Uromycladium acaciae, the cause of a sudden, severe disease epidemic on Acacia mearnsii in South Africa

Abstract

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.