A Neotropical rust of the Myrtaceae, Puccinia psidii Winter, was described from Psidium guajava L., or guava, in Brazil in 1884 (1). It was first discovered in Hawaii on potted Metrosideros polymorpha Gaud. on Oahu in April 2005 (2) with pathogenicity and identity established (3). It spread quickly, and by January 2006, severe outbreaks of this rust occurred statewide on new leaves of Syzygium jambos (L.) Alston, or rose apple. Rose apple, a native to South and Southeast Asia, was introduced to Hawaii in 1825 and is locally abundant to invasive from just above sea level to as high as 1,000 m in elevation in wet sites. Healthy, reddish green immature leaves on new twigs become deformed, yellow-red, and covered with masses of yellow urediniospores following infection. As the disease progresses, infected leaves are blackened and defoliate, with no functional leaves formed. Stem tips and branches are killed and the canopy becomes progressively smaller. Repeated mortality of juvenile leaves was observed to kill 8 to12 m tall trees in the Haiku area of Maui. Wind dispersal of urediniospores resulted in heavy infection of even small groups of S. jambos isolated by 1 km or more and billions of urediniospores covered the ground under diseased trees. On Hawaii, Maui, and Oahu, trees with many dead branches are becoming common with concerns about the fire hazard of these dead trees surrounded by dry grasses. At low humidity levels, or on more mature leaves characterized by soft expanded yellow-green tissue, fewer, mostly circular spots are formed that do not expand. S. jambos is an example of a highly vulnerable host in Hawaii and represents one of approximately 3,500 species of Myrtaceae outside the Neotropics growing in Australasia, Southeast Asia, the Pacific, and tropical Africa, which have evolved unexposed to P. psidii. Severely infected S. jambos plants have been the major source of spores in the environment, exposing many Myrtaceae hosts to P. psidii. The pathogenicity of P. psidii has been consistent among and within islands with S. jambos severely infected and M. polymorpha, Melaleuca quinquenervia, Rhodomyrtus tomentosa, Myrtus communis, and Eugenia species commonly infected. Other hosts such as S. cumini, S. malaccense, and Myriciaria cauliflora are also infected, although guava and Eucalyptus spp. are rarely infected. Strain differences within P. psidii are suspected (4). In the tropics, it is rare for mature trees to be killed by a foliar pathogen, but given the devastation of new growth, death of more S. jambos trees is likely.
References: (1) T. A. Coutinho et al. Plant Dis. 82:819, 1998. (2) E. M. Killgore and R. A. Heu. New Pest Advisory No. 05-04. Hawaii Department of Agriculture, 2007. (3) J. Y. Uchida et al. Plant Dis. 90:524, 2006. (4) S. Zhong et al. Mol. Ecol. Res. 8:348. 2008.
