A dramatic increase in the incidence of late blight and changes within populations of Phytophthora infestans were observed recently in Canada. The occurrence of several new genotypes with associated phenotypes that dominated pathogen populations in various regions was documented. Genotype US-23 that was previously detected only among isolates from the United States, dominated in the western Canadian provinces of British Columbia (BC), Alberta (AB), Saskatchewan (SK), and Manitoba (MB) (1). Although US-23 infects both potato and tomato, it was more aggressive on tomato and was the only genotype recovered from commercial garden centers. There appear to be both changes and similarities in the genetic composition of the late blight pathogen in 2010 and 2011.

2011

Introduction Materials and methods

Results and discussion

Conclusions

References

Larry Kawchuk1, Jim Lynn1, Mel Kalischuk2 and Ron Howard3

1Agriculture and Agri-Food Canada, Lethbridge Research Centre, Lethbridge, AB, T1J 4B1; 2University of Lethbridge, Department of Biological Sciences, Lethbridge, AB, T1K 3M4;

3Alberta Agriculture and Rural Development, Crop Diversification Centre South, Brooks, AB, T1R 1E6

Agriculture and Agri-Food Canada

Agriculture et Agroalimentaire Canada

Figure 1. (A) Typical leaf and stem lesions produced by P. infestans on potato

and tomato. (B) Distinctive tuber necrosis caused by P. infestans is observed

below the periderm .

Figure 3. Distribution of Phytophthora

infestans in Alberta during 2011. Only

the relatively non-aggressive Ridimol

sensitive/intermediate US-11 A1 and

US-24 A1 has been detected in AB

during 2011. The US-23 genotype

was successfully eliminated from AB

in 2011.

1. L. Kawchuk et al. (2011) Plant Disease 95: 873.

2. S. Goodwin et al. (1992) Current Genetics 22: 107.

3. P. Kathiria et al. (2010) Plant Physiology 153: 1859.

Acknowledgements

We thank the Potato Growers of Alberta, Maple Leaf Potatoes, McCain Foods Ltd, and Growers Supply Ltd for their support. This work was funded in part by the Canadian Horticultural Council and AAFC Abase Project 1751.

Genetic Composition of Phytophthora infestans in Alberta and Canada

Genotype US-8, previously dominant throughout Canada, was only detected in isolates from eastern Canadian provinces of Quebec (QC), New Brunswick (NB) and Prince Edward Island (PE) in 2010. Other genotypes detected in Canada included US-11 in AB, US-24 in MB, and US-22 in Ontario (ON). An additional genotype was detected in ON, US-22a, which appears to be a derivative of US-22 that may have arisen through sexual reproduction. However, clonal reproduction dominated and opportunities for sexual reproduction were probably limited because of a surprising separation of the A1 and A2 mating types geographically. Long-distance movement in seed tubers and garden center transplants appears to have contributed to the rapid and region specific spread of the genotypes across Canada.

Isolates of P. infestans representing most areas of Canada were obtained from potato or tomato plants, tubers, or fruits infected during 2009 to 2011. Metalaxyl sensitivity was determined using amended agar or floating leaf-disks and mating type of each isolate was ascertained by pairing isolates with a characterized A1 and A2 isolate on RSA. Procedures for DNA restriction fragment length polymorphism (RFLP) analyses were as described previously (2). The multilocus P. infestans specific RFLP probe RG57 from US-11 was used to determine the genotype of each isolate.

The new genotypes present a challenge to control with distinctive epidemiology including host preference and fungicide sensitivity. Global movement of seed and transplants provided an unprecedented opportunity of rapid migration of new genotypes into areas with environmental conditions for the disease. As a result, epidemics may occur more often, earlier in the season, and be more difficult to control. Continued monitoring of the pathogen across North America will provide early warning of the anticipated genotypes for the development of prevention strategies (3).

Figure 2. Locations of Phytophthora infestans populations and mating type distribution in major potato

production areas of Canada in 2010. A dichotomy of mating types was observed with US-23 A1

dominating in the west and US-8 A2 in the east.

A B