|Infection and colonisation of pyrethrum leaves by Stagonosporopsis tanaceti|
M. A. H. BHUIYAN (1), T. Groom (2), P. W. J. Taylor (1). (1) The University of Melbourne, Melbourne, Australia; (2) Botanical Resources Australia Pty Ltd, Tasmania, Australia
Pyrethrum (<i>Tanacetum cinerariifolium</i>) plants are used to produce biopesticide pyrethrins commercially in Australia. Ray blight caused by <i>Stagonosporopsis tanaceti</i> is a major disease of pyrethrum. Successful implementation of disease control methods depends on knowledge of the infection process by the pathogen. Histopathology is an important tool in studying host-pathogen relationships especially with biological stains that differentiate the pathogen from host cell tissues. The first fully expanded leaves were inoculated with a spore suspension of <i>S. tanaceti</i> and incubated in high humidity to induce spore germination and infection. Leaf sections were sampled, fixed in FAA, embedded with paraffin and sectioned (6 µm) using a rotary microtome. Sections were then stained with the modified Johansen’s quadruple stain technique. Adhesion of conidia to the leaf surface was followed by direct penetration of the cuticle by germ tubes from 3 - 12 hours after inoculation. Infection hyphae penetrated the epidermis within 1 - 2 days after inoculation (dai), and mesophyll tissues within 3-7 dai. Secondary infection of adjacent mesophyll cells then occurred leading to disintegration of the colonized cells. Pycnidia formation, maturation and spore release occurred between 7-9 dai in the cells of both the epidermal and mesophyll tissues. Although hyphae were detected in the leaf petioles no vascular tissue was colonized.