Surveys for pathogens of monoecious hydrilla in 2013

Approved for public release; distribution is unlimited.
\nERDC/TN APCRP-BC-34
\nOctober 2014
\nSurveys for Pathogens of Monoecious
\nHydrilla in 2013
\nby Judy F. Shearer
\nPURPOSE: This technical note describes 2013 survey results for pathogenic agents on monoecious
\nhydrilla.
\nINTRODUCTION: There are two biotypes of Hydrilla verticillata (L.f.) Royle (hydrilla) in the United
\nStates. The pistillate dioecious hydrilla biotype was introduced from Sri Lanka into Florida in the
\n1950’s (Schmitz et al. 1990). It has spread throughout the Southeast United States, as far west as Texas
\nand into parts of California (Madeira et al. 2004). Monoecious hydrilla was first discovered in
\nDelaware in 1976 and later in the Potomac River (Haller 1982, Steward et al. 1984). It has now
\nexpanded its distribution through the Atlantic States and northward to Maine (Madeira et al. 2004).
\nSeparate populations have been reported in Iowa, Ohio, Indiana, Wisconsin, Kansas, Missouri,
\nCalifornia, and Washington State (Nonindigenous Aquatic Species (NAS) 2011). The Washington
\nState population no longer exists due to an aggressive eradication program.1 It is believed that
\npopulations in Iowa and Wisconsin have also been eradicated.2 Recent invasions have appeared in
\nLake Cayuga and at Tonawanda Creek/Erie Canal, both in upstate New York (Cornell Coopertive
\nExtension (CCE) 2011, Lansing Star 2012). Shortly after its discovery in 1982, Steward et al. (1984)
\npredicted that monoecious hydrilla had the potential to invade all of the lower 48 states and southern
\nand central Canada. Balciunas and Chen (1993), after surveying for biocontrol agents in Asia and
\nexamining herbaria in the region, also indicated that hydrilla could become widespread in North
\nAmerica, including Canada and parts of Alaska. Although not known to exist in Minnesota, Maki and
\nGalatowitsch (2008) ran a CLIMEX model (a software program that predicts the effect of climate
\nchange on species distribution) that indicated the state was at risk for invasion of monoecious hydrilla.
\nThe growth forms of dioecious and monoecious hydrilla biotypes are different. Compared to the
\nmonoecious biotype, dioecious plants tend to have more vigorous growth extending vertically to the
\nwater surface then spreading laterally and forming a mat (Van 1989). Madeira et al. (1997)
\nhypothesized that this growth form was an adaptation to deep water generated from monsoons on the
\nIndian subcontinent. Tubers (i.e. vegetative propagules) can be found up to 30 cm deep in the sediment
\n(Langeland 1996) but in North Carolina, Harlan et al. (1985) found the majority of monoecious tubers
\nat depths of 0 to 8 cm in the sediment. Tubers of the dioecious biotype are larger than those of the
\nmonoecious biotype (Spencer et al. 1987) and are formed under short-day conditions (Van 1989). In
\ncontrast, tubers of the monoecious biotype are produced under long-day photoperiods and are smaller
\n1Personal Communication. 2012. J. Parsons, Aquatic Plant Specialist, State of Washington Department of Ecology, Olympia,
\nWA.
\n2Personal Communication. 2013. M. Netherland, Research Biologist, U. S. Army Engineer Research and Development Center,
\nGainesville, FL.
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\nSurveys for Pathogens of Monoecious Hydrilla in 2013
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\nVicksburg, MS: U.S. Army Engineer Research and Development
\nCenter,Vicksburg,,MS,39180
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\n(Spenser et al. 1987). When they sprout, stems tend to grow laterally and generate new root crowns
\nalong the sediment surface resulting in high shoot densities (Van 1989). When the monoecious hydrilla
\nmat declines in the fall, it breaks loose and fragments containing numerous axillary propagules (i.e.
\nturions) drift in water currents dispersing the plant (Steward and Van 1987). Madeira et al. (1997)
\nhypothesized that this growth habit suggested a temperate origin of the plant that was consistent with
\nits probable Korean origin.
\nWhile dioecious hydrilla has been surveyed for pathogenic agents periodically over the past 25 years
\n(Joye and Cofrancesco 1991, Shabana and Charudattan 1996, Shabana et al. 2003, Shearer 2012),
\nmonoecious hydrilla has received little attention. In part, this was due to its limited distribution in a few
\neastern states, but the expansion of monoecious hydrilla in recent years to widely different geographic
\nregions of the United States has given it new status as an invasive species of note.
\nMonoecious hydrilla management is primarily through chemical control using endothall (Poovey and
\nGetsinger 2010), fluridone, and a combination of copper and diquat.1 Grass carp (Ctenopharyngodon
\nidella) are a potential biological control agent as a non-specific feeder of aquatic plants and in all
\nlikelihood would feed on monoecious hydrilla. Grass carp have been released in Lake Gaston along the
\nNorth Carolina/Virginia border where they have tried to maintain target population densities of 3.2 to
\n20.5 fish per hydrilla-ha; however, there has been little evidence to show that grass carp have
\ncontributed to hydrilla control in the lake (Dick et al., in review). While the ephydrid fly Hydrellia
\npakistanae, a biocontrol insect, has successfully established populations on the dioecious hydrilla
\nbiotype, there are no published records of establishment on the monoecious hydrilla biotype even after
\nconcerted release efforts. Both greenhouse and outdoor-pond studies have documented that Hydrellia
\nflies have reduced survival rates and longer developmental times on monoecious than dioecious
\nhydrilla (Grodowitz et al. 2010). In north Texas, Hydrellia flies survive and overwinter as larval stages
\nin stems of dioecious hydrilla (Harms and Grodowitz 2011). This overwintering survival mechanism
\nmay preclude fly establishment on monoecious hydrilla populations because the plant survives as
\nsubterranean tubers and turions (Steward and Van 1987) and not as aboveground biomass.
\nSeveral pathogens have been researched as potential biological control agents for management of
\ndioecious hydrilla, including Mycoleptodiscus terrestris (Joye and Cofrancesco 1991; Joye 1990; Joye
\nand Paul 1991; Nelson et al. 1998; Netherland and Shearer 1996; Shearer 1998; Shearer 2009a, 2009b;
\nShearer and Nelson 2002; Shearer and Jackson 2006), Fusarium culmorum (Charudattan et al. 1984),
\nand Plectosporium tabacinum (Smither-Kopperl et al. 1999). To date, no pathogens have been
\nresearched as potential biological control agents for management of monoecious hydrilla. The purpose
\nof the study presented herein was to survey some known populations of monoecious hydrilla and
\nisolate potential fungal pathogens.
\nMATERIALS AND METHODS: During the summer/fall of 2013, monoecious hydrilla was collected
\nin the field from Strom Thurmond Reservoir in South Carolina/Georgia, Lake Guntersville in Alabama,
\nand Lake Cayuga and Tonawanda Creek/Erie Canal in upstate New York. Samples were also received
\nfrom the Center for Aquatic and Invasive Plants (CAIP) in Gainesville, Florida where monoecious
\nhydrilla plants collected in Missouri, Kansas, South Carolina/Georgia (Strom Thurmond Reservoir),
\nand North Carolina/Virginia (Lake Gaston) had been cultured. All samples were shipped overnight to
\n1Netherland, M. 2013. Research Biologist. U.S. Army Engineer Research and Development Center, Gainesville, FL.
\nERDC/TN APCRP-BC-34
\nOctober 2014
\n3
\nthe biomanagement laboratory located at the U.S. Army Engineer Research and Development Center
\n(USAERDC) in Vicksburg, Mississippi. U

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