M.S. Melzer

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.

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.

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.

CHV3-TYPE DSRNAS AND THE GH2 GENOTYPE IN A POPULATION OF CRYPHONECTRIA PARASITICA IN ONTARIO

CCP19, a hypovirulent isolate of Cryphonectria parasitica collected from a site near Bothwell, Ontario, in 1987, has shown potential for use in biological control of chestnut blight. CCP19 contains dsRNA that hybridizes with hypovirus CHV3 from isolate GH2 (CHV3-GH2), a hypovirulent isolate of C. parasitica from Michigan. In 1995, 26 isolates of C. parastica were collected at the Bothwell site, and 22-23 (85-89%) of these 26 isolates had significantly smaller lesion diameters in an apple virulence assay than virulent control isolate EP155. Twenty-two of the 26 isolates (85%) contained dsRNA. Segments of dsRNA in all 22 isolates hybridized with cDNA prepared from CHV3-type dsRNA in CCP19. The CHV3-type dsRNA was present in all three MCGs identified in the population. DNA fingerprint analysis revealed a highly clonal population, Twenty isolates had DNA fingerprint genotypes the same as, or similar to, GH2 and 6 isolates had genotypes similar to CCP19. Similarities, including presence of CHV3-type dsRNA, hig...

Asymptomatic infection, and distribution of Ophiostoma mitovirus 3a (OMV3a), in populations of Sclerotinia homoeocarpa

One hundred and sixteen isolates of Sclerotinia homoeocarpa from nine locations in southwestern Ontario and one location in Nova Scotia were assessed for the presence of a hypovirulence-associated mitovirus, Ophiostoma mitovirus 3a (OMV3a). Two detection methods, nucleic acid extraction with gel electrophoresis (NAE–GE) and reverse transcription – polymerase chain reaction (RT–PCR), were employed. Phenotype, growth rate, and virulence of isolates were also assessed. OMV3a was detected in four isolates (3.4%), using both the standard NAE–GE method and the RT–PCR method. The four isolates displayed reduced colony growth and virulence, and they were considered to be symptomatic, hypovirulent isolates infected by OMV3a. In the remaining 112 isolates, OMV3a was detected in 53 isolates by RT–PCR only. These isolates displayed typical colony growth and virulence, and they were considered to have asymptomatic infections by the virus. OMV3a was not detected by either method in 59 isolates that displayed typical colony growth and virulence; these isolates were considered to be free of infection by the virus. OMV3a was detected from 8 of 10 locations, all in Ontario, and in 17% to 100% of S. homoeocarpa isolates at these locations. Vegetative compatibility groups (VCGs) were previously determined for these isolates, and OMV3a was detected in 66%, 54%, and 19% of the isolates from VCGs B, A, and G, respectively, which were the three most prevalent. The results demonstrated that OMV3a was present in isolates with and without the characteristic symptoms of hypovirulence and that it was widely distributed in southwestern Ontario and in isolates from the three most prevalent VCGs detected among these isolates. This is the first report of widespread asymptomatic infections by a hypovirulence-related virus in populations of a fungal plant pathogen.

Characterization and pathogenicity of Rhizoctonia spp. from field crops in Canada

Abstract A survey for the presence of Rhizoctonia species on barley, bean, canola, corn, pea, soybean and wheat in 170 fields in Canada was completed during 2009–2011. Rhizoctonia solani was recovered from 52 (31%) of the 170 fields surveyed. A total of 191 isolates of R. solani belonging to anastomosis groups (AG) AG 2-1, AG 2-2, AG 4, AG 5, AG 9 and AG 11 were recovered. Fifty-two binucleate isolates of Rhizoctonia spp. were also recovered from 35 (21%) of the crops surveyed. In growth room pathogenicity studies, isolates belonging to AG 2-1 were pathogenic primarily on canola while isolates of other anastomosis groups tended to have wider host ranges. Symptoms included pre- and post-emergence damping off, crown rot and, to a lesser extent, root rot. The majority of binucleate isolates were not pathogenic or only had low aggressiveness on the hosts tested. There was considerable variation in host range among isolates of R. solani within individual AGs, and isolates recovered from asymptomatic seedlings of some hosts were pathogenic to other hosts often used in the same rotation. Collectively, the results are important for documenting the widespread occurrence of R. solani in seedlings of field crops in Canada, for characterizing variation in AGs and host range, and for identifying effective crop sequences used in crop rotations.