Colletotrichum acutatum in French walnut orchards – improving knowledge on the behaviour of the fungus and its host

Since 2011, fruit losses due to the fungus Colletotrichum have been a great concern for French walnut growers.
Two main species of the complex Colletotrichum acutatum are involved: C. godetiae and C. fioriniae. In order to find methods to manage the disease in the fields, studies were first carried out to improve knowledge on the pathogen biology and host susceptibility: conditions for spore germination, varietal and ripening stage susceptibility.
The effect of temperature and wetness duration on conidia germination was investigated on cellulose acetate discs placed on wet filter paper.
The two species an optimal germination between 25 to 30°C, with 100% of germinated conidia after 15 h of wetness.
Varietal and ripening stage susceptibility tests were performed on detached fruits under controlled conditions.
Between the three most planted cultivars in France, ‘Franquette’ turned out to be less sensitive than ‘Fernor’ and ‘Lara’. In addition, fruits became more susceptible as they ripened.
These results, combined with field trials, aim to improve management and timing of fungicide applications.

IX International Symposium on Walnut and Pecan

C. Masson, M. Barbedette, L. Leurs

walnut, Colletotrichum acutatum, temperature, wetness, germination, varietal susceptibility

https://doi.org/10.17660/ActaHortic.2025.1420.31