First report of Puccinia psidii caused rust-disease epiphytotic on the invasive shrub Rhodomyrtus tomentosa in Florida

Authors: Rayamajhi, Min; Pratt, Paul; KLOPFENSTEIN, NED - Us Forest Service (FS); ROSS-DAVIS, AMY - Us Forest Service (FS); RODGERS, LEROY - South Florida Water Management District

Submitted to: Plant Disease
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 4/17/2013
Publication Date: 10/1/2013
Publication URL: http://apsjournals.apsnet.org/doi/abs/10.1094/PDIS-03-13-0225-PDN
Citation: Rayamajhi, M.B., Pratt, P.D., Klopfenstein, N.B., Ross-Davis, A.L., Rodgers, L. 2013. First report of Puccinia psidii caused rust-disease epiphytotic on the invasive shrub Rhodomyrtus tomentosa in Florida. Plant Disease. 97(10):1379-1380.2013.

Interpretive Summary: Downy-rose myrtle of south Asian origin is an invasive shrub that has formed monotypic stands in Florida. During the winter and spring of 2010-2012, a rust disease of epidemic proportion was observed on young foliage, stem terminals and immature fruits of this shrub in natural areas of Martin and Lee counties, Florida, USA. Expanding leaves and succulent stems formed chlorotic flecks on the surface that formed pustules and ruptured to discharge urediniospores. Symptomatic leaves and stems developed severe necrotic spots and resulted in tissue distortion, defoliation and stem dieback. Based on these symptoms, urediniospore morphology and DNA analysis, the causal agent was identified as a rust-fungus called Puccinia psidii. This rust-fungus is believed to be of Neotropical origin and is capable of infecting 129 species in 33 genera within the family Myrtaceae. This type of severe rust-disease of downy-rose myrtle has not been previously reported in Florida, even though invasive trees melaleuca growing in adjacent areas were severely infected by this fungus. In December 2011, urediniospores were collected from downy-rose, suspended in water and sprayed on potted downy-rose myrtle plants. Sprayed plants were maintained in 100% ambient humidity, at 20oC, with a 12-h light cycle for 72 h. Same kind of plants mock-inoculated with water served as the negative control. Disease symptoms, including chlorotic flecks and raised surfaces, appeared on leaf lamina in 3 to 6 days on rust-fungus inoculated plants, while control plants remained symptomless. Raised surfaces developed into distinct pustules and eventually erupted to discharge urediniospores within 6 to 12 days of inoculation. Tests were repeated once during March-April of 2012 with the same results. To our knowledge, this is the first confirmed report of severe disease caused by the rust- fungus on downy-rose myrtle plants in Florida. This raises critical questions as to why this rust disease is so severe and widespread on this host after decades of presumed exposure to this guava rust-fungus in Florida. Because this rust pathogen has emerged as a major invasive threat to many myrtaceous species around the world, further genotyping and cross-inoculation studies are needed to determine the host specificity and potential origin of the rust-fungus isolates derived from downy-rose myrtle.

Technical Abstract: Rhodomyrtus tomentosa (Aiton) Hassk. (downy-rose myrtle, Family: Myrtaceae) of south Asian origin is an invasive shrub that has formed monotypic stands in Florida. During the winter and spring of 2010-2012, a rust disease of epiphytotic proportion was observed on young foliage, stem terminals and immature fruits of this shrub in natural areas of Martin and Lee counties, Florida, USA. Expanding leaves and succulent stems formed chlorotic flecks on the surface that developed into pustules and ruptured to discharge urediniospores. Symptomatic leaves and stems developed severe necrotic spots and resulted in tissue distortion, defoliation and stem dieback. Based on symptoms and urediniospore morphology and dimensions [17.7-26.1(22.1+0.3) x 14.7-21.1(17.7+0.2) µm; N=51], the causal agent was identified as Puccinia psidii Winter. This identification was confirmed by a GenBank BLAST of internal transcribed spacer (ITS) sequences (GenBank Nos. KC607876 and KC607877) that showed 99% identity with 42 sequences of P. psidii from diverse host species and locations. P. psidii is believed to be of Neotropical origin and has a host range of 129 species in 33 genera within Myrtaceae (1). However, P. psidii caused disease of downy-rose myrtle has not been previously reported in Florida, even though severe infections occurred on another invasive tree, Melaleuca quinquenervia (Cav.) S.F. Blake, growing in adjacent areas. In December 2011, urediniospores were collected from downy-rose myrtle, established in aqueous suspension (45K spores/ml), and spray inoculated on potted downy-rose myrtle plants (n=3), which were maintained in 100% ambient humidity, at 20oC, with a 12-h light cycle for 72 h. Plants mock-inoculated with water served as the negative control. Disease symptoms, including chlorotic flecks and raised surfaces, appeared on leaf lamina in 3 to 6 days on P. psidii-inoculated plants, while control plants remained symptomless. Raised surfaces developed into distinct pustules and eventually erupted to discharge urediniospores within 6 to 12 days of inoculation. Tests were repeated once during March-April of 2012 with the same results. The latent and incubation periods reported herein are within the previously reported range for P. psidii. To our knowledge, this is the first confirmed report of P. psidii epiphytotic on downy-rose myrtle populations in Florida. The recent occurrence of P. psidii epiphytotic on downy-rose myrtle raises critical questions as to why this myrtle rust disease is so severe and widespread on this host after decades of presumed exposure to P. psidii in Florida. Because this rust pathogen has emerged as a major invasive threat to many myrtaceous species around the world, further genotyping and cross-inoculation studies are needed to determine the host specificity and potential origin of the P. psidii isolates derived from downy-rose myrtle.