Abstract
Late blight is a devastating and worldwide potato and tomato disease caused by Phytophthora infestans (Mont.) de Bary. The aim of the study was to compare P. infestans isolates collected from potato and tomato plants by determining their virulence, aggressiveness, and sporulation intensity on both hosts in reciprocal testing with the goal of elucidating possible host specialization of the pathogen. Isolates were multiplied on leaflets of their primary hosts. In total, 76 potato-derived P. infestans (P) isolates and 100 tomato-derived P. infestans (T) isolates collected from 2005 to 2007 were studied in detached-leaflet assay experiments. Virulence was assessed on Black’s potato differentials R1–R11 and cv. Bzura, as well as on the set of six tomato genotypes, namely cv. New Yorker (Ph-1 gene), Solanum pimpinellifolium L 3708 (Ph-3), West Virginia’63 (Ph-2), West Virginia 700 (Ph-1, Ph-2), Ottawa 30 (Ph-1, Ph-2), and BALU-30 (Ph-1, Ph-2, fruit resistance). Aggressiveness scored as disease severity and sporulation intensity assessed on a 0–3 grade scale were evaluated on leaflets of susceptible cultivars of both hosts. All 176 of the tested isolates were pathogenic to both hosts, indicating that extreme host specialization has not occurred. However, we did observe differences between P and T virulence spectra and their race structures supporting the notion of host adaptation of P. infestans. P and T isolates were more frequently virulent to differentials of their own hosts. P isolates had higher sporulation intensity on their own hosts than on tomato hosts, while for T isolates a similar sporulation intensity was observed on both hosts. Significantly stronger severity of the disease development on T testers was evoked by T isolates. All isolates showed stronger aggressiveness on susceptible potato than on susceptible tomato hosts, and the latter finding indicates a relatively greater suitability of potato host tissue for P. infestans pathogenicity. The host adaptation observed in this study was not conditioned by an ability to evoke disease, but rather by quantitative differences in pathogenic fitness. Genetic characterization of P and T populations is needed to place the present findings in a fuller context.
Highlights
Late blight is a devastating disease of potato and tomato worldwide caused by the oomycete Phytophthora infestans (Mont.) de Bary
Isolates of P. infestans were collected from host potato and tomato plants and the influence of the source host species identity on pathogenicity has been analyzed since the mid-20th century (Mills 1940; Gallegly 1952)
All of the 76 P. infestans isolates collected from potatoes (P) were virulent to the Black’s differentials R1, R3, R4, R7, R10 and R11
Summary
Late blight is a devastating disease of potato and tomato worldwide caused by the oomycete Phytophthora infestans (Mont.) de Bary. Aggressiveness and virulence of isolates from potato and tomato have 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). Most isolates have been observed to be more aggressive to their original host species (Legard et al 1995; Oyarzun et al.; 1998; Lebreton et al 1999; Vega-Sánchez et al 2000), with very rare exceptions (Blandón-Diaz 2011)
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