Presence of Fusarium spp. in Air and Soil Associated with Sorghum Fields

Funnell-Harris, D. L., and Pedersen, J. F. 2011. Presence of Fusarium spp. in air and soil associated with sorghum fields. Plant Dis. 95:648-656. Sorghum grain, valuable for feed, food, and bioenergy, can be colonized by several Fusarium spp.; therefore, it was of interest to identify possible sources of conidia. Analysis of air and soil samples provided evidence for the presence of propagules from Fusarium genotypes that may cause grain infections. Soil population estimates of members of the Gibberella fujikuroi species complex, that includes sorghum pathogens and other Fusarium spp., suggested that adequate inoculum for systemic infections was present. Conidia in air samples within two sorghum fields were collected by passive trapping for 2 years. Subsampled Fusarium isolates indicated that numbers of G. fujikuroi increased from anthesis through maturity, which coincides with grain development stages vulnerable to Fusarium spp. Genotyping using translation elongation factor 1-α gene sequences revealed that spore trap isolates included members of G. fujikuroi that are sorghum pathogens: Fusarium thapsinum, F. verticillioides, F. proliferatum, and F. andiyazi. Also detected were F. graminearum, F. subglutinans, and several F. incarnatum-F. equiseti species complex haplotypes that colonize sorghum asymptomatically. All commonly found grain colonizers were detected from air samples in this study. Sorghum grain is valuable for feed, food, and bioenergy. A C4 crop that is drought and heat tolerant, sorghum can be grown in tropical, subtropical, and warm-temperate climates under subsistence farming conditions (45). In the United States, grain is produced primarily for livestock feed but, in other cultures, it is a food staple (45). Because it is gluten-free and contains nutrients and protective phytochemicals unique among grasses, food-quality sorghum is produced in the United States as an alternative grain (3,56). Sorghum grain can also be used in ethanol production for biofuels (59), and the distiller’s grain coproduct is useable for ruminant livestock feed (1). An advantage to growing sorghum grain for food or feed is that production of mycotoxins as a result of contaminating fungi can be considerably less than that on maize kernels (4,7). This may be due to reduced total colonization by mycotoxigenic fungi (4) or lower toxin production by genotypes infecting sorghum (11,25,35). Thus, sorghum grain not only has many uses, it also has the potential to be advantageous for food and feed under less-than-optimal growing or storage conditions. Nonetheless, dangerous levels of mycotoxins have resulted in sorghum, especially in stored grain (23). Natural infection of sorghum grain by Fusarium spp. can result in production of mycotoxins such as fumonisin B1, moniliformin, zearalenone, T-2, and deoxynivalenol (7,20,27,29). Previously, we reported 10 Fusarium spp. or genotypes within Fusarium species complexes in grain of elite sorghum lines grown in Nebraska fields (16). Eight of these species have been reported to produce mycotoxins: five members of the Gibberella fujikuroi species complex, two genotypes in the Fusarium incarnatum-F. equiseti species complex (FIESC; formerly known as F. bullatum Sherb. and F. pallidoroseum (Cooke) Sacc.), and F. graminearum Schwabe. Fusarium spp. most commonly recovered are within G. fujikuroi (Sawada) Wollenw., particularly F. thapsinum Klittich, J.F. Leslie, P.E. Nelson & Marasas (16). It is of great interest to define potential sources for Fusarium spp. colonizing sorghum grain. Fusarium spp. are major components of the complex that causes sorghum grain mold, a disease common in humid tropical and subtropical regions (5,54). It results in reduced grain yields but equally serious are affects on physical appearance and biochemical content of the grain (54,58) and the presence of mycotoxins (29). Sorghum is particularly vulnerable to grain mold pathogens at anthesis (flowering) through grain hard-dough stage but exposed parts of the grain can be infected until post maturity, as long as weather conditions are conducive to grain diseases and conidia of pathogens are present (6,12,14,33,34,39,52). However, previous work with maize provided evidence that F. verticillioides (Sacc.) Nirenberg, a member of G. fujikuroi, at a low rate via systemic infection, could colonize kernels of plants grown from inoculated seed (37); it is conceivable that infected grain could also result from systemically infected plants arising from endophytically infected sorghum seed or from seedlings infected through roots by soilborne fungi. Therefore, we screened soils for Fusarium spp. from sorghum research plots and production fields in Nebraska and monitored conidia from within and above the canopies of sorghum fields, using passive spore trapping on two fungal media, throughout two growing seasons. In this way, the availability of potential inoculum in soil, that included organic matter, and in air was determined. Isolates were identified by morphological traits (soil and air) and molecular identification using the 5′ portion of the translation elongation factor 1-α (TEF) gene (air) (19). In this way it was possible to determine whether infections by Fusarium spp. identified from sorghum grain grown in Nebraska could be associated with Fusarium genotypes found in soil or air. Materials and Methods Soil screens. Soil from sorghum research plots at the University of Nebraska, Lincoln, Field Laboratories (UNL) on the east side of Lincoln and near Ithaca, NE, was sampled pre-planting in 2005 and post harvest in 2005 and 2006. Soil also was sampled at Lincoln at harvest 2004 and one time during the winter in 2005. Fields Corresponding author: D. L. Funnell-Harris, E-mail: [email protected] GenBank accessions: JF270168 to JF270311. Mention of trade names or commercial products in this article is solely for the purpose of providing specific information and does not imply recommendation or endorsement by the United States Department of Agriculture. USDA is an equal opportunity provider and employer. Accepted for publication 5 February 2011. doi:10.1094 / PDIS-09-10-0671 This article is in the public domain and not copyrightable. It may be freely reprinted with customary crediting of the source. The American Phytopathological Society, 2011.