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Abstract Crop varieties carrying qualitative resistance to targeted pathogens lead to strong selection pressure on parasites, often resulting in resistance breakdown. It is well known that qualitative resistance breakdowns modify pathogen population structure but few studies have analyzed the consequences on their quantitative aggressiveness‐related traits. The aim of this study was to characterize the evolution of these traits following a resistance breakdown in the poplar rust fungus, Melampsora larici‐populina. We based our experiment on three temporal populations sampled just before the breakdown event, immediately after and four years later. First, we quantified phenotypic differences among populations for a set of aggressiveness traits on a universally susceptible cultivar (infection efficiency, latent period, lesion size, mycelium quantity, and sporulation rate) and one morphological trait (mean spore volume). Then, we estimated heritability to establish which traits could be subjected to adaptive evolution and tested for evidence of selection. Our results revealed significant changes in the morphological trait but no variation in aggressiveness traits. By contrast, recent works have demonstrated that quantitative resistance (initially assumed more durable) could be eroded and lead to increased aggressiveness. Hence, this study is one example suggesting that the use of qualitative resistance may be revealed to be less detrimental to long‐term sustainable crop production.

Foliar pathogens face heterogeneous environments depending on the maturity of leaves they interact with. In particular, nutrient availability as well as defense levels may vary significantly, with opposing effects on the success of infection. The present study tested which of these factors have a dominant effect on the pathogen’s development. Poplar leaf disks of eight maturity levels were inoculated with the poplar rust fungus Melampsora larici-populina using an innovative single-spore inoculation procedure. A set of quantitative fungal traits (infection efficiency, latent period, uredinia size, mycelium quantity, sporulation rate, sporulation capacity, and spore volume) was measured on each infected leaf disk. Uninfected parts of the leaves were analyzed for their nutrient (sugars, total C and N) and defense compounds (phenolics) content. We found that M. larici-populina is more aggressive on more mature leaves as indicated by wider uredinia and a higher sporulation rate. Other traits varied independently from each other without a consistent pattern. None of the pathogen traits correlated with leaf sugar, total C, or total N content. In contrast, phenolic contents (flavonols, hydroxycinnamic acid esters, and salicinoids) were negatively correlated with uredinia size and sporulation rate. The pathogen’s fitness appeared to be more constrained by the constitutive plant defense level than limited by nutrient availability, as evident in the decrease in sporulation.

Abstract Since the 1990s, a new Phytophthora species hybrid has been jeopardizing the natural population of alders throughout Europe. This new Phytophthora, P. alni, has been suggested as a natural hybrid between two closely related species of Phytophthora. Little is known about the epidemiology of this pathogen, because its direct isolation is not always satisfactory. In this study we developed three pairs of Polymerase Chain Reaction (PCR) primers derived from Sequence Characterized Amplified Regions (SCAR) that allow discrimination among the three subspecies of P. alni: P. alni subsp. alni, P. alni subsp. uniformis and P. alni subsp. multiformis. These molecular tools were successfully used to detect P. alni directly in different substrates such as infested river water and soil, and necrotic alder bark, without the need for any prior baiting or isolation stages. An Internal Amplification Control (IAC) was included to help discriminate against false negative samples due to the potential presence of inhibitory compounds in DNA extracts. These molecular tools should be useful for epidemiological studies on this emerging disease. [ABSTRACT FROM AUTHOR]