First Report of Puccinia psidii (Myrtle Rust) in Eucalyptus Plantations in Australia.

Abstract

Puccinia psidii Winter (myrtle rust, eucalyptus rust) is a significant pathogen of Eucalyptus plantations in Brazil, causing reduced growth, stem malformation, and in severe cases, tree death (3). It has a wide host range in the Myrtaceae, with over 445 species in more than 72 genera (4). As such, P. psidii has long been a threat to Australia, where many ecosystems are dominated by Myrtaceae and industries are reliant on myrtaceous hosts, including almost 1 million hectares of eucalypt plantations. In April 2010, P. psidii was detected in Australia (2) and is now established along the east coast from southern New South Wales to far north Queensland (1,5). Although known to cause severe damage to eucalypt seedlings and coppice in native forests (5), it had not been found affecting eucalypt plantations in Australia. Surveys for P. psidii were thus initiated in eucalypt plantations in NSW from the central coast (33°06'40.0″ S, 151°18'13.8″ E) to the NSW-Queensland border, encompassing 55 plantations. Two to four 100-tree transects were conducted per plantation, during spring and summer. Symptoms were first detected in December 2011 in a 6-month-old Eucalyptus agglomerata Maiden plantation on the central coast. Further surveys until summer 2014 identified P. psidii on E. pilularis Sm., E. cloeziana F. Muell., and E. grandis (Hill) Maiden in young plantations from the central coast to the north coast (30°24'20.2″ S, 152°55'57.9″ E) of NSW. Necrotic lesions and yellow pustules typical of P. psidii were present on immature leaves and shoots, often causing leaves to buckle and die. Urediniospores were globose to subglobose, yellowish brown, 15 to 20 × 18 to 23 μm, single-celled, and finely echinulate, with a prominent tonsure on the majority of spores. Teliospores were cylindrical to ellipsoidal with a rounded apex, tan brown, 25 to 45 × 15 to 25 μm, and two-celled, with remnants of a pedicel. These morphological characteristics are consistent with those of P. psidii from Australia and elsewhere (5). Simple Sequence Repeats developed from genome sequencing of an Australian isolate of P. psidii revealed no variation among 15 isolates of P. psidii from Australia and Hawaii, including an isolate from an E. pilularis plantation in NSW (K. S. Sandhu and R. F. Park, unpublished), corroborating the morphological identification. This is the first report of this significant pathogen in eucalypt plantations in Australia. P. psidii was found in only five plantations during the current surveys, in trees 6 months to 2 years old, with only a low incidence (1%) per plantation. Repeat surveys revealed no ongoing disease in the same plantations after trees were three years of age. Moreover, P. psidii was found only in plantations surrounded by native forest stands, which harbor a large reservoir of susceptible hosts, such as Rhodamnia rubescens (Benth.) Miq. The strain of P. psidii that entered Australia is currently not causing serious disease in eucalypt plantations. However, there is a need to continue quarantine restrictions to reduce the chance of another, more aggressive strain of P. psidii entering Australia.