Cross-pathogenicity of Phytophthora infestans genotypes and impact on their interaction with potato and tomato
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Date
2019-04-02
Authors
Alkher, Hassna
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Abstract
Late blight is one of the most devastating diseases on potato and tomato, which are important crops worldwide. The disease is caused by an oomycete, a fungus- like organism, Phytophthora infestans. The disease can result in a loss of an entire yield within a few days if left unchecked. Historically, late blight caused the famous Irish famine in the 1840s and continues to affect potato and tomato. The most recent pandemics of late blight that occurred in North America were caused by new P. infestans genotypes known as US-22, US-23 and US-24, and resulted in significant yield losses of both potato and tomato. In this study, isolates from infected potato and tomato plants collected in Canadian provinces during 2012 were identified, and some of the work was performed in collaboration with other scientists across Canada. Our results concluded a great shift of P. infestans populations within a single year, from US-24 and US-8 in 2011 to US-23 in 2012. Isolates of some populations showed increased insensitivity to mefenoxam as well as recombination. We confirmed an independent segregation of either Gpi, mating type or RG57 loci in a number of the recombinant isolates including CA12. Cross-pathogenicity of different isolates belonging to different genotypes were evaluated on susceptible and moderately resistant cultivars of potato and tomato. Isolates of genotypes US-8, US-11, US-22, US-23 and US-24 were assessed for their pathogenicity on both tomato and potato leaves as well as on tubers, at different days post inoculation (dpi). This allowed us to investigate their pathogenicity on both the original and alternative hosts of each tested isolate. Area under disease progress curve (AUDPC) was measured at 3, 5, and 7 dpi on plant leaves. We revealed higher infection caused by most US-24 isolates in all tested cultivars. Observation of more blight in Russet Burbank, the susceptible potato cultivar was recorded, followed by Sun Rise and Ultra Sweet, the susceptible and the moderately resistant
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tomato cultivars, respectively. Among all tested cultivars, Kennebec, the moderately resistant potato cultivar showed less disease. Interestingly, tubers of both potato cultivars showed contrasting pathogenicity results when infected with the same isolates, with larger diseased areas on tuber slices of Kennebec, the moderately resistant cultivar and smaller ones on Russet Burbank, the susceptible cultivar. Among all of the tested genotypes, US-24 was the most aggressive on potato and tomato. However, variation was observed in the aggressiveness of US-24 isolates, where Pi19.1.11 and Pi4.3.11 were highly aggressive, whereas, Pi688, Pi166 were weakly aggressive on both hosts. Isolates of US-22 and US-23 genotypes, which were obtained from tomato, were more pathogenic to tomato. In an attempt to gather more information on virulence factors that determine P. infestans effects on potato and tomato, putative virulence genes (effector genes) were assessed for their role in the aggressiveness of the tested P. infestans isolates on potato and tomato. The expression of five RXLR putative effector genes of P. infestans was evaluated using qRT-PCR during the interaction of six US-24 isolates, varying in their aggressiveness with potato and tomato susceptible and moderately resistant cultivars. Gene expression was assessed 3 and 6 dpi. Among the tested candidate effector genes, DL119 was the only gene detected during the interaction between P. infestans and both plant species. A higher expression of DL119 was recorded in all tested isolates during their interaction with tomato cultivars compared to the potato ones. The expression of DL119 carried the same pattern in most of the treatments, with up-regulation at 3dpi then decrease at 6dpi. DL119 could play a role in pathogenicity since it showed higher expression during interaction of the isolates with tomato cultivars. The gene expression results are in line with our cross-pathogenicity result, which indicates greater aggressiveness of isolates of US-24 on tomato more than potato.
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Plant pathology
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