Symptom development and latent period of Austropuccinia psidii (myrtle rust) in relation to host species, temperature, and ontogenic resistance

Abstract Myrtle rust (Austropuccinia psidii) is an invasive species causing damage to Myrtaceae species in natural and managed ecosystems in many countries. To better understand myrtle rust epidemiology we studied latent period (LP) and ontogenic resistance in relation to temperature on three susceptible hosts (Metrosideros excelsa, Lophomyrtus bullata × L. obcordata and Syzygium jambos). The latent period curve was U-shaped, with latent development >0 from between 8 and 10 °C, depending on the host, to 32 °C. Optimum range was 22–28 °C with minimum LP of 5–7 days. Peak spore production occurred over about 2 weeks, starting about 1 week after the LP ended. Some spore production continued for 1–2 months. Comparison of the LP data with field temperatures indicated that the uredinial stage of A. psidii can overwinter in the latent phase in temperate areas of New Zealand and southern Australia and, therefore, uredinial or telial reinfection is not required during winter. The LP information was used to correct the LP function in a New Zealand myrtle rust climatic risk model. The transition of emergent leaf and stem tissues in susceptible Myrtaceae genotypes from susceptible to immune (ontogenic resistance) was characterized in terms of uredinium density and LP. Onset of ontogenic resistance was closely linked to the degree of leaf expansion, with fully expanded leaves being immune to infection. Because ontogenic resistance restricts infection to periods when growth flushes occur, understanding it is crucial for explaining the seasonality of myrtle rust development in the natural environment.

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