The cause of Cryphonectria canker on Eucalyptus in Florida reconsidered.

Pham, N. Q.*¹, Wingfield, B. D.¹, Marincowitz, S.¹, Brawner, J. T.^2,3,4, Hulcr, J.⁴, Wingfield, M. J.¹

¹ Department of Biochemistry, Genetics and Microbiology, Forestry and Agricultural Biotechnology Institute (FABI), University of Pretoria, Pretoria, South Africa
² Department of Plant Pathology, University of Florida, Gainesville, Florida, USA
³ School of Forest, Fisheries and Geomatics Sciences, University of Florida, Gainesville, Florida, USA
⁴ Genics PTY, Level 5/60 Research Rd, St Lucia QLD 4067, Australia

Cryphonectria canker is one of the most important diseases of plantation-grown Eucalyptus spp. in the tropics and Southern Hemisphere. The disease has been known in Florida, USA for many years and the causal agents attributed to two known canker pathogens, Chrysoporthe cubensis (= Cryphonectria cubensis) and Microthia havanensis (= Endothia havanensis). These identifications were based on morphological characteristics, which in many recent studies have been shown not to recognize cryptic species in the Cryphonectriaceae. In this study, we visited various sites in Florida where cankers were present on Eucalyptus grandis and E. amplifolia. Isolations were made from fungal structures associated with the diseased material. Cultures were identified based on DNA sequences of the ITS region of the rDNA, and fragments of the β-tubulin gene. The results showed that two species were present including Chrysoporthe doradensis and Microthia havanensis. Interestingly C. cubensis was not isolated. We assume that the previous record of that species was incorrect due to morphological characteristics not accurately reflecting species boundaries as they are currently understood. Chrysoporthe doradensis was first discovered causing cankers on Eucalyptus in Ecuador; subsequently on native Tibouchina granulosa and Rhynchanthera grandiflora (Melastomataceae) in Brazil. It has recently also been found on both Eucalyptus and native Henriettea seemanii (Melastomataceae) in Colombia. This study represents the first record of any Cryphonectriaceae on E. amplifolia, a Eucalyptus species that is poorly studied in terms of disease. The origin of C. doradensis and M. havanensis in Florida is unknown. They could have been accidentally introduced on Eucalyptus from South America or have undergone a host shift to Eucalyptus from unknown Florida Myrtales hosts. Whole genome sequences are currently being produced for these fungi to facilitate future population genetic studies that will be focused on determining the origin and pathways of movement of these pathogens.

Keywords: canker disease, Chrysoporthe, forest pathology, Myrtales, plantation forestry, tree disease