Host adaptation to potato and tomato within the US−1 clonal lineage of Phytophthora infestans in Uganda and Kenya

Abstract

Twenty isolates of Phytophthora infestans from potato and twenty-two from tomato, collected in Uganda and Kenya in 1995, were compared for dilocus allozyme genotype, mitochondrial DNA (mtDNA) haplotype, mating type and restriction fragment length polymorphism (RFLP) fingerprint using probe RG57. Based on RFLP fingerprint and mtDNA haplotype, all isolates were classified in the US21 clonal lineage. Nonetheless, isolates from potato differed from isolates from tomato in several characteristics. Isolates from potato had the 86/100 glucose-6-phosphate isomerase (Gpi) genotype, while those from tomato were 100/100, which represents a variant of US21 that had been identified previously as US21.7. Furthermore, while pure cultures of the pathogen were acquired from infected potato leaflets by first growing the isolates on potato tuber slices, this approach failed with infected tomato tissue because the isolates grew poorly on this medium. Tomato isolates were eventually purified using a selective medium. Six isolates from each host were compared for the diameter of lesions they produced on three tomato and three potato cultivars in one or two detached-leaf assays (four isolates from the first test were repeated in the second). On potato leaflets, isolates from potato caused larger lesions than isolates from tomato. On tomato leaflets, isolates from that host caused larger lesions than did isolates from potato, but the difference was significant in only one test. The interaction between source of inoculum (potato or tomato) and inoculated host (potato or tomato) was significant in both tests. Isolates from tomato were highly biotrophic on tomato leaflets, producing little or no necrosis during the seven days following infection, even though abundant sporulation could be seen. In contrast, isolates from potato sporulated less abundantly on tomato leaflets and produced darkly pigmented lesions that were most visible on the adaxial side of the leaflets. Nonetheless, all isolates infected and sporulated on both hosts, indicating that host adaptation is not determined by an ability to cause disease but rather by quantitative differences in pathogenic fitness. Assessment of Gpi banding patterns, mtDNA haplotype and RFLP fingerprint of 39 isolates from potato collected in Uganda and Kenya in 1997 indicated that the population had not changed on this host. The population of P. infestans from Kenya and Uganda provides an interesting model for the study of quantitative host adaptation.

Foliar virulence of isolates of Phytophthora infestans sensu lato on detached leaves of two Solanum betaceum cultivars

“…These quantitative differences were represented on IF, SI and IP. Previous reports from Uganda and Kenya (Vega-Sanchez et al, 2000) did not show any significant difference in IF between host-isolates and their hosts (S. lycopersicum and S. tuberosum). However, low values of IP and high values of IF can explain pathogen fitness on host (Lebreton et al, 1999).…”

“…Also differential virulence on S. betaceum of isolates of P. infestans from S. betaceum and S. tuberosum (EC-1 clonal lineage) was found. This quantitative hostspecificity can be considered an additional example of differential virulence previously reported in P. infestans s.l., in which host-isolates are best adapted to their original host (Garry et al, 2005;Legard et al, 1995;Oyarzun et al, 1998;Suassuna et al, 2004;Vega-Sanchez et al, 2000). These quantitative differences were represented on IF, SI and IP.…”

Virulence and aggressiveness of Phytophthora infestans isolates collected in Poland from potato and tomato plants identified no strong specificity

“…Quantitative assessments of the fitness parameters of P. infestans have been conducted mostly with detached-leaf or leaflet assays (Goodwin et al 1995;Legard et al 1995;Świeżyński et al 2000). Although P. infestans isolates are obtained from both potato and tomato, aggressiveness and virulence tests of the isolates have been evaluated primarily on potato differentials (Lebreton and Andrivon 1998;Dorrance et al 1999;Vega-Sánchez et al 2000;Derie and Inglis 2001;Klarfeld et al 2009;Blandón-Diaz et al 2012;Harbaoui et al 2013). Aggressiveness and virulence of isolates from potato and tomato have also been compared on leaflets of the two host plants (Wilson and Gallegly 1955;Oyarzun et al 1998;Lebreton et al 1999;Jaime-Garcia et al 2000;Cohen 2002; Knapova and Gisi 2002;Blandón-Diaz 2011).…”

“…However, our understanding of the pathogenic specialization of the late blight pathogen is incomplete. Most isolates have been observed to be more aggressive to their original host species (Legard et al 1995;Oyarzun et al;Lebreton et al 1999;Vega-Sánchez et al 2000), with very rare exceptions (Blandón-Diaz 2011).…”

“…infestans isolates are often stored on artificial media and/or multiplied on potato tuber slices or leaves before being subjected to host adaptation experiments, even if the original host species of the isolate was tomato (Legard et al 1995;Lebreton and Andrivon 1998;Dorrance et al 1999;Vega-Sánchez et al 2000). These procedures may influence aggressiveness and virulence results (Zarzycka 1995(Zarzycka , 1996.…”