Seasonal development of septoria leaf blight in young Eucalyptus nitens plantations in New Zealand

Abstract

Summary The leaf spotting fungus Phaeophleospora eucalypti (synonyms Kirramyces eucalypti, Septoria pulcherrima) occurs on eucalypt hosts throughout much of New Zealand. In parts of the country it is associated with a serious leaf-cast disease in young plantations of Eucalyptus nitens, often in association with infection by Mycosphaerella cryptica. Studies were undertaken over three seasons at one inland and three coastal sites in the Bay of Plenty-Taupo region to investigate disease epidemiology. Shoots on study trees were monitored monthly for production of new leaves, degree of fungal leaf spotting, and defoliation. Monthly records were also kept of fungal spore production, and glasshouse-raised potted seedlings were placed in the field at monthly intervals to determine the infection period of P. eucalypti. Eucalyptus nitens produced leaves throughout most of the year, with least emergence during winter (when over a short period juvenile leaves were not produced at all), and greatest production during spring and summer. New leaves emerging in spring became infected by P. eucalypti, evidently from spores released from infected, residual, previous season's foliage. Most infection, in the seedling study, was restricted to the first few weeks after leaf appearance. Fertile pycnidia were present as soon as symptoms became visible, and infected leaves continued to produce conidia throughout the growth season, while still retained. Under suitable conditions an epidemic condition appeared to develop as successive sets of leaves emerged, became infected early, and in turn began producing new conidia, resulting in a sustained increase in the quantity of available inoculum. Seasonal infection decelerated in autumn, as the production of new leaves and spores declined. Infection by M. cryptica paralleled that by P. eucalypti, except that spotting was reduced on juvenile leaves and lacking on adult foliage, and infection of new season's foliage could also occur from ascospores produced in leaf litter. The disease progressed more vigorously at warmer, lower elevation coastal sites, where there was also more inoculum available at the start of the growth season.