The distribution of Gymnosporangium fuscum and its implication on pear cultivation in Sweden

Sammanfattning: The European pear rust is caused by the fungus Gymnosporangium fuscum which like rusts in general require a winter host for survival, in this case species within the genera of Juniperus. The infections are perennial on the juniper host on which it in spring at favour climate develops the characteristic telial horns. These occur as yellow-brown tongue-shaped swellings and distribute spores at moist conditions infecting young pear leaves within a distance of 500m. The symptom on the pear leaves begin as yellow spots which gradually increase during the summer and become bright orange with a red boarder. Opposite this lesion, on the underside of the leaf, are cluster cups formed in groups onto small swollen areas of tissue. From these are spores distributed that cause primary infections on the juniper host and thereby enclosing the life-cycle of the pathogen. The pear infections are only annual and spores developed from the spore stage on the pear leaf may not re-infect within the same host. Severe infections on the tree repeated during several years may decrease its vitality and affect its fertility rate resulting in poor fruit set or premature fruit drop. This is a literature study to achieve a broad picture of the pathogen G. fuscum and its biology, host plants, environmental demands and control measures. As a compliment, a questionnaire study was implemented to evaluate the geographical spread and the general estimation of its symptoms and control measures. It also requested the supply of susceptible junipers in nurseries and garden centres. The supply of susceptible junipers could be considered as the major cause of spread. Old plantings but also newly established ones since the supply of these junipers still occur according to the result of the questionnaire. Single observations of pear rust have been done in Arvika and Gävle which represents zone III respectively IV of plant hardiness. Other factor affecting the development and the infection pressure of the pathogen is the environmental conditions during spring when the telial horns develop and distribute spores. Thus, the only possibility to reduce the spread of pear rust is to rupture its life-cycle by eliminate the infected juniper host and avoid new plantings of these. The supply of resistant cultivars could be considered large. However, a voluntary cooperative effort of must be made to prohibit the development of new infestations.