Progress 10/01/01 to 09/30/06
Outputs
Dr. Hallett has been studying two fungal plant pathogens of weeds with potential as bioherbicides; Myrothecium verrucaria, a toxin-producing soilborne plant pathogen with a broad host range and Microsphaeropsis amaranthi, a host-specific pathogen of Amaranthus spp. He has shown that Myrothecium verrucaria causes severe and rapid damage to a wide range of weed species from a number of different families, and that the mechanism of this broad spectrum activity is its production of secondary metabolites. These findings are important demonstrations of the potential impact of fungal secondary metabolites upon the efficacy and safety of plant pathogens investigated and developed as bioherbicides. This is significant to the field of bioherbicides research because it proposes an avenue of product development with a key genus of strongly herbicidal fungi, but at the same time challenges the safety of the bioherbicide approach. These findings have opened up opportunities for the
further evaluation of Myrothecium spp. as a source of phytotoxic molecules or valuable genetics that could be transferred to more benign organisms. The results of this research have been reported at national and international meetings and published in Weed Science. Microsphaeropsis amaranthi is a virulent pathogen of a number of important weedy Amaranthaceae, causing severe foliar and stem necroses that can lead to plant death. Dr Hallett has studied a number of aspects of this pathogen with a view to developing it as a bioherbicide product for the control of common waterhemp (Amaranthus rudis) in midwestern corn and soybean production. A sequence of publications has been submitted examining 1) the variable responses of common waterhemp to the herbicide glyphosate (Weed Technology), 2) the climatic constraints on M. amaranthi (Phytopathology), optimal spray application parameters (Weed Technology), and the interactions between M. amaranthi and chemical herbicides (Proceeding of the
BCPC International Congress: Crop Science and Technology; Weed Science). Research with this system has developed a detailed understanding of the key opportunities and limitations for the development of a host restricted bioherbicide into a major field crop. Current research is using novel methods for the genetic and cultural manipulation of the fungus to enhance its virulence and impact under suboptimal climatic conditions and the mass production and formulation of the M. amaranthi product.
Impacts
Myrothecium verrucaria: The demonstration that the herbicidal activity of this fungus is closely linked to its production of a suite of toxic secondary metabolites is important. This information guides researchers developing this fungus with respect to the safe handling of the fungus, and the likely molecular components of the fungus that might have potential in future genetically modified products. Microsphaeropsis amaranthi: The development of this bioherbicide is a highly relevant applied research activity for the North Central Region. Common waterhemp and pigweeds are troublesome, have quickly adapted to changes in management tactics and developed resistance to several herbicide modes of action. The incorporation of another weed management technique into the IWMS toolbox would be extremely valuable in a number of agronomic and horticultural crops and for herbicide resistance management in common waterhemp. The development of M. amaranthi as a bioherbicide will have
considerable value for the field of bioherbicide technology. Although the market for the product will be small compared to glyphosate use in midwestern soybean production, it is expected to be significant, and it will be the most prominent bioherbicide developed to date, anywhere in the world. A product development strategy for this bioherbicide is in the pre-planning stages and the results from this project will provide critical information.
Publications
- Smith, DA, DA Doll, D Singh & SG Hallett. 2006. Climatic constraints to the potential of Microsphaeropsis amaranthi as a bioherbicide for common waterhemp. Phytopathology 96:308-312.
- Smith, DA & SG Hallett. 2006. Interactions between chemical herbicides and the candidate bioherbicide Microsphaeropsis amaranthi. Weed Sci. 54:532-537.
- Smith, DA & SG Hallett. 2006. Variable response of common waterhemp (Amaranthus rudis Sauer) to glyphosate. Weed Technol. 20:466-471.
- Anderson, KI & SG Hallett. 2004. Herbicidal spectrum and activity of Myrothecium verrucaria. Weed Sci. 52:623-627.
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Progress 10/01/04 to 09/30/05
Outputs
Microsphaeropsis amaranthi isolated from Amaranthus sp. was obtained from Dr. G. Weidemann, Univ. Arkansas. We have shown that M. amaranthi is a virulent pathogen of common waterhemp (A. rudis Sauer), one of the key weeds of the Midwest. Our research aims to investigate the potential for M. amaranthi to be integrated into production systems for the control of common waterhemp. The optimum and limiting conditions for activity of M. amaranthi have been determined in a series of lab and field experiments. The growth and sporulation of the fungus is limited at temperatures below 15C and above 30C. The fungus required a period of leaf wetness of more than 12 h for optimum activity, and infection was limited with a dew period of less than 10h at a temperature below 18C or above 24C. In the field, spray applications of M. amaranthi caused damaging infections on days characterized by temperatures and dew periods within the range above, and infection failed when conditions
were not within this range. We have shown that M. amaranthi has a strong positive interaction with the herbicide glyphosate. When plants were sprayed sequentially with sub-lethal rates of glyphosate and M. amaranthi, plants were killed. This may have particular value for common waterhemp control in glyphosate resistant soybean - we have shown a variable response in the response of common waterhemp to glyphosate, indicating that the weed may be evolving glyphosate resistance in the region. A number of adjuvants commonly used with herbicides inhibited germination in M. amaranthi which may preclude the mixing of conidia into the spray tank for combined applications in an integrated weed management system. We have investigated the mass production and spray application of M. amaranthi with a view to developing methodologies for field application. The fungus does not sporulate in submerged culture, and requires light for sporulation. However, the fungus grows and sporulates freely on a
range of crude agricultural products. Spray application is most efficient when conidia of M. amaranthi are applied using spray tips that generate fine, swirling droplets. When sprayed with a hollow cone nozzle tip (such as is typically used for fungicide applications) at high pressure, rather than a flat fan nozzle tip (such as is typically used for herbicide applications) the fungus caused elevated levels of disease on the stem base of common waterhemp plants. These stem infections resulted in higher levels of plant mortality.
Impacts
This research evaluated the potential for a bioherbicidal fungus to be integrated into midwestern production systems for the control of common waterhemp. Limiting factors have been determined: inhibition by adjuvants and poor performance under dry conditions. Progress has been made: improved production of conidia, optimized applications system and evaluation of herbicide synergies.
Publications
- Doll, DA, PE Sojka and SG Hallett. 2005. Factors affecting the efficacy of spray applications of the bioherbicidal fungus Microsphaeropsis amaranthi. Weed Technol. 19:110-115.
- Hallett, SG. 2005. Where are the bioherbicides? Weed Sci. 53:404-415.
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Progress 10/01/03 to 09/29/04
Outputs
Studies of the bioherbicidal activity of Myrothecium verrucaria conducted to date have been reported in the journal Weed Science (Anderson & Hallett, 2004). Briefly, these studies showed that M. verrucaria has broad herbicidal activity via the production of secondary metabolites. We have continued our studies of Microsphaeropsis amaranthi, a candidate bioherbicide for the control of weeds in the genus Amaranthus. We have investigated the interactions between M. amaranthi and the herbicide glyphosate. Interestingly, the interaction is completely different when the bioherbicide and the herbicide are applied to waterhemp plants at different times. When the bioherbicide is applied first, the effect of glyphosate is reduced, and we hypothesize that the presence of fungal infection limits the absorption and translocation of the chemical herbicide. When the chemical herbicide is applied first it predisposes waterhemp to attack by the bioherbicide resulting in increased
levels of control of the weed. We hypothesize that this predisposition is a result of the suppression of plant chemical defenses as a result of an inhibition of the shikimic acid pathway - the target pathway of the chemical herbicide. Further research is on-going to characterize these interactions in more detail. In the last year we obtained field permits and have tested the efficacy of M. amaranthi in the field. We sprayed waterhemp and redroot pigweed seedlings with conidial suspensions of M. amaranthi at 6 pm every Thursday from April 22 to August 5, 2004. We found that M. amaranthi has the potential to cause severe and damaging symptoms to common waterhemp and redroot pigweed when the nighttime temperature is between approximately 12 to 25 celsius and there is a leaf wetness duraton of at least nine hours. Infection severity was limited with less conducive conditions. As a result, of the fourteen weeks the bioherbicide was tested, it was completely successful three times,
partially successful four times and failed seven times. Future research will investigate formulations and other methods to alleviate the environmental requirements of the bioherbicide. We have investigated application technology for the deployment of M. amaranthi. The bioherbicide was applied as a foliar spray using a range of different hydraulic nozzles. Spray volumes and spore concentrations were kept constant. We found dramatic differences in efficacy from different types of sprays. In particular, efficacy was poor for sprays that generated large droplets (larger than 200 um) and efficacy was very high for sprays that generated small droplets (less than 100 um). Of the nozzles tested, the most effective was a hollow cone nozzle that generated swirling clouds of very fine droplets that penetrated the plant canopy and resulted in stem lesions as well as leaf lesions on the target. Interestingly, this is a type of nozzle that is normally used for the application of fungicides and
insecticides rather than herbicides.
Impacts
We continue to develop Microsphaeropsis amaranthi as a bioherbicide candidate. We have shown that the fungus has potential in the field, and that it has the potential to be integrated into cropping systems. Future research will investigate the mechanism of interactions between the bioherbicide and chemical herbicides, develop improved deployment methods and conduct further field testing. The expected impact is a bioherbicide that that will give midwestern farmers an additional tool with which to battle troublesome weeds in the genus Amaranthus.
Publications
- Anderson, KI and SG Hallett. 2004. Herbicidal spectrum and activity of Myrothecium verrucaria. Weed Science 52:623-627.
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Progress 10/01/02 to 09/30/03
Outputs
(1) M. verrucaria isolated from sicklepod (Cassia obtusifolia) was obtained from Dr. HL Walker of Louisiana Tech Univ. Our research was designed to determine the efficacy of M.verrucaria for the control of a range of key weeds of the Midwest and to investigate the importance of toxic metabolites in the infection process of the pathogen. Our results demonstrated that crude preparations of M. verrucaria conidia (2 x 107 conidia ml-1 in 0.01% (v/v) Silwet L-77) were strongly herbicidal to a wide range of economically important weed species. This herbicidal activity, however, was shown to be caused almost entirely by toxins leached into the culture filtrate during growth of the fungus. Culture filtrates without spores had the same herbicidal activity as culture filtrates plus spores. Washed spores of the fungus were unable to cause infection alone. This, coupled with the fact that M. verrucaria can produce mammalian toxins, demonstrates that M. verrucaria has no potential
as a bioherbicide. (2) Microsphaeropsis amaranthi isolated from Amaranthus sp. was obtained from Dr. G. Weidemann, Univ. Arkansas. We have shown that M. amaranthi is virulent against waterhemp (A. tuberculatus), a key weed of the Midwest. Our research aims to investigate the potential for integrating M. amaranthi into Midwestern production systems. We are investigating a positive interaction that exists between M. amaranthi and the herbicide glyphosate. Applications of M. amaranthi (106 conidia/ml) or glyphosate (0.32 kg ae/ha) caused little mortality when applied alone, but significant mortality when applied sequentially. We hypothesize that this interaction is mediated by the suppression of the plant's phytoalexin-based defense reponses as a result of inhibition of the shikimic acid pathway by sublethal doses of glyphosate. While this positive interaction exists between glyphosate and M. amaranthi, we also found that mixture of conidia of M. amaranthi with glyphosate products
dramatically reduced their germinability. We found that this impact was caused not by glyphosate salts, but by the surfactants with which glyphosate is formulated in products. During this investigation of M. amaranthi, we have performed preliminary tests with different accessions of waterhemp from Indiana, Iowa, Missouri and Illinois that demonstrate very large differences in response to the herbicide glyphosate. We have not established the inheritance of these traits, but are cautiously characterizing some of these plants as glyphosate resistant.
Impacts
This research evaluates the bioherbicide potential of two fungi. In the case of Myrothecium verrucaria, we have shown that efficacy of this fungus is dependent upon the activity of toxic secondary metabolites. In the case of Microspaheropsis amaranthi, we have demonstrated virulence against waterhemp and a positive interaction with the herbicide glyphosate, although conidia of the fungus are negatively impacted by glyphosate products.
Publications
- No publications reported this period
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Progress 10/01/01 to 09/30/02
Outputs
Steve Hallett began a new research program in weed biological control in 2001. Research was initiated to study two fungal plant pathogens as part of the S-1001 project; Myrothecium verrucaria (1) and Microsphaeropsis amaranthi (2), and surveys were conducted for pathogens with potential as bioherbicides of weeds of the Midwest (3). (1) Myrothecium verrucaria (Objective 1b). M. verrucaria isolated from sicklepod (Cassia obtusifolia) was obtained from Dr. HL Walker of Louisiana Tech University. Our research was designed to determine the efficacy of M.verrucaria for the control of a range of key weeds of the Midwest and to investigate the importance of toxic metabolites in the infection process of the pathogen. Our results demonstrated that crude preparations of M. verrucaria conidia (2 x 107 conidia ml-1 in 0.01% (v/v) Silwet L-77) were strongly herbicidal to a wide range of economically important weed species, including Amaranthus tuberculatus, Xanthium strumarium,
Abutilon theophrasti, Kochia scoparia, Chenopodium album, Ambrosia trifida and Solanum ptycanthum. This herbicidal activity, however, was shown to be caused almost entirely by toxins leached into the culture filtrate during growth of the fungus. Culture filtrates without spores had the same herbicidal activity as culture filtrates plus spores. Washed spores of the fungus we unable to cause infection alone. This, coupled with the fact that M. verrucaria can produce mammalian toxins, demonstrates that M. verrucaria has no potential as a bioherbicide. (2) Microsphaeropsis amaranthi (Objective 1d). M. amaranthi isolated from Amaranthus sp. was obtained from Dr. G. Weidemann, University of Arkansas. We have shown that M. amaranthi is virulent against waterhemp (A. tuberculatus), a key weed of the midwest. Our research aims to investigate the potential for integrating M. amaranthi into Midwestern production systems. Biotypes of waterhemp resistant to glyphosate, ALS-inhibitor, diphenylether
or s-triazine herbicides will be treated with M. amaranthi in combination with a wide range of different herbicides. This will allow us to evaluate any synergies or antagonisms that may exist, suggesting use strategies for the bioherbicide in Midwestern cropping systems. Initial results show that spores of M. amaranthi are rapidly killed when incubated in solutions of commercial formulations of glyphosate. On the other hand, unformulated solutions of glyphosate caused very little damage to spores. (3) Pathogen Surveys. Field surveys have been conducted for diseases of key weeds of the Midwest, including Amaranthus tuberculatus and other Amaranthus spp., Xanthium strumarium, Abutilon theophrasti, Digitaria spp., Kochia scoparia, Chenopodium album, Ambrosia trifida, Setaria spp. and Solanum ptycanthum. Pathogens have been collected from each species and placed into storage. Many pathogens, including Alternaria, Colletotrichum and Fusarium spp. have been evaluated for virulence against
their respective hosts. We have identified a number of species causing disease symptoms on key weeds, but none of these has shown sufficient damage to be pursued towards development as bioherbicides.
Impacts
This research evaluates the bioherbicide potential of two fungi and reveals limitations to their continued development. In the case of Myrothecium verrucaria, we have shown that efficacy of this fungus is dependent upon the activity of toxic secondary metabolites. In the case of Microspaheropsis amaranthi, we have demonstrated virulence against waterhemp, and we have shown that conidia of this fungus are inhibited from germinating by commercially available formulated glyphosate products.
Publications
- No publications reported this period
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