The life cycle and field epidemiology of Uromycladium acaciae (Pucciniales) on Acacia mearnsii in South Africa

Abstract

Uromycladium acaciae has damaged plantations of Acacia mearnsii in southern Africa since 2013. Uredinia of a species of Uromycladium have been known on A. mearnsii in South Africa since the 1980s. However, the recent damage is associated with telia and spermogonia. Uredinia and telia were previously treated as conspecific with a phylogenetic species concept. However, uredinia did not form after previous artificial inoculation experiments with teliospores. Controlled studies identified the optimum conditions for basidiospore infection, but the optimum conditions for sporulation and dispersal have not been identified. To investigate the life cycle and field epidemiology of Uromycladium on A. mearnsii, spores were trapped weekly and development of disease symptoms and plant phenology were monitored monthly at three plantations. Telia and spermogonia developed independently from uredinia, and nucleotide polymorphisms between rDNA of uredinia and telia were fixed based on high throughput sequencing and PCRs. All three weather variables measured had a significant effect on teliospore abundance at two of the three sites. Teliospore abundance was greatest during trapping periods when mean relative humidity was high, mean rainfall was 4–5 mm day−1 and mean temperature was 15–16°C. Teliospore counts peaked at the end of summer, potentially the result of epidemic build‐up. Results support the hypothesis that despite sharing a most recent common ancestor, uredinia on A. mearnsii in southern Africa are independent to the life cycle of the telial rust, which likely constitutes a new introduction. Furthermore, teliospores of U. acaciae disperse under wet conditions, and the wet season between October and March is the optimal period for wattle rust development.

Hyperparasitism by Sphaerellopsis macroconidialis may lower over‐wintering survival of Uromycladium acaciae

“…mearnsii and the main epidemic period for U . acaciae (Fraser et al, 2021). Foliage and rust pustules collected during this period were therefore older.…”

“…mearnsii infected by U . acaciae in Mpumalanga were selected for monitoring of disease symptom development (Fraser et al, 2021). All sites were in the south‐east of Mpumalanga near the border with Eswatini, an area with a temperate climate characterized by dry winters and wet summers.…”

“…Uromycladium acaciae causes a damaging rust disease on Acacia mearnsii (black wattle) in southern Africa (Fraser et al, 2021). First observed in the KwaZulu‐Natal Midlands in 2013, this disease is now found throughout the KwaZulu‐Natal and Mpumalanga Provinces of South Africa, as well as in neighbouring Eswatini (formerly Swaziland) (Fraser et al, 2021).…”

“…Uromycladium acaciae causes a damaging rust disease on Acacia mearnsii (black wattle) in southern Africa (Fraser et al, 2021). First observed in the KwaZulu‐Natal Midlands in 2013, this disease is now found throughout the KwaZulu‐Natal and Mpumalanga Provinces of South Africa, as well as in neighbouring Eswatini (formerly Swaziland) (Fraser et al, 2021). Uromycladium acaciae produces spermogonia and telia on all above‐ground plant parts, with symptoms and signs including rachis and rachilla malformation, production of chocolate‐brown telia that become ‘slimy’ under wet conditions, matting of leaves and stunting of growth (Fraser et al, 2021).…”

First report of phyllode rust on Acacia crassicarpa outside its native range

“…In the case of Acacia spp., phyllode rust caused by Endoraecium digitatum (Endoraeciaceae, Pucciniales) is a well‐known disease of A. mangium in Southeast Asian countries including Indonesia, Malaysia, and Vietnam (Berndt, 2011; Doungsa‐ard et al., 2018; Mohd Farid et al., 2023; Old et al., 2000). In recent years, rust caused by Uromycladium acaciae (Raveneliaceae, Pucciniales) has emerged as a threat to A. mearnsii in South African plantations (Fraser et al., 2021; McTaggart, Doungsa‐ard, Wingfield, & Roux, 2015; Morris et al., 1988) and most recently also in Ethiopia (Pham et al., 2023). In all cases, these Acacia rust pathogens, Endoraecium and Uromycladium , are known on their native hosts in Australia (Doungsa‐ard et al., 2018).…”