Atypical growth of Phoma koolunga on cultural media and on plants artificially inoculated in environmentally controlled conditions

Abstract

Phoma koolunga is a recently recognised causal agent in the ascochyta blight complex (blackspot) of field pea in Australia. Several isolates of this fungus exhibited atypical morphology, including a rhizoid growth habit, and unusual reproductive behaviour on potato dextrose agar (PDA). Otherwise-typical cultures of P. koolunga frequently gave rise to rhizoid sectors on PDA during routine sub-culture and rhizoid mycelium was also isolated directly from field pea seeds on PDA amended with streptomycin. Rhizoid growth confounds the identification of this fungus. Little is known about this growth form, so experiments were undertaken to examine the growth and pathogenicity of rhizoid cultures. Five rhizoid colonies were purified by single hyphal tip isolation from sectors in cultures of P. koolunga or from atypical growth from seeds and were confirmed as P. koolunga by a DNA test using P. koolunga-specific primers. Colonies of an atypical culture were smaller than those of the typical parent culture on PDA, oat agar, malt extract agar and Sach’s agar. Dark pycnidium-like structures formed on these atypical colonies as well as on inoculated plant material. These were similar to normal pycnidia of P. koolunga in terms of shape, size and colour, however, each pycnidium-like structure contained thousands of round, hyaline guttulae of 0.4–12.5 μm diam. Inoculation of field pea plants with an atypical culture produced small lesions resembling ascochyta blight symptoms on leaflets and stems.