Survival of Conidia of the Boxwood Blight Pathogen Calonectria pseudonaviculata Under Different Relative Humidity Conditions

Abstract

Boxwood blight, caused by Calonectria pseudonaviculata, is a significant disease affecting both nursery production and landscape boxwood plantings. C. pseudonaviculata conidia are produced in clumps in an extracellular mucilaginous matrix. Local spread of the pathogen is by dispersal of conidia in water splash or direct contact dispersal, but the effects of relative humidity (RH) on survival and germination of dispersed conidia over time are unknown. We investigated the effects of 15, 40, 80, and 100% RH on the survival of individual conidia or conidia aggregated in clumps over time. Conidia were transferred to a dry glass surface and scored for the incidence of survival and percentage germination over 3, 6, and 9 days. Conidia in clumps had higher incidence of survival and percentage germination than individual conidia. RH significantly and positively influenced germination and survival, as measured by the presence or absence of germination by individual conidia, but not by clumps of conidia. Individual conidia survived for at least 6 days at all RH tested. No germination was observed for any individual conidium exposed to 15% RH for 9 days, whereas conidia in clumps survived and germinated at all RH levels for at least 9 days. This study indicates that simply pruning or working in infected boxwoods under dry conditions may be insufficient to limit the spread of the pathogen and, ultimately, disease. Further measures such as frequent disinfestation of tools with alcohol or fungicide application before or after possible spread of conidial clumps may be necessary.