Greg J. Boland

Index of plant hosts of Sclerotinia sclerotiorum

(1994). Index of plant hosts of Sclerotinia sclerotiorum. Canadian Journal of Plant Pathology: Vol. 16, No. 2, pp. 93-108.

A review of the non-target effects of fungi used to biologically control plant diseases

Abstract Biological control agents for plant diseases are currently being examined as alternatives to synthetic pesticides due to their perceived increased level of safety and minimal environmental impacts. Fungal biological control agents have several mechanisms of action that allow them to control pathogens, including mycoparasitism, production of antibiotics or enzymes, competition for nutrients and the induction of plant host defences. While effective in the control of plant diseases, these mechanisms may pose risks to non-target species including mycorrhizal and saprophytic fungi, soil bacteria, plants, insects, aquatic and terrestrial animals, and humans. Non-target effects including mycoparasitism of mycorrhizae, reduction in plant root colonisation by mycorrhizal fungi, disorders in commercial mushrooms and nodulation by Rhizobium spp., and changes in plant growth have been associated with fungal biological control agents, such as Trichoderma spp. Also, the genera Trichoderma and Gliocladium have been linked to respiratory disorders and shellfish toxicity in humans, respectively. Biological control agents, such as Pythium oligandrum, Talaromyces flavus, Coniothyrium minitans and Ampelomyces quisqualis have modes of action which may pose risks to non-target fungi, bacteria, plants and animals. There is need for future research into ecological impacts associated with the release of any biological agent and methods of determining possible non-target effects. Adequate monitoring and the use of molecular techniques to identify and follow the movement of biological control agents are needed to examine and mitigate negative biological impacts.

Climate change and plant diseases in Ontario

Current models predict that expected climate change in Ontario will significantly affect the occurrence of plant diseases in agriculture and forestry in the coming years. Direct, multiple effects on the epidemiology of plant diseases are expected, including the survival of primary inoculum, the rate of disease progress during a growing season, and the duration of epidemics. These effects will positively or negatively influence individual pathogens and the diseases they cause. Changes in the spectra of diseases are also anticipated. Abiotic diseases associated with environmental extremes are expected to increase, and interactions between biotic and abiotic diseases might represent the most important effects of climate change on plant diseases. The management of plant diseases will also be affected. In agriculture, plant breeding programs are expected to adapt many crops to increased duration of growing seasons and, concurrently, to develop drought and stress tolerance. There will be opportunities for new crops and cultivars to be introduced, but effective systems must be in place to detect new pathogens and prevent them from entering with these new crops. Because of the long-lived nature of trees, forests are slow to adapt, and the impact of climate change will have to be considered in forest management plans. Adaptations in agriculture and forestry have been occurring in Ontario for over 100 years, but these may need to occur at an accelerated rate because of rapid changes in climate. It is critical that the infrastructure of agricultural and forestry research remains strong to ensure successful transition and adaptation.

Compost: a study of the development process and end-product potential for suppression of turfgrass disease.

The relationships among the chemical, physical and biological aspects of compost and their role in suppression of turfgrass pathogens are reviewed. The composting process, mediated by microbial activity, is affected by physical and chemical characteristics which include temperature, aeration, moisture content, C:N ratio and pH. In the absence of parameter restrictions, the microbial community follows a predictable successional pattern resulting in the re-colonization of compost with metabolically active mesophilic populations that can be suppressive towards plant pathogens. Although mechanisms of suppression are not fully understood, those postulated include physiochemical and biological characteristics. The physiochemical characteristics of composts can alter suppressive properties through direct effects on pathogens and antagonistic microorganisms, or indirect effects on host systems through the supply of nutrients, improvement of soil structure, porosity and water retention capabilities, along with other factors. Biological characteristics centre on microbial community involvement in suppressive mechanisms, which can include one or a combination of competition for nutrients, antibiosis, lytic and other extracellular enzyme production, parasitism, predation and host-mediated induction of resistance. As a result of the potential benefits of compost, there is considerable interest in determining the capacity for composts to suppress turfgrass pathogens. Although the exact mechanisms of suppression are largely unknown, there appear to be several factors that play an integrated role. The use of composts that successfully suppress turfgrass diseases will permit a reduction in the use of chemical controls, and slow the development of fungicide resistance.

Microbial studies of compost: bacterial identification, and their potential for turfgrass pathogen suppression

Composting is the degradation of organic materials through the activities of diverse microorganisms. This research examined microbial community dynamics, population levels and identification of bacteria throughout the composting process and in storage. In addition, an evaluation was performed to determine the potential for dominant bacterial isolates to suppress selected turfgrass pathogens: Sclerotinia homoeocarpa, Pythium graminicola, Typhula ishikariensis, and Microdochium nivale, responsible for causing the turfgrass diseases dollar spot, pythium blight, typhula blight, and fusarium patch, respectively. Composts supported high population levels of bacteria with 78% of cultures tested being Gram-negative. Proteolytic activity, found in 29% of cultures tested is a potential mechanism of suppression or competition with other microorganisms. Although the Biolog system did not identify a wide range of bacteria, the main Gram-negative genera identified in mature compost were Pseudomonas (28%), Serratia (20%), Klebsiella (11%), and Enterobacter (5%). Twenty-one percent of isolates tested were not identified by Biolog, and many more had similarity indexes < 0.50. The microbial identification system, based on whole cell fatty acid analysis, identified a wide range of bacteria, with a higher proportion of similarities than the Biolog system. Genera common to both testing procedures included Pseudomonas, Serratia, and Enterobacter. All Gram-positives were identified as Bacillus spp. Phospholipid fatty acid analysis, used to estimate the diversity of microbial communities, was useful in monitoring changes in microbial population in storage and during composting, as well as estimating levels of compost maturity. Plate challenge experiments revealed a number of cultures with antagonistic activity against turfgrass pathogens. There were 52, 31, 32 and 19% of the bacterial isolates tested that exhibited antagonistic activity against S. homoeocarpa, P. graminicola, T. ishikariensis, and M. nivale, respectively. Improved understanding of microbial populations and their dynamics in composts will expand their potential to act as suppressants on pathogenic fungi or turfgrass.

Transmissible hypovirulence in Sclerotinia minor

Thirty isolates of Sclerotinia minor were collected from a 4 ha commercial lettuce crop at Holland Marsh, Ontario, and examined for the presence of transmissible hypovirulence. Three slow-growing isolates with atypical colony morphology were less virulent (P = 0.05) on detached lettuce leaves than other isolates and were considered hypovirulent. These thirty isolates comprised three mycelial compatibility groups with the five least virulent isolates producing variable interactions. Hypovirulence was transmissible among some isolates in culture and on lettuce tissue. When a mycelial suspension of a hypovirulent isolate was sprayed on lesions on lettuce leaves caused by a compatible virulent isolate, there was a reduction in lesion size and number of sclerotia produced in comparison to control treatments. Virulent isolates converted to hypovirulence retained the hypovirulent phenotype after repeated subculturing. All isolates were tested for the presence of double-stranded ribonucleic acid (dsRNA), which is associated with transmissible hypovirulence in Sclerotinia sclerotiorum. Eleven isolates, including some virulent and all hypovirulent isolates, tested positive for dsRNA. However, conclusions could not be drawn concerning associations between certain dsRNA segments and hypovirulence in S. minor because of the variability in the recovery of dsRNA. Results of this study suggest that transmissible hypovirulence has potential for use as a management strategy for diseases caused by S. minor.

Interactions of air temperature, relative humidity and biological control agents on grey mold of bean

The interactions ofBotrytis cinerea and seven biological control agents (BCAs) were examined in controlled environments to determine the influence of selected relative humidities (RH) (90,95, and 100%) and air temperatures (20,24 and 28 ‡C) on grey mold of bean. All main effects and interactions were significant (P≤0.05) among the 72 treatments. In the control, lesions of grey mold developed under all environmental conditions but were largest at 24 ‡C×95 and 100% RH, and 28 ‡C×95% RH. Interactions of environment, BCAs and grey mold were complex.Alternaria alternata, Drechslera sp.,Myrothecium verrucaria, Trichoderma viride, Gliocladium roseum and an unidentified pink yeast were all highly dependent on environment for biological control efficacy, and changes of 4 ‡C or 5% RH were associated with variability in disease suppression that ranged from ≤ 15 to 100%. Efficacy ofEpicoccum purpurascens appeared independent of environment and this BCA suppressed disease by 100% in all of the environmental treatments. Suppression of grey mold by many of the BCAs was most effective under environmental conditions least conducive to disease. Therefore, evaluations of potential BCAs in environmental conditions that are marginal for disease can overestimate their efficacy in field environments. Assessments of biological control efficacy in various environments can be used to more accurately assess the potential of BCAs.

Mycelial growth and production of oxalic acid by virulent and hypovirulent isolates of Sclerotinia sclerotiorum

Abstract Hypovirulent dsRNA-containing, and virulent dsRNA-free, isolates of Sclerotinia sclerotiorum were compared for mycelial growth and oxalic acid production in liquid culture. When harvested at 14 days post-inoculation, hypovirulenr isolates 91 and 215 produced less dry mycelium than virulent isolates 191 and 275C in potato-dextrose broth (PDB) and PDB amended with sodium succinate (PDBSS). All isolates produced more oxalic acid per gram dry mycelium When grown in PDBSS than in PDB. In a time course study, hypovirulent isolates 91 and 275 produced less mycelium (0.23 and 0.03 g dry wt.) and oxalic acid (0.22 and 0.01 mg/mL) than virulent isolates 191 and 275C (0.81 and 0.91 g dry wt., and 0.50 and 0.63 mg oxalic acid per millilitre, respectively) at 15 days post-inoculation. The amount of oxalic acid produced per gram dry mycelium reached a maximum for virulent isolates 191 and 275C at 3 and 5 days post-inoculation. Hypovirulent isolate 91 reached a maximum at 9 days post-inoculation, and hypovirule...

Influence of crop rotation and tillage on production of apothecia by Sclerotinia sclerotiorum

Field experiments were conducted at Marden and Arkell, in southwestern Ontario, from 1995 to 1998 to assess the influence of crop rotation and tillage on production of apothecia by Sclerotinia sclerotiorum. Treatments included five 2-year crop rotations of continuous soybean, corn–soybean, soybean–corn, winter wheat – soybean, and soybean – winter wheat in combination with minimum tillage, no tillage with residue chopped after harvest, or no tillage with no disturbance of residue after harvest. Crop rotation had a significant effect on the production of apothecia in all years at both sites (except for Arkell in 1998) and, in general, lower mean numbers of apothecia and (or) clumps of apothecia were recorded in plots planted with corn or winter wheat than with soybean. In all years, and at both sites (except Arkell in 1995), there were fewer apothecia and clumps of apothecia in the no-tillage treatments than in minimum tillage, although the difference was not statistically different in all years. Interactions between crop rotation and tillage were only significant in 1996 and 1997 at Marden where apothecia production was greatest in minimum-tillage plots planted with soybean, regardless of the preceding crop. Within the two no-tillage treatments, plantings of continuous soybean produced the highest number of apothecia and clumps of apothecia, and there was a 46.4 and 80.6% reduction in the numbers of apothecia at Marden in 1996 and 1997, respectively, in the no-tillage with chopped residue treatments when any crop rotation was followed compared with continuous cropping of soybean. The results of this study established that crop rotation and no tillage of soybean was the most useful combination of treatments that reduced the primary inoculum (e.g., apothecia) of S. sclerotiorum in infested soybean fields.

Vegetative compatibility and transmission of hypovirulence-associated dsRNA in Sclerotinia homoeocarpa

One hundred and sixteen isolates of Sclerotinia homoeocarpa, the causal agent of dollar spot of turfgrass, were collected from golf courses at nine locations in southern Ontario and one location in Nova Scotia. Isolates were assessed for vegetative compatibility in culture, and four types of reactions were observed. These reactions were examined for transmission of hypovirulence-associated double-stranded RNA (dsRNA) and the results indicated that vegetative incompatibility in S. homoeocarpa ranged from fully to partially incompatible. The fully incompatible reaction strongly restricted transmission of hypovirulence-associated dsRNA whereas the partially incompatible reactions allowed limited spread between vegetative compatibility groups (VCGs), suggesting that vegetative incompatibility is not an absolute barrier to the transmission. Intra- and inter-location VCG tests detected vegetative incompatibility at 7 of the 10 locations, but there were only four unique VCGs among the 116 isolates. These VCGs were compared with representative isolates of six VCGs identified in Michigan, Illinois, and Wisconsin. Three VCGs in the present study correspond to VCG-A, -B, and -E, respectively, and one VCG, named VCG-G, was a new group. In southern Ontario, VCG-B was the most commonly recovered VCG, comprising 56.0% of the sampled isolates, and was present at nine locations. VCG-A and -G comprised 11.2 and 31.9% of the sampled isolates and were present at three and seven locations, respectively. VCG-E was only detected at one location. These results support the hypothesis that there is limited diversity among VCGs in S. homoeocarpa. The transmission of hypovirulence-associated dsRNA among VCGs and the relatively simple genetic structure, as reflected in the small number of VCGs within populations of S. homoeocarpa, is consistent and supportive of the use of hypovirulence as a biocontrol agent for dollar-spot management.Key words: Sclerotinia homoeocarpa, dollar spot, turfgrass, vegetative compatibility, hypovirulence, double-stranded RNA, dsRNA.