Western gall rust (Peridermium harknessii J. P. Moore (syn. Endocronartium harknessii (J. P. Moore) Y. Hiratsuka) is potentially a serious threat to exotic Pinus radiata D. Don plantations of New Zealand although the pathogen has not been recorded here. Mechanisms that may have prevented invasion of the t pathogen include geographic isolation, biological characteristics of the' fungus, stand management, and regulatory mechanisms affecting transport and establishment. Major factors may include a low probability of importation of infected seedlings, unlikely spore transport in the atmosphere across the tropics, and asynchrony of rust sporulation and pine susceptibility in North America and New Zealand. The outbreak or "wave year" phenomenon in the native range of western gall rust demonstrates that both biological and microclimatic conditions must be suitable for establishment to occur. We conclude that the probability of invasion of New Zealand by western gall rust is very low; however, if the pathogen were to become established in New Zealand, the long-term effects may be large.
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