Host range of Puccinia psidii, a potential biological control agent of Melaleuca quinquenervia in Florida

The rust fungus Puccinia psidii infects the foliage and causes dieback of a ctively growing tips on several myrtaceous plants in South and Central Amer ica. It has recently been discovered in south Florida causing a similar dis ease on Metaleuca quinquenervia. We therefore evaluated P. psidii as a pote ntial biological control agent of this invasive tree. Typical disease sympt oms on M. quinquenervia included distortion and abscission of young foliage and dieback of severely infected tips. Young stems with living bark develo ped lesions and localized swellings. The stems became brittle and prone to breakage at the point of these swellings. Often, flowers and young seed cap sules also developed eruptive pustules. Host range tests were performed on IS species in 11 genera of Myrtaceae by inoculating expanding leaves with u redospores of two P. psidii isolates: MISOL, obtained from M. quinquenervia , and PISOL, obtained from Pimenta dioica. Results showed Callistemon vinti nalis, Eugenia reinwardtiana, M. decora, M. quinquenervia, Myrcianthes frag rans, Alyrciaria cauliflora, P. dioica, and Psidium guajava to be susceptib le to both Isolates. Eucalyptus grandis, Eugenia paniculatum, and Syzygium cumini manifested chlorotic halos that developed into brown leaf spots but had no sporulation and were therefore considered resistant. The remaining s even species (Calyptranthes pallens, Eugenia confusa, Eugenia foetida, Euge nia uniflora, Feijoa sellowiana Psidium cattleianum, and S. jambos) exhibit ed no symptoms and were considered immune to both isolates. The ability of these isolates to initiate pustules on susceptible hosts differed significa ntly. Overall, both isolates induced more pustules on M. quinquenervia, E. reinwardtiana, and P. dioica than on other susceptible species. Based on ho st range, both Florida isolates of P. psidii appear similar to one that inf ects Pimenta spp. in Jamaica. Our studies included a limited number of plan t species grown under optimal conditions for disease expression. Field test s will be needed to ascertain their susceptibility under more natural condi tions. The P. psidii and M. quinquenervia pathosystem probably represents a "new association," because of the disparate origins of the two species inv olved and their adventive status in Florida. (C) 2001 Academic Press.