Conidial morphology, structure and development in Valdensinia heterodoxa

Abstract

As a pathogen and possibly biological control agent, the morphological development process of the conidia of Valdensinia heterodoxa is studied under a controlled environment. Conidial production in culture can be controlled by exposure to light. No conidia are produced at 20°C in the dark on weak oatmeal agar, but after placing in an growth chamber with a 12°C /19°C (night/day) diurnal temperature cycle, mature conidia form from initial cells within 24 h and discharge by 48 h. The initial cell is obovate with a thick peridium. Preceding conidial differentiation, the generative cell breaks through the peridium, forming a central axillary plate cell on an obovate conidiophore cell. The axillary cell develops radiating conical protrusions, which initially curve upward and then develop turgid, plicated evaginations on their upper and flank sides. The radiating protrusions elongate into arms that ultimately bend downwards toward the surface of medium. There are normally four or five arms per conidium, but some have three to six arms. The axillary plate cell also forms numerous radiating clavate cells in the central area surrounded by the arms, eventually forming a hemispherical conidial centre. With continuing maturation, involving inflation of the clavate apical cells and the plicated evaginations on the arms, the arms straighten and contract to the centre of the conidiophore, pushing downward on the substrate. A further pressure-increase eventually ruptures the joints between conidiophores and conidia, resulting in forcible discharge of the conidia. Conidia are star-shaped prior to dispersal, but become tear-drop-shaped when discharged due to the depressed arms. Conidial size varies with culture media and substrate. Single conidia cause infection on detached leaves of salal (Gaultheria shallon). Viscous granules on the conidial arms and apex help with adhesion to host plants and also play an important role in the infection process.