Champignon du mois – Octobre 2021

Octobre 2021


Venturia inaequalis

REGNE : Eumycota (Fungi)
PHYLUM : Dikarya, Ascomycota
CLASSE : Dothideomycètes
ORDRE : Venturiales
FAMILLE : Venturiaceae

Le genre Venturia décrit pour la première fois en 1844 renferme 52 espèces morphologiques, responsables de la maladie de la tavelure, et capables d’infecter les herbacées comme les ligneux. Les espèces les plus importantes sur le plan économique sont celles qui infectent le pommier (V. inaequalis) (Figure 1), le poirier, (V. pyrina), le pêcher (V. carpophila) ou encore le pacanier (V. effusa). La présence de symptômes sur pomme n’étant pas acceptée dans les circuits de distribution, les arboriculteurs ont recours à l’utilisation de fongicides (jusqu’à 30 applications par an) ce qui fait de la tavelure du pommier la principale maladie en verger dans l’ensemble des pays présentant un climat tempéré.

Figure 1 : Symptômes typiques provoqués par V. inaequalis sur pomme gala

Le cycle de ce champignon se caractérise par deux phases bien distinctes : une phase saprotrophe au cours de laquelle a lieu la reproduction sexuée aboutissant à la production de méiospores (ascospores) (Figure 2) et une phase parasitaire pendant laquelle se déroulent plusieurs cycles de reproduction asexuée sur jeunes feuilles et sur fruits menant à la production de conidies (Figure 3). Les épidémies débutent au printemps lorsque les conditions de température et d’humidité déclenchent la projection d’ascospores à partir des fructifications sexuées qui se sont développées dans la litière foliaire. Les ascospores, au nombre de 8 par asques (Figure 2), de taille comprise entre 13-18 µm x 6-8 µm et septées dans leur tiers supérieur, sont responsables de la dissémination de la maladie ; les conidies, plus grandes (12-30 µm x 6-10 µm), septées ou non et portées par des conidiophores (Figure 3), assurent les cycles secondaires de la maladie dans la canopée jusqu’à la récolte.

Figure 2 : Asques de V. inaequalis renfermant 8 ascospores.
Figure 3 : Conidiophores portant des conidies de V. inaequalis (microscopie électronique à balayage).

Des travaux de génomique des populations ont montré que le champignon partage en Asie Centrale le même centre de diversité que Malus sieversii, l’ancêtre du pommier cultivé (https://www.youtube.com/watch?v=2wocn2TB6dg&t=21s). Par ailleurs, les généticiens du pommier ont identifié une dizaine de gènes de résistance à la tavelure au sein des pommiers sauvages dont certains ont été introduits par croisements récurrents dans des variétés de pommiers. Récemment, des adaptations du champignon à ces gènes de résistance ont été décrites pointant le rôle de l’habitat sauvage comme source de virulence. Grâce à des travaux de génétique moléculaire, le gène de V. inaequalis impliqué dans le contournement des gènes de résistance du pommier a pu être identifié (https://www.youtube.com/watch?v=rXXBqgm-lLw).

Comprendre comment le champignon s’adapte aux facteurs de résistance du pommier doit permettre de proposer des stratégies de gestion durable des résistances en verger, en optimisant notamment à l’aide d’algorithmes la répartition spatiale des variétés résistantes dans l’environnement (https://www.youtube.com/watch?v=Md–q5NLlKk).

Pour plus d’informations, consulter :

  1. Le Cam B, Sargent D, Gouzy J, Amselem J, Bellanger MN, Bouchez O, Brown S, Caffier V, De Gracia M, Debuchy R, Duvaux L, Payen T, Sannier M, Shiller J, Collemare J, Lemaire C. Population genome sequencing of the scab fungal species Venturia inaequalis, Venturia pirina, Venturia aucupariae and Venturia asperata. G3. 2019;9(8):2405–14. doi: 10.1534/g3.119.400047
  2. Feurtey A, Guitton E, De Gracia Coquerel M, Duvaux L, Shiller J, Bellanger MN, Expert P, Sannier M, Caffier V, Giraud T, Le Cam B, Lemaire C. Threat to Asian wild apple trees posed by gene flow from domesticated apple trees and their “pestified” pathogens. Mol Ecol. 2020;29(24):4925–41. doi: 10.1111/mec.15677.
  3. https://www6.angers-nantes.inrae.fr/irhs/La-science-et-vous2/Mediation-scientifique/videos-tavelure

 

Bruno Le Cam
(Institut de Recherche en Horticulture et Semences (IRHS), INRAE, Angers)

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