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 actively growing tips on several myrtaceous plants in South and Central America. It has recently been discovered in south Florida causing a similar disease on Melaleuca quinquenervia. We therefore evaluated P. psidii as a potential biological control agent of this invasive tree. Typical disease symptoms on M. quinquenervia included distortion and abscission of young foliage and dieback of severely infected tips. Young stems with living bark developed lesions and localized swellings. The stems became brittle and prone to breakage at the point of these swellings. Often, flowers and young seed capsules also developed eruptive pustules. Host range tests were performed on 18 species in 11 genera of Myrtaceae by inoculating expanding leaves with uredospores of two P. psidii isolates: MISOL, obtained from M. quinquenervia, and PISOL, obtained from Pimenta dioica. Results showed Callistemon viminalis, Eugenia reinwardtiana, M. decora, M. quinquenervia, Myrcianthes fragrans, Myrciaria cauliflora, P. dioica, and Psidium guajava to be susceptible 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 seven species (Calyptranthes pallens, Eugenia confusa, Eugenia foetida, Eugenia uniflora, Feijoa sellowiana, Psidium cattleianum, and S. jambos) exhibited no symptoms and were considered immune to both isolates. The ability of these isolates to initiate pustules on susceptible hosts differed significantly. Overall, both isolates induced more pustules on M. quinquenervia, E. reinwardtiana, and P. dioica than on other susceptible species. Based on host range, both Florida isolates of P. psidii appear similar to one that infects Pimenta spp. in Jamaica. Our studies included a limited number of plant species grown under optimal conditions for disease expression. Field tests will be needed to ascertain their susceptibility under more natural conditions. The P. psidii and M. quinquenervia pathosystem probably represents a “new association,” because of the disparate origins of the two species involved and their adventive status in Florida.