Three “other” diseases impacting avocado productivity in Australia

24 INTRODUCTION Phytophthora root rot is undoubtedly the single most important constraint to orchard productivity in Australia (and probably South Africa too). However, in recent years the industry in Australia has also faced significant tree deaths and/or yield reductions resulting from other fungal pathogens, viz. black root rot, brown root rot and branch dieback. The purpose of this article is to heighten awareness in the South African industry to these disease problems. Black root rot is a severe disease of young avocado trees caused by soilborne fungal pathogens in the Nectriaceae family (Dann et al., 2012; Ramírez Gil, 2013; Vitale et al., 2012). The most severe symptoms of black root rot include leaf wilt and chlorosis and rapid decline and death of young orchard transplants. Affected roots have distinct brown to black, sunken lesions. There have been several reports of fungi associated with avocado black root rot, however, there is considerable confusion with respect to taxonomy and nomenclature (names) of causal agents. Ilyonectria destructans associated with avocado was first reported by Joe Darvas (1978) and subsequently others as Cylindrocarpon destructans (Besoain and Piontelli, 1999; Ramírez Gil, 2013) and as Neonectria radicicola (Zilberstein et al., 2007). Species confirmed by pathogenicity tests as the cause of black root rot in avocado include Calonectria ilicicola in Australia (Dann et al., 2012), which also caused severe stunting, and Dactylonectria macrodi­ dyma (as Ilyonectria macrodidyma) in Italy (Vitale et al., 2012). Ilyonectria liriodendri, and an undescribed Gliocladiopsis sp. were not pathogenic to avocado seedlings in glasshouse pathogenicity tests (Dann et al., 2012). A clear understanding of the taxonomy and pathogenicity of these fungi is necessary for accurate diagnosis and management strategies. Brown root rot is caused by the basidiomycete fungus Phellinus noxius, and is characterised by rapid leaf wilting and tree death, with a fungal crust or “stocking” sometimes evident on the trunk (Dann et al., 2013). This fungus has an extremely wide host range amongst woody species in tropical and subtropical environments, and has been reported in many African countries including Cameroon, Kenya and the Democratic Republic of the Congo, but not, Three “other” diseases impacting avocado productivity in Australia