Lygodium microphyllum (Cav.) R.Br. (Old World climbing fern), in the family Schizaeaceae, is one of the most invasive (Category I in Florida) weeds in Florida. It has invaded more than 50,000 ha of wetlands and moist habitats in southern Florida and is rapidly spreading in new areas of the Everglades (3). The search and evaluation of biocontrol agents for this fern is currently in progress. Puccinia lygodii (Har.) Arth. (Uredinales) (1), previously recorded on L. volubile Sw. and L. venustum Sw. in South America (2), attacks foliage and severely damages L. japonicum Thunb. (Japanese climbing fern) vines in northern and central Florida (4). We hypothesized that since L. japonicum occurred mainly in northern and central Florida, P. lygodii did not have opportunity to interact with L. microphyllum, which primarily occurs in southern Florida. Therefore, we used two inoculation methods to test the possible pathogenicity of P. lygodii on the new host, L. microphyllum. Method-I was designed to imitate a seminatural inoculation technique in which three containerized (0.45-L capacity) L. microphyllum test plants (15- to 30-cm-high sporelings) were intermixed among a group of containerized (5.0-L capacity) P. lygodii-infected L. japonicum plants (source of inoculum) in a glasshouse. In Method-II, uredospores obtained from pustules on diseased L. japonicum foliage were adjusted to 1 × 106 uredospores/ml and then misted on three L. microphyllum sporelings (same size as in Method-I) until foliage was completely wet. The plants were then covered individually with a plastic bag for 3 days to facilitate spore germination and infection. In both methods, three L. japonicum sporelings of similar size as L. microphyllum were intermixed among diseased L. japonicum plants as a positive control. All test and infected plants were placed on 6-cm-high trays filled two-thirds with water and exposed to diffused daylight and a temperature range of 20 to 35°C in a glasshouse. These plants were monitored for the development of rust symptoms (halos and rust pustules) development for 8 weeks. Minute cinnamon flakes that developed into eruptive pustules were seen on the lower surface of the pinnules approximately 42 and 28 days after treatment initiation (in both methods) for L. microphyllum and L. japonicum (positive control), respectively. Each method was repeated twice. Dimensions (29.7 [±3.7] × 23.5 [±2.6] μm) and morphology of urediniospores from pustules on inoculated L. microphyllum were similar to those reported for P. lygodii on other host systems (1,2,4). To our knowledge, this is the first report demonstrating the infection of P. lygodii on L. microphyllum. The potential use of P. lygodii as a classical bio-control agent of L. microphyllum in southern Florida will be further investigated.
References: (1) J. C. Arthur. Bull. Torrey Bot. Club 51:55, 1924. (2) J. W. McCain et al. Mycotaxon 39:281, 1990. (3) R. W. Pemberton. SIDA 20:1759, 2003. (4) M. B. Rayachhetry et al. Plant Dis. 85:232, 2000.
