W. G. D. Fernando

Head Blight Gradients Caused by Gibberella zeae from Area Sources of Inoculum in Wheat Field Plots

ABSTRACT The spread of Fusarium head blight of wheat from a small area inoculum source was examined in wheat plots (100, 625, or 2,500 m(2)) inoculated in the center with Gibberella zeae-colonized corn kernels or macro-conidia sprayed on heads at anthesis. With the first inoculation method, disease foci were produced from ascospores released from perithecia formed on inoculated kernels. With the second inoculation method, disease foci were produced by macroconidia directly applied to the heads. Some plots were misted during anthesis. Plots were divided into grids, and disease incidence on spikelets and seeds was assessed at the grid intersections. Isopath contour maps were constructed using an interpolation procedure based on a weighted least squares method. Disease gradients were constructed from the isopath contours in the direction parallel to average nightly wind vectors using an exponential model. This study was conducted over a 3-year period at two sites: one in Quebec and one in Ontario. Both inoculation methods resulted in a discrete, primary focus of head blight in each plot, with one or two smaller secondary foci in some plots. The highest incidence of disease on spikelets or seed was commonly displaced somewhat from the inoculum source, usually downwind. The gradient slopes of seed and spikelet infection ranged from -0.10 to -0.43 m(1) in plots with ascospore inoculum and from -0.48 to -0.79 m(1) in plots inoculated with macroconidia. Seed infection declined to 10% of the maximum within 5 to 22 m from the focal center in asco-spore-inoculated plots, and within 5 m in a macroconidia-inoculated plot. Gradients were usually steeper upwind compared with downwind of the inoculum source. In misted plots, incidence of disease was higher and more diffuse than in nonirrigated plots. Based on gradients and dispersal patterns, disease foci in plots inoculated with G. zeae-colonized corn kernels probably arose from airborne ascospores rather than from splash-borne macroconidia and were the result of infection events that occurred over a short period of time. Comparison of conidial- and ascospore-derived disease gradients indicated a lack of secondary infection, confirming that Fusarium head blight is primarily a monocyclic disease.

Population Structure, Chemotype Diversity, and Potential Chemotype Shifting of Fusarium graminearum in Wheat Fields of Manitoba

This study was to investigate the variation of acetyl ester derivative of DON at 15-position oxygen (15ADON) and acetyl ester derivative of DON at 3-position oxygen (3ADON) chemotypes and potential chemotype shifting of Fusarium graminearum based on the population structure of this species in Manitoba. The study was conducted in 15 locations with wheat cvs. Superb and AC Barrie in Manitoba from 2004 to 2005. Percentages of chemotypes 3ADON and 15ADON of F. graminearum ranged from 0 to 95.7 and 4.3 to 100%, respectively. The 3ADON chemotype was distributed in the southern part of Manitoba and predominant in Morris and Horndean. The two chemotypes almost shared the same percentage in Portage la Prairie. The 15ADON chemotype was predominant in the other locations. Significant gene flow was found among the populations from Sanford, Portage la Prairie, Hamiota, Plumas, Rapid City, and Virden; the populations from Cartier, Rivers, Killarney, and Souris; and the populations from Morris, Kenville, and Dauphin. There were no differences between the populations from two wheat cultivars and two chemotypes. The great variation of chemotype likely resulted from the great genetic diversity of F. graminearum. Sexual recombination, population age, and cropping system could result in genetic and chemotypic diversities. Wheat seed shipment and long-distance spore transportation of F. graminearum potentially caused the genetic migration and chemotype shifting in Manitoba.

Comparative screening of bacteria for biological control of potato late blight (strain US-8), using in- vitro, detached-leaves, and whole-plant testing systems

Forty-three bacteria were isolated from the phylloplane and (or) rhizosphere of potato and canola plants and tested for their ability to control the pathogen Phytophthora infestans (strain US-8) causing late blight on potato. This study revealed the benefit of using more than one system when searching for biocontrol activity. In this regard, the complementarity of the three systems chosen (in-vitro culture media, detached leaves, and whole plants) in selecting and identifying potential modes of action, provided a useful insight into the different types of biocontrol activity present. Bacteria with biocontrol activity were from the genera Bacillus, Pseudomonas, Rahnella, and Serratia. Mechanisms of inhibition characterized included those occurring directly, through antibiosis, and (or) indirectly, through the induction of plant defense systems.

Induction of plant defence compounds by Pseudomonas chlororaphis PA23 and Bacillus subtilis BSCBE4 in controlling damping-off of hot pepper caused by Pythium aphanidermatum

Abstract Bacillus subtilis strain BSCBE4 and Pseudomonas chlororaphis strain PA23 (=P. aureofaciens) were effective biocontrol agents against Pythium aphanidermatum, the causal agent of damping-off of hot pepper (Capsicum annum L.) in greenhouse vegetable production systems. Application of strains BSCBE4 and PA23 at the rate of 20 g kg−1 of seed significantly increased the growth of hot pepper seedlings. The efficacies of various carriers in sustaining the population of these strains in storage were assessed. Both the antagonists survived up to 180 days of storage in peat and talc-based formulations. The two bacterial strains induced development of plant defence-related enzymes including phenylalanine ammonia lyase, peroxidase, polyphenol oxidase, phenol content, suppressed incidence of damping-off and increased growth of hot pepper seedlings.

Seasonal and Diurnal Patterns of Spore Dispersal by Leptosphaeria maculans from Canola Stubble in Relation to Environmental Conditions

Seasonal and diurnal patterns of spore dispersal by Leptosphaeria maculans, which causes blackleg disease of canola, were studied in two consecutive field seasons using a 7-day Burkard spore sampler and rotorod impaction spore samplers. Ascospores of L. maculans were trapped from mid-June to the end of July, whereas pycnidiospores were trapped from mid-July until the end of July or early August. Ascospores and pycnidiospores were trapped between 9:00 P.M. and 4:00 A.M., when air temperatures were 13 to 18°C and relative humidity was >80%. Peak ascospore and pycnidiospore dispersal was associated with rain events. Peak ascospore dispersal was found to occur several hours after rainfall ≥2 mm, and ascospore dispersal continued for approximately 3 days after such events. Peak pycnidiospore dispersal occurred during the same hours as rain events. More ascospores and pycnidiospores were carried in the direction of prevailing winds than in other directions. To the south of the inoculated area, the gradients of disease incidence and stem disease severity were -19.2 and -0.8 m-1, respectively. Disease incidence and stem severity declined by 50% 12.5 and 5.5 m from the inoculated area, respectively. To the north of the inoculated area, the gradients of disease incidence and stem severity were -21.5 and -0.7 m-1, respectively. Disease incidence and stem severity declined by 50% 14.0 and 5.2 m from the inoculated area, respectively. In 2001, ascospores and pycnidiospores were trapped within 25 m of the inoculated area, whereas pycnidiospores were trapped up to 45 m from the inoculated area.

Epidemiology and biological control of Gibberella zeae / Fusarium graminearum

A decade of losses and damage due to fusarium head blight in cereals in North America and other parts of the world has resulted in great efforts to understand the factors that cause and intensify the disease. This review considers our current understanding of the importance of the contribution of cultural practices to the increase or decrease of inoculum levels, spore dispersal, and biological control of spore production and fusarium head blight. Perithecia of Gibberella zeae develop on aboveground residues, and on maize and wheat kernels rather than maize stems and wheat spikes, at temperatures of 15 and 25 °C, but not below 15 °C or above 30 °C. Ascospores are released during the evening, in response to rising relative humidity, and there is evidence for both local and long-distance dispersal. The effect of rotation and tillage system on the development of inoculum and FHB requires further research, but several studies indicate that weather may be the principal factor in development of the disease. Several fungal and bacterial species have been reported to inhibit hyphal and perithecial formation of G. zeae. These are discussed with an appraisal of the conditions and requirements to produce an effective biological control of G. zeae.

Genetic Diversity of Gibberella zeae Isolates from Manitoba

Gibberella zeae (anamorph Fusarium graminearum) causes Fusarium head blight, one of the most important diseases of cereals in the Canadian prairies for the last decade. In 2002, 60 isolates of G. zeae were collected and single spored from naturally infected spikes of wheat from Carman and Winnipeg in Manitoba. These isolates were compared using vegetative compatibility analysis and polymerase chain reaction (PCR)-based sequence related amplified polymorphisms (SRAP). Sixteen vegetative compatibility groups (VCG) were found among the 50 isolates tested. Five VCGs were found in the two locations, five in Carman and six in Winnipeg. Eight SRAP primer pairs amplified 90 polymorphic DNA fragments from 60 isolates and identified 59 distinct haplotypes. Among seven pairs of isolates, each pair from a distinct spike, four had isolates with different VCGs and six comprised different SRAP haplotypes. Principal component analysis and UPGMA separated the dataset into two main groups, each with isolates from both locations. The analysis of molecular variance also revealed that 75 and 20% of the variance was associated with differences among individual isolates and varieties sampled, respectively. Geographic location was not a significant source of variation at P = 0.05 and accounted for only 4% of total variance. A low correlation between VCG and SRAP marker data was detected. This study showed that, although genetic diversity is high among G. zeae isolates, Carman and Winnipeg collections have a similar genetic makeup and are likely part of the same population. The significant proportion of variance accounted by the variety compared with the geographic origin of isolates suggests that seedborne inoculum might have contributed to the genetic diversity within the G. zeae collection under study.

Seasonal and daily variation in the airborne concentration of Gibberella zeae (Schw.) Petch spores in Manitoba

The aerial concentration of ascospores and macroconidia of Gibberella zeae (anamorph Fusarium graminearum) within small inoculated plots was measured over 2 years (1999 and 2000) in Manitoba. In July 1999, a 30 m × 30 m area was inoculated with corn kernels infested with G. zeae. Ten Rotorod spore samplers were set up in a line transect, trapping airborne ascospores of G. zeae from 1800 to 0200. Ascospore concentration was high 1 to 4 days after rainfall exceeding 5 mm. Daily ascospore concentrations ranged from 0 to 214 m−3. Daily macroconidial concentrations ranged from 0 to 42 m−3. In 2000, a 3 m × 3 m area was inoculated with corn kernels infested with G. zeae, and a Burkard 7-day spore sampler was set up in the centre. Higher numbers of both ascospores and macroconidia were trapped in 2000 than in 1999. The airborne concentration of ascospores started to increase between 1500 and 1700 and coincided with the lowest daily relative-humidity values. Ascospores continued to be trapped until 0400. The highest concentration of ascospores occurred at 2100, with a maximum of 15 233 m−3. Fewer ascospores were trapped between 0500 and 1400, ranging from 0 to 167 m−3. As in 1999, high ascospore concentrations were detected after rainfall exceeding 5 mm. Compared with the number of ascospores, fewer macroconidia were trapped, with daily concentrations ranging from 0 to 567 m−3. An increased understanding of airborne inoculum is important for developing local-risk assessment models and strategies for managing fusarium head blight.

PGPR MEDIATED MANAGEMENT OF STEM BLIGHT OF PHYLLANTHUS AMARUS (SCHUM AND THONN) CAUSED BY CORYNESPORA CASSIICOLA (BERK AND CURT) WEI

Bacillus subtilis (BSCBE4), Pseudomonas chlororaphis (PA23), endophytic P. fluorescens (ENPF1) inhibited the mycelial growth of stem blight pathogen Corynespora casiicola (Berk and Curt)Wei under in vitro. All these bacterial isolates produced both hydroxamate and carboxylate type of siderophores. But the siderophore production was maximum with the isolate ENPF1. Delivering of talc based formulation of BSCBE4 through seedling dip and foliar application effectively reduced stem blight disease incidence and increased the dry matter production under pot culture and field conditions. Application of BSCBE4, PA23 and ENPF1 increased the defense related enzymes such as peroxidase, polyphenol oxidase, chitinase and β-1,3 glucanase in P. amarus up to ten days after challenge inoculation with C. cassicola. Native gel electrophoretic analysis revealed that challenge inoculation of pathogen with BSCBE4 and PA23 induced both peroxidase and polyphnol oxidase isoforms.

Virulence and Genetic Variability Among Isolates of Mycosphaerella pinodes

Fifty-eight isolates of Mycosphaerella pinodes, collected from western Canada, New Zealand, France, Australia, the United Kingdom, and Ireland, were analyzed for pathogenic and genetic variation according to their virulence on six differential cultivars of field pea (AC Tamor, Bohatyre, Danto, Majoret, Miko, and Radley) and amplified fragment length polymorphism markers. The 56 isolates were classified into 15 pathotypes. Pathotype 1 consisted of 31 isolates that were virulent on all six pea differential cultivars. Pathotypes 14 and 15 consisted of eight isolates that were avirulent on all six differential cultivars or virulent on one of six differential cultivars. The analysis of molecular variance showed that 57.2% of the total variation was caused by differences among populations, and 42.8% was due to molecular diversity within populations. Phylogenetic analysis of molecular variation of isolates showed that most of the Canadian isolates and four Australian isolates formed two clustered groups, respectively, regardless of virulence on the six differential cultivars. Isolates from New Zealand were geographically clustered into two groups. However, the isolates from France, Ireland, and the United Kingdom were clustered with the Canadian isolates.