Methods of inoculating cypress with Seridium species to screen for resistance and pathogen variability : a thesis presented in partial fulfilment of the requirements for the degree of Master of Applied Science in Plant Health at Massey University, Palmerston North, New Zealand

Abstract

The cypress species are grown for their timber value, ornamental beauty and shelter. Their existence is threatened by the presence of cypress canker disease caused by fungal pathogens of the genus Seiridium. The long term solution for controlling this disease is to breed for cypress clones that are resistant to cypress canker. Screening for resistance is conducted by artificially inoculating cypress plants with the pathogen's inoculum. This study aimed at developing reliable methods of artificial inoculation that are suitable for New Zealand's climatic conditions. Infection of cypress plants in nature is caused by conidia but mycelial inocula are more commonly used in artificial inoculation. Several methods of inducing sporulation of Seiridium species were investigated Addition of plant substrates was shown to increase sporulation of cultures of Seiridium isolates. Studies comparing the two types of inocula (mycelial plugs and conidial suspensions) showed that mycelium inocula caused a higher percentage of canker lesions than spore inocula. Conidial inocula offer a more consistent pathogenicity. Experiments to determine the effective spore load revealed that the percentage of canker increased with the increase of inoculum load. Pathogenicity varied between species and individual colonies of Seiridium isolates. Infection of cypress in nature is thought to occur through wounds and in this work, wounding was required for infection under both glasshouse and outdoor conditions. Inoculation of the main stem and side branches showed disease symptoms develop more rapidly on side branches than on the main stem. Investigations on in vitro inoculation of tissue cultured plants and excised side shoots showed the possibility of screening cypress ramets under different environmental conditions. Temperature and percentage relative humidity were found to influence the percentage of successful inoculations on cypress plants