First report of guineagrass smut caused by Conidiosporomyces ayresii in North America

Authors: Rosskopf, Erin; ABBASI, MEHRDAD - Purdue University; AIME, CATHERINE - Purdue University

Submitted to: Plant Disease
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 1/16/2019
Publication Date: 2/11/2019
Citation: Rosskopf, E.N., Abbasi, M., Aime, C. 2019. First report of guineagrass smut caused by Conidiosporomyces ayresii in North America. Plant Disease. PDIS-08.

Interpretive Summary: Guineagrass is an invasive species of grass that is native to Africa. In some regions of the world it is used as a livestock forage, but in the United States it is considered a significant weed problem in crops and pastures. It has been the focus of biological control programs in the past. Guineagrass with symptoms of a fungal infection of the seed heads was observed in several counties in Florida. The fungus caused flowers to become inflated and filled with a powdery gray mass of spores. The fungus was isolated and identified as Conidiosporomyces ayresii and was confirmed as the cause of the observed symptoms by infecting new plants with the fungus, which resulted in the recreation of the observed symptoms. 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 Kriebal Herbarium, Purdue University (PUL F2881).

Technical Abstract: Guineagrass 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 of in several areas of the southern United States and has been the focus of previous efforts for biological control. Guineagrass with inflated florets filled with grey-brown spore masses was found in multiple locations in Saint Lucie and Okeechobee counties in Florida, USA 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 inflorescence, inflated, ovoid or cylindrical, 3–8 x 2–3 mm, composed of an apically open, sac-like, leathery peridium, and a central mass of semi-powdery mixture of spores, sterile cells, and balls of conidia. Peridium was thick, dark olivaceous at the base, light grayish-brown at its opened end. Spore mass was grey-brown, with sterile cells and conidia between the spores. Spores globose to subglobose, 11.5–17 x 10.5–16.5 µm, yellowish brown to dark brown, finely to coarsely verrucose, verrucae about 1–1.5 µm high, sometimes fused to each other. Sterile cells globose, subglobose, ellipsoidal, hyaline to pale yellow, 13–21.5 x 12–19.5 µm, wall 1–1.5 µm thick, finely, moderately to densely verruculose. Balls of conidia were composed of many loosely connected conidia. Conidia thin-walled, smooth, hyaline, variable in shape, usually Y-shaped, but also triangular, T-shaped, club-shaped or somewhat branched, 9–19.5 µm long, usually 2.5–4 µm wide. The morphological characteristics of the fungus were consistent with those of Conidiosporomyces ayresii and disease symptoms were similar to those previously reported. Sequencing of the 28S rDNA using primer set LROR/LR7 (GenBank accession MH378444) confirmed the morphological identification with 99% sequence homology to C. ayresii accessions AY819017.1 and AY818984.1. Five one-month old, flowering guineagrass seedlings were inoculated with a 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 one 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 Kriebal Herbarium, Purdue University (PUL F2881).