News and Reviews

Biological Control at the Weed Science Society of America Meeting

The Weed Science Society of America (WSSA) held its annual meetings in February at Chicago. A session on biological control of weeds was dominated by presentations on bioherbicides and the use of plant pathogens for biocontrol of weeds.

The first presentation was on using Myrothecium verrucaria to control Amaranthus (pigweed and water hemp) species. Amaranthus species are common throughout midwestern cropping systems and are developing resistance to common herbicides. Drs. Yang and Brenner determined the virulence of five isolates of M. verrucaria on 28 species of Amaranthus. Powell amaranth (A. powellii) was severely damaged by two isolates. Another isolate severely damaged tumble pigweed (A. albus), prostrate pigweed (A. blitoides), Palmer amaranth (A. palmerei), and redroot pigweed (A. retroflexus). An oil carrier such as corn oil or Myvacet, increased both the virulence and range of Amaranthus species damaged by isolates of M. verrucaria.

There were two presentations from Dr. Boyette's group at the USDA-ARS research station in Stoneville, MS. They discussed the biocontrol of common cocklebur (Xanthium strumarium) with Alternaria helianthi and the effect of pesticides on biocontrol of hemp sesbania (Sesbania exaltata) by Colletotrichum truncatum. Cocklebur is common in the southern portions of the midwest but hemp sesbania is not found in the region. In the study of A. helianthi for control of cocklebur, formulations were compared under field conditions. Conidia in water caused little or no damage. Conidia in a combination of Silwet L-77 and unrefined corn oil caused 100% mortality of cocklebur after 14 days. Drs. Gronwald, Johnson, and Wyse from the USDA-ARS and University of Minnesota also reported that Silwet L-77 with Pseudomonas syringae pv. tagetis controls sunflower and Canada thistle without injuring soybeans. Soybean resistance relates to trichomes trapping the fungi on the leaf surface and reduced susceptibility of soybean to tegetitoxin.

Presentations by Wenmin Zhang and colleagues from Agriculture and Agri-Food Canada and Sarah Green and Glen Sampson from Nova Agriculture College also looked at bioherbicide formulations. Dr. Zhang reported findings from an extensive study that characterized the compatibility of surfactants on fungi species or strains from Colletotrichum, Phoma, Fusarium, and Alternaria. They were able to identify surfactants compatible with all species in a genus. In general, Tween 40 and Tween 80 were compatible with most genera. Tween 80 released the self-inhibition of conidium germination in Colletotrichum even at the greatest inoculum density of 1 x 107 conidia/ml. The optimum surfactant concentration increased as inoculum density increased. Fungi in the Coelomycetes were more sensitive to surfactants than those in Hyphomycetes. Green and Sampson studied wettable powder formulations of Myrothecium roridum and Plectosphaerella cucumerina which use grain both as the growth substrate and carrier. The fungi were evaluated for control of dandelion (Taraxacum officinale). Silwet L-77 at 0.1% increased infection by M. roridum and the combination of SeaSpen (a gel-forming polysaccharide extracted from red seaweed) at 0.5% and oil at 10% enhanced infection by P. cucumerina. These treatments also reduced each isolate's dew point requirements for infection.

Robert Kremer reported on his studies which combined cover crops with a soil-applied deleterious rhizobacteria (DRB). Barley, rye, or wheat cover crops reduced total weed biomass (mostly velvetleaf and foxtail) by 90% compared to weedy checks. Brassica species, ryegrass, and sweet clover reduced weed biomass to a greater extent when combined with soil-applied DRB. Cover crops for establishing high DRB densities in the soil could be selected. Kremer feels that integrating two or more approaches will enhance the efficacy of biological control.

Fabian Menalled, Karen Renner, and Douglas Landis at Michigan State University presented the only paper in the session on biocontrol of weeds using invertebrates. They compared the effect of landscape complexity in no-till corn fields on post-dispersal weed seed removal by invertebrates. Fields located in the complex landscape were smaller and had wider more diverse hedgerows than simple landscapes. Weed seed removal was 52% in the complex landscape and 32% in the simple landscape. They concluded that landscape and field level variables will influence the effectiveness of invertebrates as seed predators.

In a future issue I will review presentations from the integrated weed management session.

Abstracts of the WSSA meeting are available for $15.00 from:

Weed Science Society of America
John Breithaupt, Exec. Secretary
810 E. 10th
St. Lawrence, KS 66044-8897.

- John Masiunus, University of Illinois

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