Variabilité phénotypique dans la phénologie de la production d’ascospores entre populations européennes d’oïdium du chêne

Abstract

Abstract• Oak powdery mildew severity (Erysiphe alphitoides) is usually mild in Europe because epidemics start late in spring, at the end of the first oak growth unit maturation. However, the disease can occasionally be very severe when strong infection occurs early during the development of the first growth unit, suggesting that host-pathogen synchrony in spring could be a critical factor in disease severity.• We studied the timing of ascospore production in a given environment for four E. alphitoides populations sampled from SW France to the Czech Republic to determine whether this trait shows variation within Europe.• Timing of ascospore production was clearly influenced by environmental factors as chasmothecia from a single origin showed very different dates of optimal ascospore production when transferred for overwintering in locations with different climate. In common garden experiments, no differences were observed between populations for the date of optimal ascospore production.• Results suggest little genetic differentiation for timing of ascospore production for E. alphitoides populations across Europe and therefore a lack of local adaptation to their host phenology. Availability in ascospore inoculum is limited during host budburst, explaining the low infection usually observed on the first oak growth unit. Mots-clés :agent pathogène / arbre forestier / Erisyphe alphitoides / Quercus / synchronisation Résumé -Variabilité phénotypique dans la phénologie de la production d'ascospores entre populations européennes d'oïdium du chêne.• L'oïdium du chêne (Erysiphe alphitoides) est généralement peu sévère en Europe, la maladie se développant tard au printemps, après le débourrement des chênes. E. alphitoides peut toutefois être sévère quand de fortes infections se produisent tôt durant le débourrement, suggérant que la synchronisation entre hôte et pathogène pourrait être un facteur déterminant la sévérité des épidémies.• Nous avons caractérisé la dynamique de production d'ascospores dans un environnement donné pour quatre populations d'E. alphitoides échantillonnées du sud-ouest de la France à la République Tchèque pour tester si ce trait est variable en Europe.• Des chasmothèces d'une même origine présentent des dates d'optima de production d'ascospores très différentes quand elles sont transférées durant l'hiver dans des zones à climats différents montrant que ce trait est sous dépendance de l'environnement. Par contre, les quatre populations ont la même date d'optimum de production d'ascospores quand elles sont placées dans un même environnement.• Les résultats suggèrent que les populations d'E. alphitoides européennes ne sont pas différentiées génétiquement pour le timing de production d'ascospores et donc qu'elles ne sont pas localement adaptées à la phénologie de leur hôte. L'inoculum ascospores est très peu présent durant le débourre-ment, expliquant en partie l'absence d'infection sur la première unité de croissance des chênes.

Are plant pathogen populations adapted for encounter with their host? A case study of phenological synchrony between oak and an obligate fungal parasite along an altitudinal gradient

“…Whereas oak leaf phenology was strongly delayed at high altitudes, a relatively low sensitivity of E. alphitoides phenology to altitude was observed in the 2 years of our experiment. The slight cline observed is of the same order of magnitude as that found in a latitudinal gradient between Bordeaux and Nancy (Marçais et al. , 2009), when expressed in days of delay per °C decrease, considering annual temperatures (approximately 3 days/°C).…”

Individual tree traits shape insect and disease damage on oak in a climate‐matching tree diversity experiment

“…We observed that in the UK, insect herbivore and powdery mildew damage are temporally and spatially separate. Almost all insect herbivore damage occurs on primary shoot leaves emerging during the first flush in late spring (April–May), while oak powdery mildew infection occurs on leaves on lammas shoots emerging during subsequent bud bursts in June–August, consistent with E. alphitoides releasing ascospores from overwintering chasmothecia in early to mid‐summer (Marçais, Kavkova, & Desprez‐Loustau, ). Oakleaf susceptibility to powdery mildew declines sharply following bud burst, so that leaves on primary shoots largely remain completely uninfected (Ayres & Edwards, ).…”

Interspecific and intraspecific diversity in oak powdery mildews in Europe: coevolution history and adaptation to their hosts

“…Host-pathogen phenological matching was shown to vary between and within sites at different altitudes. Moreover, common garden experiments did not give any evidence of genetic differentiation for fungal phenology among populations from different altitudes or geographic origins (Marçais et al 2009). These data therefore did not support local adaptation of the fungus for host encounter at population level.…”

“…The occurrence of powdery mildew fungi on the model plant Arabidopsis thaliana has prompted many studies on the plant side but few on pathogen populations (Adam et al 1999;Micali et al 2008). The E. alphitoidesQuercus robur interaction was recently investigated in two studies, in Finland and France (Roslin et al 2007;Marçais et al 2009, Desprez-Loustau et al 2010a. A common objective of these studies was to test the hypothesis of a local adaptation of powdery mildew to its host.…”