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Pathogenicity Assessment of Puccinia lygodii, a Potential Biological Control Agent of Lygodium japonicum in Southeastern United States

Lygodium japonicum (Thunb.) Swartz (Family, Schizaeaceae) is naturally distributed from Asia to Australia and has naturalized in the United States from Texas to the Carolinas and Florida (4). Recently, it has been declared a Category I weed (the most invasive group) by the Florida Exotic Pest Plant Council. A foliar rust fungus was observed on a population of L. japonicum growing under the canopy of a pine/hardwood forest in Gainesville, FL. The lower surfaces of almost all the pinnules (foliage) were covered with cinnamon-brown eruptive pustules. Necrotic areas developed around mature, erupted, and coalesced pustules. Severely infected foliage were wilted and dried. Microscopic observations of the pustules and spore morphology revealed these eruptive structures to be uredinia. The dimensions (24.6 [+ 2.2] × 29.7 [+ 3.5] μm) and morphology (ellipsoid or obovoid, pale cinnamon-brown, and echinulate with indistinct pores) of urediniospore were similar to those reported for Puccinia lygodii (Har.) Arth. (Uredinales) (1). Therefore, the rust was identified as P. lygodii and confirmed by J. Hennen. P. lygodii is native to South America, where it has been recorded from L. volubile and L. venustum (2). This rust was previously identified as Milesia and Uredinopsis spp. on L. japonicum from Louisiana and Florida, respectively (3). Herein, we report the performance of Koch's postulates for P. lygodii on L. japonicum. Excised foliage bearing uredinia from plants collected near Gainesville were placed in a flask, flooded with deionized distilled water, shaken vigorously for a few minutes, and the suspension strained through four layers of cheesecloth. Urediniospores suspended in the filtrate were concentrated to 1.0 × 10⁶ spores/ml, using sedimentation technique, and then misted onto 3-week-old foliage of fully expanded fronds of four juvenile L. japonicum plants grown in pots, until the foliage were completely wet. The plants were then covered with a plastic bag and placed in dappled shade. After 3 days, the bags were removed and the water-filled containers were placed around L. japonicum plants to maintain high ambient humidity. During the remaining 4-week experimental period, the temperature and relative humidity under the shaded areas ranged from 23 to 38°C and 38 to 93%, respectively. The plants were monitored daily for development of symptoms characteristics of P. lygodii. Minute cinnamon-brown flecks appeared on the foliage 20 days after inoculation. Within 3 to 5 days, these flecked areas expanded, erupted, and formed uredinia on the lower surface of the symptomatic foliage. The morphology and size range of the uredinia and urediniospores were the same as those of the P. lygodii applied in this test. This is the first report confirming pathogenicity of P. lygodii on L. japonicum. P. lygodii may be a potential biological control agent of L. japonicum in the Southeast United States.

References: (1) J. C. Arthur. Bull. Torrey Club 51:55, 1924. (2) J. F. Hennen and J. W. McCain. Mycologia 85:970--986, 1993. (3) J. W. McCain, J. F. Hennen, and Y. Ono. Mycotaxon 39:281--300, 1990. (4) R. W. Pemberton and A. P. Ferriter. Am. Fern J. 88:165--175, 1998.