Submitted to: Plant Pathology
Publication Type: Peer reviewed journal
Publication Acceptance Date: 12/10/2008
Publication Date: 8/1/2009
Citation: Young, G.K., Cooke, L.R., Kirk, W.W., Tumbalam, P., Perez, F.G., Deahl, K.L. 2009. The influence of competition and host plant resistance on selection of Phytophthora infestans populations in Michigan State and Northern Ireland. Plant Pathology. 58(4) 703-714. Interpretive Summary: Late blight, caused by Phytophthora infestans, is the most disastrous and widespread disease of potato. One of the most effective means of controlling late blight is through the use of resistant cultivars, but newly developed resistant cultivars often lose their resistance after a few years of commercial production. We characterized patterns of fitness and diversity of late blight strains in the UK and compared them with patterns found in one region of the United States. This study revealed large differences between US and UK populations of the late blight pathogen in the frequency of most virulences examined, and particularly virulences for resistance genes originating from the UK. This study is important in documenting how different isolates from various geographic regions group together based on several attributes, and may be useful to other potato pathologists, mycologists, and potato breeders who have an interest in potato fungal pathogens. Pathogen population structure and variability in isolate aggressiveness may be important considerations in disease-management systems and serious consideration should be given to these findings when planning control strategies for this disease.
Technical Abstract: The current investigation used multiple cultivars of potato and genotypes of Phytophthora infestans to study the P. infestans genotype x cultivar interaction, and investigate the potential influence of competition and level of field resistance on selection of the surrounding population of P. infestans. Multiple isolates of several genotypes of P. infestans were chosen from the local populations in Northern Ireland (UK) and the state of Michigan (US) for inoculation onto separate field trials planted in 2003, 2004 and 2005. Four cultivars with differing levels of field resistance to late blight were planted in each trial. Equal concentrations of inoculum consisting of zoosporangia and sporangia of representative isolates from each genotype were inoculated separately onto potato late blight susceptible spreader potato plants in each trial and epidemics were initiated and assessed. Single-lesions of P. infestans were collected from leaves at 1% infection, characterized using pre-assigned markers and re-assigned to their respective genotypes. Extreme selection occurred within the populations of genotypes of P. infestans at both locations in each year. In Northern Ireland, the effect of the potato cultivar was clear with different genotypes dominating infection of different cultivars. Selection was greatest on the more resistant cultivars, but the effect was observed on all cultivars tested. By contrast, in the US the US-8 genotype dominated infection in all cultivars, and only rarely, were other genotypes detected. It is clear that the US-8 genotype has greater fitness compared to other genotypes which have been recently found in Michigan.