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Abstract: Both race-specific (RS) and race-non-specific (RNS) resistances exist in potato against the late blight pathogenPhytophthora infestans. Because these resistance types do not have the same epidemiological effects, their presence, alone or combined, in potato genotypes can be deduced from the analysis of disease progress curves from field experiments, a type of data commonly available to potato breeders, and their comparison with those of standard reference cultivars. The identification of RS resistance is based on the presence of a delay in epidemic onset compared to a susceptible cultivar, whereas the identification of RNS resistance is translated into a reduction of apparent infection rates. These parameters can be easily computed after linearization of the disease progress curves. This paper assesses the reliability of this identification using sets of experimental data, discusses its limitations, and highlights potential applications for breeding and cultivar assessment purposes.

Abstract: Lipopolysaccharides (LPS), ubiquitous cell surface components of Gram‐negative bacteria, are directly implicated in plant/pathogen interactions. However, their perception by the plant, the subsequent signal transduction in both compatible and incompatible interactions, as well as the defence reactions induced in compatible interactions are as yet poorly understood. We focused on biochemical and physiological reactions induced in cell suspensions of three Solanaceae species (tobacco, tomato, and potato) by purified lipopolysaccharides from Pectobacterium atrosepticum (Pa),a pathogen of potato, and Pseudomonas corrugata (Psc),a pathogen of tomato. LPS Paand LPS Psccaused a significant acidification of potato, tomato, and tobacco extracellular media, whereas laminarin (a linear β□1,3 oligosaccharide elicitor) induced an alkalinisation in tobacco and tomato, but not in potato cell suspensions. None of the two LPS induced the formation of active oxygen species in any of the hosts, while laminarin induced H2O2production in cells of tobacco but not of tomato and potato. In tomato cells, LPS Paand LPS Pscinduced a strong but transitory stimulation of lipoxygenase activity, whereas laminarin induced a stable or slightly increasing LOX activity over the first 24 h of contact. In tobacco, LOX activity was not triggered by either LPS, but significantly increased following treatment with laminarin. In potato, neither LPS nor laminarin induced LOX activity, in contrast with concentrated culture filtrate of Phytophthora infestans(CCF). These results demonstrate that LPS, as well as laminarin, are perceived in different ways by Solanaceaespecies, and possibly cultivars. They also suggest that defence responses modulated by LPS depend on plant genotypes rather than on the type of interaction.

Mycelial growth on clarified V8 agar of the potato late blight pathogenPhytophthora infestans was inhibited when either aluminum chloride (AlCl3, 6 H2O) or aluminum sulfate (Al2(SO4)3, 18 H2O) was added to the culture medium at concentrations of 2.5–100 mg.l-1 Al3+. Toxicity of Al3+ varied among the fiveP. infestans isolates tested, but toxicity of sulfate and chloride salts was similar for a given isolate. Overall sporangial production was affected in all five isolates by both Al3+ forms. Al3+ also decreased sporangial germination at concentrations equal to or greater than 10 mg.l-1 in two isolates. These data support the hypothesis of aluminum toxicity as a major factor in soil suppressiveness toP. infestans.