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First Report of Guineagrass Smut Caused by Conidiosporomyces ayresii in North America

Rosskopf, E. N., Abbasi, M., Aime, M. C.
Plant disease 2019 v.103 no.4 pp. 760
Megathyrsus maximus, agar, biological control, biological control agents, conidia, crop production, florets, flowering, forage, fungi, herbaria, introduced species, invasive species, livestock, malt, oligodeoxyribonucleotides, panicles, plant ovary, plastic bags, ribosomal DNA, seedlings, sequence analysis, spikelets, tropics, weeds, Africa, Florida
Guineagrass (Panicum maximum Jacq., syn = Megathyrsus maximus [Jacq.] B. K. Simon & S. W. L. Jacobs) is native to Africa but has been introduced to many tropical regions for use as livestock forage. It is a significant weed problem for crop production in several areas of the southern United States and has been the focus of previous efforts for biological control (Chandramohan and Charudattan 2001). It is listed by the Florida Exotic Pest Plant Council as a category II invasive species (FLEPPC 2017). Guineagrass with inflated florets filled with gray-brown spore masses was found in multiple locations in Saint Lucie and Okeechobee counties in Florida, U.S.A., in May 2015. When observed, nearly all spikelets on individual plants and all plants in the area were affected. Sori in some ovaries of infected inflorescences were inflated, ovoid or cylindrical, 3 to 8 × 2 to 3 mm, composed of an apically open, sac-like, leathery peridium, and contained a central mass of semipowdery mixture of spores, sterile cells, and balls of conidia. Peridium was thick, dark olivaceous at the base, and light grayish-brown at its opened end. Spore mass was gray-brown, with sterile cells and conidia between the spores. Spores were globose to subglobose, 11.5 to 17 × 10.5 to 16.5 µm, yellowish brown to dark brown, finely to coarsely verrucose, verrucae about 1 to 1.5 µm high, and sometimes fused to each other. Sterile cells were globose, subglobose, ellipsoidal, hyaline to pale yellow, 13 to 21.5 × 12 to 19.5 µm, wall 1 to 1.5 µm thick, finely, moderately to densely verruculose. Conidia were thin-walled, smooth, hyaline, variable in shape, usually Y-shaped, but also triangular, T-shaped, club-shaped or somewhat branched, 9 to 19.5 µm long, usually 2.5 to 4 µm wide, loosely connected into balls of conidia. The morphological characteristics of the fungus were consistent with those of Conidiosporomyces ayresii (Berk.) K. Vánky & R. Bauer (Vánky and Bauer 1992), and disease symptoms were similar to those reported by Tsukiboshi et al. (2012). Sequencing of the 28S rDNA using primer set LROR (Cubeta et al. 1991) and LR7 (Vilgalys and Hester 1990; GenBank accession MH378444) produced a 1,366-bp product that shared 99% (1,301/1,305 bp) sequence identity with C. ayresii accession AY819017 (Castlebury et al. 2005). Five 1-month-old, flowering Guineagrass seedlings were inoculated with a 10⁶ conidial suspension prepared from 0.2% malt agar cultures of the fungus. Five control plants were treated with sterile deionized water. Panicles were covered with opaque plastic bags in which sterile water was applied with an atomizer. Approximately 1 month after application of conidia, inoculated plants were symptomatic, produced no seed, and the fungus was reisolated from infected spikelets. Control plants remained healthy and produced seed. To our knowledge, this is the first report of Guineagrass smut in North America, the causal agent of which could potentially be used as a biological control agent for this invasive species. A voucher specimen of the fungus was deposited at The Kriebel Herbarium, Purdue University (PUL F2881).