RESUMO
Myrtle rust, caused by the fungus Austropuccinia psidii, is a serious disease, which affects many Myrtaceae species. Commercial nurseries that propagate Myrtaceae species are prone to myrtle rust and require a reliable method that allows previsual and early detection of the disease. This study uses time-series thermal imagery and visible-to-short-infrared spectroscopy measurements acquired over 10 days from 81 rose apple plants (Syzygium jambos) that were either inoculated with myrtle rust or maintained disease-free. Using these data, the objectives were to (i) quantify the accuracy of models using thermal indices and narrowband hyperspectral indices (NBHI) for previsual and early detection of myrtle rust using data from older resistant green leaves and young susceptible red leaves and (ii) identify the most important NBHI and thermal indices for disease detection. Using predictions made on a validation dataset, models using indices derived from thermal imagery were able to perfectly (F1 score = 1.0; accuracy = 100%) distinguish control from infected plants previsually one day before symptoms appeared (1 DBS) and for all stages after early symptoms appeared. Compared with control plants, plants with myrtle rust had lower and more variable normalized canopy temperature, which was associated with higher stomatal conductance and transpiration. Using NBHI derived from green leaves, excellent previsual classification was achieved 3 DBS, 2 DBS, and 1 DBS (F1 score range = 0.89 to 0.94). The accurate characterization of myrtle rust during previsual and early stages of disease development suggests that a robust detection methodology could be developed within a nursery setting. [Formula: see text] Copyright © 2023 The Author(s). This is an open access article distributed under the CC BY 4.0 International license.
RESUMO
A locally severe crown disease of exotic plantation Eucalyptus trees has been recorded periodically in New Zealand since 1986. Symptoms include leaf spots, petiole infection and twig and small branch lesions. Outbreaks of disease are episodic and individual trees may show marked variation in crown symptoms ranging from unaffected to total defoliation. Two previously unknown species of Phytophthora are associated with the disease. These are described and formally designated here as P. captiosa, from Eucalyptus botryoides and E. saligna; and P. fallax, from E. delegatensis, E. fastigata, E. nitens and E. regnans. Both P. captiosa and P. fallax have non-papillate, non-caducous sporangia and both are self-fertile. Phylogenetic analysis on the basis of ITS rDNA sequence data indicates they are closely related to each other but evolutionarily distant from the majority of described Phytophthora taxa. They share a common ancestor with another assemblage of Phytophthora lineages that includes P. insolita, P. macrochlamydospora and P. richardiae. Sporulation of P. captiosa and P. fallax has not been observed in the field. The mode of infection and spread of these non-caducous Phytophthora species in the eucalypt tree canopy remains unknown. This issue, and the possible geographic origins of these two Phytophthora species are discussed.
