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PHYTOTOXICITY ASSESSMENT FOR POTENTIAL BIOLOGICAL CONTROL
OF LEAFY SPURGE BY SOILBORNE MICROORGANISMS
R.J. Kremer A and T. Souissi B
A USDA, Agricultural Research Service and University of Missouri, Columbia, MO USA. Email: kremerr@missouri.edu
B Institut National Agronomique de Tunisie, Tunis, Tunisia
ABSTRACT. Leafy spurge (Euphorbia esula-virgata), a native of Eurasia, is a serious invasive weed of western grasslands
of North America. It is very difficult and cost-prohibitive to control with herbicides; control by insect biological control
agents and cultural practices are minimally effective in suppressing vegetative growth and seed production. Current
biological control of leafy spurge with pathogens is primarily withmycoherbicides,which require specific environmental
conditions and repeated applications to be effective. Alternative biological control approaches using selected
microorganisms to attack roots and adventitious shoots may effectively decrease vigor of leafy spurge without
environmental manipulations to assure control efficacy. Our objectives were to survey leafy spurge accessions and their
native soils for associated microorganisms, andto assess thesemicroorganisms for potential biological control. Preliminary
lettuce seedling bioassays indicated that 62 and 54% of rhizosphere and endorhizal bacteria significantly (P=0.05) inhibited
root growth, causing necrotic lesions. Over 60% of fungal isolates bioassayed on rice agarsignificantly inhibited root
growthof lettuce seedlings. The most effective microbial isolates, based on preliminary bioassays, were screened directly on
leafy spurge cuttings. Culture filtrates of 40% of fungi caused complete chlorosis and leaf wilting. The most effective fungi
originated from leafy spurge adventitious shoots. Only intact cells of bacteria were detrimental to leafy spurge, indicating
that host-bacterial contact was required for pathogenicity. Results of the survey suggest that leafy spurge rhizospheres and
adventitious shoots are good sources of potential biological control microorganisms,which should be considered for
inclusion in comprehensive management programs for leafy spurge.
INTRODUCTION
Leafy spurge (Euphorbia esula-virgata) is an invasive,
deep-rooted perennial weed reproducingboth by seed and
by vegetative propagation. The weed infests
approximately two million ha of land in the U.S.
northern Great Plains and prairie provinces of Canada.
Leafy spurge resists weed management tactics because
crown and root buds regenerate new plants after foliar
herbicide treatment, mowing, biological control, burning
or grazing. Biological control focuses on use of a suite of
insect natural enemies that readily establish but have
little measurable impact on infestations (Caesar 2003).
However, successful biological control often coincided
with soilborne pathogens associated with root-feeding
larvae of flea beetles(Aphthona spp.).European
accessions of microorganisms from leafy spurge in its
native range revealed that virulent fungal strains and
plant-growth suppressive rhizobacteria originated from
plants with insect-feeding damage (Kremer et al. 2006).
Potential biological control activity of
microorganisms associated with leafy spurge is
associated with numerous physiological traits including
excess auxin, hydrogen cyanide, and polysaccharide
production and apparent cell wall- and membranedegrading
enzymes detected via tissue culture bioassays
(Souissi and Kremer 1998; Caesar 2003; Kremer et al.
2006;). Other phytotoxic compounds are likely involved
in pathogenicity of the potential biological control
microorganisms through translocation from the
producing microorganism throughout the plant causing
disease (Yang et al. 1990). Our objectives were to survey
soilborne and rhizosphere microorganism associated with
leafy spurge for phytotoxicity using stem cuttings.
MATERIALS AND METHODS
Intact leafy spurge plants and soil were collected from
infestations in Minnesota, Missouri, North Dakota USA
and Saskathewan Canada. Shoot, root, and crown
components were macerated and plated on a battery of
selective media to isolate bacteria or fungi. Isolates were
screened for phytotoxic activity using lettuce seedling
bioassays (Souissi and Kremer 1998); culture filtrates
were prepared and bioassayed on leafy spurge stem
cuttings (Yanget al. 1990 ). Phytotoxic effects rated and
evaluated for potential biological control activity.
RESULTS AND DISCUSSION
Bbioassays of selected rhizobacteria on lettuce seedlings
correlated well (R 2 =0.68) with stem cutting assays using
intact cells. Bacterial culture filtrates were inconsistently
effective, suggesting need for intact cells for phytoxicity.
Fungal isolates inhibiting root growth also highly
damaged stem growth (Table 1). Survey of microbial
community diversity associated with leafy spurge is a
valuable approach for discovery of new and effective
biological agents of this invasive weed.
Table 1. Response of leafy spurge cuttings to culture
filtrates of fungal accessions.
Isolate A Source Injury rating B
Fusarium sp. 9.50 Rhizosphere 3.2
Fusarium sp. 10.50 Rhizosphere 2.5
Fusarium sp. 17.50 Rhizosphere 3.0
Fusarium sp. 18.37 Rhizosphere 3.5
Fusarium sp. 18.44 Rhizosphere 3.8
Fusarium sp. 18.45 Rhizosphere 3.1
Fusarium sp. 18.47 Rhizosphere 3.4
Fusarium sp. 18.18 Endorhizal 2.6
Fusarium sp. 18.19 Endorhizal 2.1
Fusarium sp. 10.52 Stem 3.6
Fusarium sp. 18.42 Stem 3.4
Fusarium sp. 18.46 Stem 3.6
Rhizoctonia sp. 43 Stem 3.5
Rhizoctonia sp. 51 Stem 2.0
LSD (P=0.05) 0.45
A Numeric code indicates accession number.
B Rating: 0=no injury; 1=chlorosis, slight necrosis on
50% plant; 2=complete chlorsis, beginning wilt;
3=leaves completely wilted, easily detached; 4=leaves
desiccated, shoots dead.
REFERENCES
Caesar, A.J. (2003).BiolContr 28: 144-153.
Kremer, R.J., Caesar, A.J. and Soussi, T. (2006).Appl
Soil Ecol 32: 27-37.
Souissi, T. and Kremer, R.J. (1998).Biocontr. Sci.
Technol. 8: 83-92.
Yang, S.M., Johnson, D.R., and Dowler, W.M. (1990).
Plant Dis 74: 601-604.
7th Australasian Soilborne Diseases Symposium 26