ETHYLENE PRODUCTION BY EUCALYPTUS CLONES IN RESPONSE TO INFECTION BY HYPOVIRULENT AND VIRULENT ISOLATES OF CRYPHONECTRIA-CUBENSIS

Ethylene production was measured in the bark of two Eucalyptus clones that were artificially inoculated with virulent and hypovirulent isola tes of Cryphonectria cubensis. Trees inoculated with the hypovirulent isolate produced extremely low amounts of ethylene. No significant dif ferences were found in ethylene production by a susceptible clone (ZG 14) and a disease-tolerant clone (TAG 5) inoculated with a hypovirulen t isolate. However clone ZG 14 produced significantly more ethylene th an TAC 5 when inoculated with the virulent isolate of C. cubensis. Thi s supports the view that trees more susceptible to fungal attack tend to produce greater amounts of ethylene than disease-tolerant clones. T he amount of ethylene produced in response to C. cubensis infection ma y thus be a convenient and rapid means of distinguishing a susceptible from a resistant response. If heritable, the ability to produce small quantities of ethylene in response to infection instead of large quan tities as in susceptible clones, may be a valuable factor in breeding for resistance to this pathogen.