A severe rust disease has caused extensive damage
to plantation grown Acacia mearnsii trees in the KwaZulu-
Natal Province of South Africa since 2013. The symptoms are
characterized by leaf spots, petiole and rachis deformation,
defoliation, gummosis, stunting of affected trees and dieback
of seedlings. The cause of this new disease was identified
using a combined morphological and DNA sequence approach.
Based on morphology, the rust fungus was identified
as a species of Uromycladium. It formed powdery, brown telia
on petioles, stems, leaves, seedpods and trunks of affected
trees. The teliospores were two per pedicel and either lacked
or had a collapsed sterile vesicle. Sequence data and morphology
showed that the collections from South Africa were conspecific,
however telia were not produced in all provinces.
Uromycladium acaciae is the most suitable name for this rust
fungus, based on morphology and phylogenetic analyses of
the internal transcribed spacer and large subunit regions of
ribosomal DNA. The rust was first identified as U. alpinum in 1988, from minor symptoms on the leaflets caused by its
uredinial stage on A. mearnsii in South Africa. It has now
become a threat to plantations of A. mearnsii, with an altered
life cycle and increased disease severity.