Calonectria pseudonaviculata Conidia Dispersal and Implications for Boxwood Blight Management

Abstract

We investigated Calonectria pseudonaviculata conidial dispersal from sporulating lesions on boxwood leaves and sporulating cultures on half-strength PDA (1/2 PDA). Botrytis cinerea-infected blossoms were used as a control. Dispersal of C. pseudonaviculata or Botrytis conidia was confirmed by capture using an Allergenco air sampler at 15 liters/min and by microscopic observation of conidia and C. pseudonaviculata growth on 15-cm-diameter 1/2 PDA Petri dishes. C. pseudonaviculata conidia were not dispersed by either dry or moist air currents directed at conidia and conidiophores from 2 mm away at air speeds of 19.8 m/s for 10 min or by a fine mist with water droplets (mean diameter 20 µm) with air speeds of 1.7 m/s. C. pseudonaviculata spores were dispersed by splash of water droplets at air speeds of 9.0 to 19.8 m/s. C. pseudonaviculata conidia released from phialides by water could not be wind dispersed after the water had evaporated. Secondary water dispersal was reduced because conidia strongly adhered to a surface after drying. Boxwood leaves dropped from heights of 15, 33, or 66 cm landed with more than 60% of leaves facing abaxial surface up. The cupped shape of most boxwood leaves may result in the abaxial surface with sporulation facing up. That orientation may also aid in retention of water films to wet and release conidia for splash dispersal. This is consistent with observations of increased disease severity in lower boxwood canopies and reinforces suggestions for best management practices including mulching and pruning lower branches to reduce the incidence and severity of disease.