quantified in controlled environment germination chambers at 9, 13, 17, 21, and 25 °C. Seedlings were harvested when the second leaf began to emerge. Coleoptiles and first seedling leaves were examined for the presence of lesions caused by S. nodorum. First leaves were incubated on Bannon's medium for 2 weeks, after which they were examined for pycnidia of S. nodorum. Transmission to the coleoptile occurred at all temperatures, but decreased from 100% to 72% as temperature increased from 9 to 25 °C. Transmission to the first leaf was less, dropping from 37% to 2% as temperature increased from 9 to 25 °C. At least 44% of infected first leaves were symptomless at all temperatures, with 96% of infected leaves showing no symptoms at 25 °C. Transmission to seedling leaves occurred over a broad temperature range. Under the high densities at which wheat is sown, a significant number of infected seedlings per unit area may originate from relatively low initial seed infection levels and transmission efficiencies.     

Control of Stagonospora nodorum and Septoria tritici in Wheat by Pre-treatment with Drechslera teres, a Non-host Pathogen

A field study is described which explored the possibility of controlling Stagonospora nodorum and Septoria tritici on wheat using a barley pathogen, Drechslera teres. Pre-treatment of wheat cv. Hussar flag leaves with D. teres resulted in a significant reduction in disease caused by S. nodorum and S. tritici, resulting in a significant increase in grain yield. When cv. Brigadier leaves were treated with D. teres prior to inoculation with S. nodorum there was an initial increase in disease expression whilst D. teres had no effect on symptoms produced by S. tritici on cv. Brigadier. There was significantly less disease on leaves of cvs. Hussar and Brigadier pre-treated with D. teres prior to inoculation with an equal mixture of S. nodorum and S. tritici compared to plants pre-treated with water. It is concluded that D. teres and other non-host pathogens show potential as biological control agents for S. nodorum and S. tritici.     

Antagonism of yeastlike phyllosphere fungi against Septoria nodorum on wheat leaves

Aureobasidium pullulans, Sporobolomyces roseus, andCryptococcus laurentii var.flavescens, added to the inoculum, reduced the superficial mycelial growth ofSeptoria nodorum and the infection of wheat leaves by 50% or more. The mycelial growth was affected similarly in vitro, on slides covered with water agar. The antagonistic effect on germination was slight. The concentration of the saprophytes on the leaves after inoculation was comparable to population densities occurring on field-grown wheat.Samenvatting Aureobasidium pullulans, Sporobolomyces roseus enCryptococcus laurentii var.flavescens toegevoegd aan een conidiënsuspensie vanSeptoria nodorum verminderden de oppervlakkige myceliumgroei vanSeptoria en de infectie, van de bladeren tot de helft of meer (Tabel 1). Het effect op de sporekieming was gering. In vitro, op zgn. agarglaasjes, werd de myceliumgroei op vergelijkbare wijze geremd (Tabel 2). De concentrative van de saprofyten na inoculatie kwam overeen met in het veld voorkomende populatiedichtheden (Tabel 3).     

SnToxA,SnTox1, and SnTox3 originated in Parastagonospora nodorum in the Fertile Crescent

The centre of origin of the globally distributed wheat pathogen Parastagonospora nodorum has remained uncertain because only a small number of isolates from the Fertile Crescent were included in earlier population genetic and phylogeographic studies. We isolated and genetically analysed 193 P. nodorum strains from three naturally infected wheat fields distributed across Iran using 11 neutral microsatellite loci. Compared to previous studies that included populations from North America, Europe, Africa, Australia, and China, the populations from Iran had the highest genetic diversity globally and also exhibited greater population structure over smaller spatial scales, patterns typically associated with the centre of origin of a species. Genes encoding the necrotrophic effectors SnToxA, SnTox1, and SnTox3 were found at a high frequency in the Iranian population. By sequencing 96 randomly chosen Iranian strains, we detected new alleles for all three effector genes. Analysis of allele diversity showed that all three effector genes had higher diversity in Iran than in any population included in previous studies, with Iran acting as a hub for the effector diversity that was found in other global populations. Taken together, these findings support the hypothesis that P. nodorum originated either within or nearby the Fertile Crescent with a genome that already encoded all three necrotrophic effectors during its emergence as a pathogen on wheat. Our findings also suggest that P. nodorum was the original source of the ToxA genes discovered in the wheat pathogens Phaeosphaeria avenaria f. sp. tritici 1, Pyrenophora tritici-repentis, and Bipolaris sorokiniana.     

Attachment and adhesion of conidia of Stagonospora nodorum to natural and artificial surfaces

Attachment and adhesion of conidia of a wheat-isolate of Stagonospora nodorum to leaf and artificial surfaces was studied. Attachment of conidia was a non-viable process, separate from adhesion, that occurred rapidly and irreversibly. Attachment involved conidial-surface carbohydrates and was partially influenced by surface hydrophobicity. The subsequent adhesion, via the secretion of extracellular matrix from conidia, was a viable process that induced the complete cover of conidia in response to wheat leaf surface components containing epi-cuticular wax and to a lesser extent to barley but inducing only partial covering on glass. Results suggest that specific surface components from the compatible host promote rapid attachment and adhesion of S. nodorum conidia.     

Development and validation of a set of standard area diagrams to aid in estimation of spot blotch severity on wheat leaves

This study aimed to develop and validate a standard area diagram (SAD) set to quantify the severity of spot blotch, caused by Bipolaris sorokiniana, on wheat leaves. The proposed SAD set includes images of leaves with 11 distinct disease severities (0·1, 1, 5, 10, 20, 30, 40, 50, 60, 70 and 83%). The SAD set was validated by 12 raters without experience in evaluating plant disease. Lin's concordance correlation analysis of estimated versus actual disease severity (based on image analysis) showed that precision and accuracy improved for all raters using the SAD set in contrast to assessments made without it. The SAD set improved accuracy (coefficient of bias, C_b = 0·88 and 0·99, without and with the SAD set, respectively) and agreement (Lin's concordance correlation coefficient, ρ_c = 0·81 and 0·96 without and with the SAD set, respectively) of the estimates of severity. The severity estimates were also more reliable when using the SAD set (coefficient of determination, R² = 0·76 unaided and R² = 0·92 with the SAD set, and intra‐class correlation ρ = 0·79 without the SAD set and ρ = 0·95 using the SAD set). The SAD set proposed in this study will improve the accuracy and reliability of estimates of spot blotch severity on wheat leaves.     

Potential for eradication of the exotic plant pathogens Phytophthora kernoviae and Phytophthora ramorum during composting

Temperature and exposure time effects on Phytophthora kernoviae and Phytophthora ramorum viability were examined in flasks of compost and in a large‐scale composting system containing plant waste. Cellophane, rhododendron leaf and peat‐based inoculum of P. kernoviae and P. ramorum isolates were used in flasks; naturally infected leaves were inserted into a large‐scale system. Exposures of 5 and 10 days respectively at a mean temperature of 35°C in flask and large‐scale composts reduced P. kernoviae and P. ramorum inocula to below detection limits using semi‐selective culturing. Although P. ramorum was undetectable after a 1‐day exposure of inoculum to compost at 40°C in flasks, it survived on leaves exposed to a mean temperature of 40·9°C for 5 days in a large‐scale composting system. No survival of P. ramorum was detected after exposure of infected leaves for 5 days to a mean temperature of ≥41·9°C (32·8°C for P. kernoviae) or for 10 days at ≥31·8°C (25·9°C for Phytophthora pseudosyringae on infected bilberry stems) in large‐scale systems. Fitted survival probabilities of P. ramorum on infected leaves exposed in a large‐scale system for 5 days at 45°C or for 10 days at 35°C were <3%, for an average initial infection level of leaves of 59·2%. RNA quantification to measure viability was shown to be unreliable in environments that favour RNA preservation: high levels of ITS1 RNA were recovered from P. kernoviae‐ and P. ramorum‐infected leaves exposed to composting plant wastes at >53°C, when all culture results were negative.     

Spatiotemporal variation in the fungal community associated with wheat leaves showing symptoms similar to stagonospora nodorum blotch

The fungal communities on wheat leaves showing symptoms similar to stagonospora nodorum blotch were analysed using terminal restriction fragment length polymorphism (T-RFLP). Collection of diseased leaves was carried out in eleven winter wheat fields located in three regions of Sweden during mid-July in 2003–2005. Fourteen different fungal species were found on the leaves out of which thirteen were identified to the species level and one to the genus level. The majority of the samples had between one and four species present of which at least one was a pathogen. Among the analysed leaves three major leaf pathogens were found: Phaeosphaeria nodorum was common during 2003 and 2004, Mycosphaerella graminicola dominated during 2005. Pyrenophora tritici-repentis was present in all fields, but sometimes in just a few samples. Phaeosphaeria nodorum and P. tritici-repentis often co-occurred on the same leaf. In addition, seven species of yeast and three saprophytes frequently occurred on the leaves every year. The variation in fungal community was largest between the different years while the region of Uppland diverged from the other two regions in species composition. No significant differences in fungal communities were found within a single field, indicating a uniform community at the lowest spatial level.     

Residues of Mecoprop in Post-emergence-Treated Wheat and Oat

The dissipation of mecoprop in wheat (Triticum aestivum L.) and oat (Avena sativa L.) was monitored over a growing season following post-emergence application of the dimethylamine salt of mecoprop to each crop at 1·1 kg ha^?1. Residues of mecoprop, as its methyl ester, were determined gas chromatographically using electrolytic conductivity detection. Initial residues in wheat (119 (±20) mg kg^?1) and oat (95·3 (± 10·0) mg kg^?1) on the day of application (four-leaf stage of wheat and four- to five-leaf stage of oat) decreased to 0·1 to 0·2 mg kg^?1, respectively, within six weeks. Residues were non-detectable in the mature seed of both crops. Recoveries of mecoprop were in the order of 90% from the green tissue and seed of both crops fortified at 0·05 mg kg^?1.     

Species composition,toxigenic potential and pathogenicity of Fusarium graminearum species complex isolates from southern Brazilian rice

This study aimed to assess the extent and distribution of Fusarium graminearum species complex (FGSC) diversity in rice seeds produced in southern Brazil. Four species and two trichothecene genotypes were detected among 89 FGSC isolates, based on a multilocus genotyping assay: F. asiaticum (69·6%) with the nivalenol (NIV) genotype, F. graminearum (14·6%) with the 15‐acetyldeoxynivalenol (ADON) genotype, and F. cortaderiae (14·6%) and F. meridionale (1·1%), both with the NIV genotype. Seven selected F. asiaticum isolates from rice produced NIV in rice‐based substrate in vitro, at levels ranging from 4·7 to 84·1 μg g^?1. Similarly, two F. graminearum isolates from rice produced mainly 15‐ADON (c. 15–41 μg g^?1) and a smaller amount of 3‐ADON (c. 6–12 μg g^?1). One F. meridionale and two F. cortaderiae isolates did not produce detectable levels of trichothecenes. Two F. asiaticum isolates from rice and two from wheat (from a previous study), and one F. graminearum isolate from wheat, were pathogenic to both crops at various levels of aggressiveness based on measures of disease severity in wheat spikes and rice kernel infection in a greenhouse assay. Fusarium asiaticum and the reference F. graminearum isolate from wheat produced NIV, and deoxynivalenol and acetylates, respectively, in the kernels of inoculated wheat heads. No trichothecene was produced in kernels from inoculated rice panicles by any of the isolates. These findings constitute the first report of FGSC composition in rice outside Asia, and confirm the dominance of F. asiaticum in rice agroecosystems.     

Evidence that wheat cultivars differ in their ability to build up inoculum of the take‐all fungus,Gaeumannomyces graminis var. tritici,under a first wheat crop

The effect of wheat cultivar on the build‐up of take‐all inoculum during a first wheat crop was measured after harvest using a soil core bioassay in field experiments over five growing seasons (2003–2008). Cultivar differences in individual years were explored by analysis of variance and a cross‐season Residual Maximum Likelihood (REML) variance components analysis was used to compare differences in those cultivars present in all years. Differences between cultivars in the build‐up of inoculum were close to or at significance in two of the five trial years (2004 P < 0·05; 2006 P < 0·07), and current commercially listed cultivars were represented at both extremes of the range. In 2007 and 2008, when environmental conditions were most favourable for inoculum build‐up, differences were not significant (P < 0·3). In 2005 the presence of Phialophora spp. at the trial site restricted the build‐up of take‐all inoculum under all cultivars. The cross season REML variance components analysis detected significant differences (range: 3·4–47·8% roots infected in the soil core bioassay; P < 0·01) between the nine cultivars present in all years (excluding 2005). This is the first evidence of relatively consistent differences between hexaploid wheat cultivars in their interactions with the take‐all fungus, and this could give an indication of those cultivars that could be grown as a first wheat crop, in order to reduce the risk of damaging take‐all in a second wheat crop. This phenomenon has been named the take‐all inoculum build‐up (TAB) trait.     

Natural occurrence of fusarium head blight,mycotoxins and mycotoxin‐producing isolates of Fusarium in commercial fields of wheat in Hubei

Combined analyses of the natural occurrence of fusarium head blight (FHB), mycotoxins and mycotoxin‐producing isolates of Fusarium spp. in fields of wheat revealed FHB epidemics in 12 of 14 regions in Hubei in 2009. Mycotoxin contamination ranged from 0·59 to 15·28 μg g^?1 in grains. Of the causal agents associated with symptoms of FHB, 84% were Fusarium asiaticum and 9·5% were Fusarium graminearum, while the remaining 6·5% were other Fusarium species. Genetic chemotyping demonstrated that F. asiaticum comprised deoxynivalenol (DON), 3‐acetyldeoxynivalenol (3‐AcDON), 15‐acetyldeoxynivalenol (15‐AcDON) and nivalenol (NIV) producers, whereas F. graminearum only included DON and 15‐AcDON producers. Compared with the chemotype patterns in 1999, there appeared to be a modest shift towards 3‐AcDON chemotypes in field populations during the following decade. However, isolates genetically chemotyped as 3‐AcDON were present in all regions, whereas the chemical 3‐AcDON was only detected in three of the 14 regions where 3‐AcDON accounted for 15–20% of the DON and acetylated forms. NIV mycotoxins were detected in seven regions, six of which also yielded NIV chemotypes. The number of genetic 3‐AcDON producers was positively correlated with amounts of total mycotoxins (DON, NIV and acetylated forms) or DON in wheat grains. Chemical analyses of wheat grains and rice cultures inoculated with different isolates from the fields confirmed their genetic chemotypes and revealed a preferential biosynthesis of 3‐AcDON and 4‐AcNIV in rice. These findings suggest the importance of chemotyping coupled with species identification for improved prediction of mycotoxin contamination in wheat.     

Biocontrol traits of two Paenibacillus polymyxa strains SQR‐21 and WR‐2 in response to fusaric acid,a phytotoxin produced by Fusarium species

The aim of the present study was to investigate the effects of the phytotoxin fusaric acid (FA) on the biocontrol traits of two biocontrol strains Paenibacillus polymyxa WR‐2 and SQR‐21. The results showed that the growth of both WR‐2 and SQR‐21 decreased with increasing FA concentration, and at 70 and 80 μg mL^?1 FA, respectively, the strains were unable to grow. The biocontrol traits of both strains were negatively affected by FA concentration higher than 2·5 μg mL^?1. However, at 2·5 μg mL^?1 FA, biofilm formation and root colonization were not affected and there was even a positive effect on the production of spores and hydrolytic enzymes (protease and β‐l,3‐glucanase). The production of fusaricidin‐type antifungal compounds was increased with an increase in FA concentration up to 50 and 60 μg mL^?1 for WR‐2 and SQR‐21, respectively. The production of antifungal volatile organic compounds by WR‐2 and SQR‐21 was increased only at 2·5 μg mL^?1 FA. The effect of FA on the overall metabolic activity of WR‐2 and SQR‐21 was also determined. This study will help to understand the response of P. polymyxa strains to FA and will help to improve their biocontrol efficiency.     

Influence of magnesium on physiological responses of wheat infected by Pyricularia oryzae

Although magnesium (Mg) is considered an essential element for wheat growth, its importance for disease control has often been overlooked, and the physiological features of diseased plants mediated by Mg remain elusive. In this study, the effect of three Mg concentrations (0·25, 2·5 and 4 mm ) on wheat resistance to leaf blast (Pyricularia oryzae), leaf gas exchange, invertase activity, cellular damage and foliar concentration of photosynthetic pigments and nutrients was investigated. Foliar Mg increased from 1·9 to 3·9 g kg^?1, whereas calcium (Ca) decreased from 7·8 to 4·9 g kg^?1 as the applied Mg increased from 0·25 to 4 mm . Blast severity increased from 11·3 to 39·6% as the applied Mg increased from 0·25 to 4 mm . Photosynthesis, stomatal conductance, transpiration and photosynthetic pigment concentrations decreased in inoculated plants compared to non‐inoculated plants regardless of the Mg concentration; however, the reductions were more pronounced for plants grown with 4 mm Mg than those grown with 0·25 mm Mg. On the other hand, a higher internal CO₂ concentration, invertase activity and malondialdehyde concentration was recorded in inoculated plants grown with 4 mm Mg compared to those grown with 0·25 mm Mg. In conclusion, reduced Ca uptake may partially explain the increased susceptibility of wheat to leaf blast with the highest Mg concentration. Mg‐induced susceptibility to leaf blast appeared responsible for the photosynthetic impairments. These were most probably due to biochemical constraints because plants grown with the highest Mg concentration suffered extensive cellular damage and degradation of photosynthetic pigments as a result of high disease severity.     

Biocontrol of sclerotinia stem rot (Sclerotinia sclerotiorum) of soybean using novel Bacillus subtilis strain SB24 under control conditions

Sclerotinia stem rot (SSR), caused by Sclerotinia sclerotiorum, is a major disease of soybean in Canada. Laboratory and greenhouse experiments were conducted to evaluate potential effectiveness of cell suspensions, cell‐free culture filtrates and broth cultures of Bacillus subtilis strain SB24 for suppression of SSR. The SB24 cell suspensions and cell‐free culture filtrates significantly reduced mycelial growth of S. sclerotiorum by 50 to 75% and suppressed sclerotial formation by > 90%. The severity on soybean was negatively correlated (r < ?0·84, P < 0·01) to the concentrations of cell suspension, cell‐free culture filtrate and broth culture applied. The cell suspension and broth culture preparations significantly (P < 0·01) reduced SSR severity by 45 to 90% at concentrations ranging from 5 × 10⁶ to 10⁹ CFU mL^?1. The most effective concentration was 5 × 10⁸ CFU mL^?1 for all three preparations, reducing the severity by 60 to 90%. The B. subtilis SB24 was most effective in reducing disease severity when applied ≤ 24 h before plant inoculation with S. sclerotiorum and a significant effectiveness was observed up to 15 days after plant inoculation. The population density of B. subtilis on soybean leaves decreased by 1·5 to 2·5 log units over 15 days under field conditions, and by 0·8 log units over 5 weeks under control conditions. The decrease in population density was significantly correlated with rainfall in the field (r < ?0·93, P < 0·01), suggesting that the biocontrol bacteria may be washed away by rain.     

Inhibition of Botrytis cinerea growth and suppression of botrytis bunch rot in grapes using chitosan

Chitosan inhibited growth of Botrytis cinerea in liquid culture and suppressed grey mould on detached grapevine leaves and bunch rot in commercial winegrapes. Germination of B. cinerea was completely inhibited in malt extract broth containing chitosan at concentrations greater than 0·125 g L^?1. However, treated conidia were able to infect detached Chardonnay leaves and pathogenicity was not affected, even after incubation for 24 h in chitosan at 10 g L^?1. When added after conidial germination, chitosan inhibited B. cinerea growth and induced morphological changes suggestive of possible curative activity. The effective concentration of chitosan that reduced mycelial growth by 50% (EC₅₀) was 0·06 g L^?1. As a foliar treatment, chitosan protected detached Chardonnay leaves against B. cinerea and reduced lesion diameter by up to 85% compared with untreated controls. Peroxidase and phenylalanine ammonia‐lyase activities were also induced in treated leaves. In vineyard studies, Chardonnay winegrapes exhibited 7·4% botrytis bunch rot severity at harvest in 2007 after treatment with an integrated programme that included chitosan sprays from bunch closure until 2 weeks preharvest, compared with 15·5% in untreated controls and 5·9% with fungicide treatment. In the following season, botrytis bunch rot severity was 44% in untreated Chardonnay at harvest and the integrated programme (21%) was less effective than fungicides (13·8%). However, in Sauvignon blanc winegrapes, the integrated and the fungicide programme each reduced botrytis bunch rot severity to 4% and were significantly different from the untreated control (11·5%). This study provides evidence that suppression of botrytis in winegrapes by chitosan involves direct and indirect modes of action.     

Erodium cicutarium density and duration of interference effects on yield of wheat,oilseed rape,pea and dry bean

Field studies were conducted to determine the effect of various densities and duration of interference of Erodium cicutarium (L.) L'Her. ex Ait. on the yield of wheat, oilseed rape, pea and dry bean. The magnitude of yield reductions caused by E. cicutarium differed among the crops. Results indicate that the ranking of crop tolerance to E. cicutarium, when established at their recommended planting densities, was wheat > oilseed rape > pea ? dry bean. Maximum yield reduction occurred at E. cicutarium densities of 100–200 plants m^?2 and were 36% for wheat, 37% for oilseed rape, 82% for dry bean and 92% for pea. Crop yield progressively decreased as the duration of E. cicutarium interference increased. Three weeks of E. cicutarium interference after emergence was sufficient to reduce the yield of all crops, indicating the importance of controlling this weed early in the growing season. The mean yield reduction for each week of E. cicutarium interference was 1·6%, 2·7%, 3·6% and 6·3% for wheat, oilseed rape, pea and dry bean respectively. E. cicutarium is therefore a weed that warrants consideration for control in annual cropping systems.