Black root rot, caused by fungi in the Nectriaceae family, has been reported from multiple countries as a significant disease, particularly of young avocado trees. In 2019, a higher than usual mortality rate in newly planted avocado trees was observed across New Zealand avocado growing regions. Symptoms from these trees included black necrotic roots, considerably smaller root systems, and in some cases internal browning at the base of the trunk. Foliar dieback was often associated with these symptoms.
Symptomatic avocado trees were submitted for disease analysis from various locations and included a range of rootstock cultivars. Isolations onto agar media from root lesions and discoloured cortex tissue at the base of the trunk consistently yielded nectriaceous fungi, primarily Cylindrocarpon-like asexual morphs. DNA sequence analysis of the histone H3 gene was conducted on 50 isolates. Subsequent phylogenetic analysis of histone H3 sequences showed that 41 isolates were placed within Ilyonectria liriodendri. In addition, one isolate was identified as Ilyonectria robusta, three as Dactylonectria pinicola and two as Dactylonectria macrodidyma. One Ilyonectria and one Dactylonectria isolate clustered with described species within their respective genera, but species-level identification could not be confirmed based on the available sequence data. A Cylindrocladiella isolate, not part of the phylogenetic analysis, was initially identified as Cylindrocladiella parva, however further analysis revealed it is likely a closely related species.
Nine isolates were selected for pathogenicity testing to represent a range of sampling locations and taxa identified. A glasshouse pot trial was set up to determine pathogenicity of the isolates on Zutano seedling rootstocks sourced from two plant propagators. Fungal inoculum of each isolate was produced on autoclaved barley grain and added to the potting media prior to planting. Sixteen weeks after inoculation, all six I. liriodendri isolates resulted in statistically higher (P<0.05) root disease scores compared to the uninoculated control. Inoculation with three of these isolates resulted in a lower root dry weight (P<0.05) whilst one isolate showed differences across all measures of disease severity including the relative height increase (P<0.05) between inoculation and assessment. The isolates of Dactylonectria and Cylindrocladiella spp. were not pathogenic. Re-isolation of I. liriodendri from necrotic roots of inoculated plants fulfilled Koch’s postulates, confirming pathogenicity. These findings suggest that I. liriodendri is associated with black root rot of avocado in New Zealand, and further research is required to develop strategies to mitigate the impact on the avocado industry.