Botanic Gardens Trust, Sydney, Australia

Leaf spot fungi systematics

Dr Brett Summerell - Pathologist/Mycologist; Director Science and Public Programs

Many of the most important plant diseases affecting both agricultural crops and plants in native ecosystems are caused by fungi in the Mycosphaerellaceae. These fungi were until recently all considered as members of the genus Mycosphaerella which numbered over 3000 different species. The number of species in the genus has grown significantly over the past ten years as new species were recovered, especially from native plants in Australia

However research by a group of mycologists including the Trustís Brett Summerell have shown that this genus is better defined if it is split into 33 separate genera. The decision to split the genus was based on large amounts DNA fingerprinting data where a number of genes were sequenced, as well as an analysis and comparison of morphological and ecological characters of a large number of species in each of the genera.

Work of this nature where there are very large numbers of name changes is not something that should be taken lightly. As many of these species of fungi cause very important plant diseases large numbers of people are familiar with the fungi and refer to the diseases by older names. In some cases legislation relating to fungicide use or quarantine refer to specific fungi names and as such will need to be changed so that they remain current. Of course such changes have a significant impact on fungal collections where the collections need to be annotated and the databases updated - again not a trivial task in a group so large.

On a positive side however, the benefits that result from the names changes and the better understanding of the phylogenetic relationships will allow us to better predict how these fungi will behave as pathogens and what the likely impact of control measures will be. 

Readeriella patrickii
Readeriella patrickii a fungus causing a leafspot on Eucalyptus amygdalin in Tasmania: a. Leaf spot; b, c. conidiogenous cells giving rise to conidia (arrows denote loci); d-g. conidia with mucoid sheaths. Scale bars = 10 μm.