Weeds Down Under — What do Rhizobacter and Vegemite Have In Common?

From March through August 2003 I was on study leave in Australia. I had two major goals for this study leave. First was enhance my skills in conducting research on plant pathogens as weed biological control agents; and second, was to learn how Australian nursery crop producers control weeds with few chemical herbicide choices. In addition I attended several conferences including two international symposia on weed biocontrol. Major accomplishments from this sabbatical leave include four publications, enhanced skills in working with plant pathogens as biocontrol agents, expanded knowledge of nursery crop and cut flower weed management practices that are less-reliant on residual herbicides, and new professional contacts that have already resulted in a new collaborative research project.

• Weed Biocontrol Research
• Nursery and Cut Flower Weed Management

Enhancing Weed Biocontrol Research:

The first four months of my sabbatical were spent conducting research evaluating plant pathogenic rhizobacteria as potential biocontrol agents for annual grassy weeds, specifically annual ryegrass (Lolium multiflorum and Lolium rigidum). Dr. Gavin Ash, of Charles Sturt University, Wagga Wagga, New South Wales, was my host.

Deleterious rhizobacteria are non-parasitic bacteria, found in the rhizosphere of plants, that suppress plant growth and development. Deleterious rhizobacteria are well suited for the biocontrol of weeds as many are host plant specific. Work in the US Pacific Northwest by Elliott and Lynch has identified Pseudomonas spp. that are detrimental to the growth of annual ryegrass. Biological control of leafy spurge (Euphorbia esula) and downy brome, (Bromus tectorum) with Pseudomonas spp. have also been reported. In Australia, annual ryegrass and wild oats cause an estimated loss of $100 million in winter crops. However, no research reports are available concerning the use of deleterious rhizobacteria for control of annual grass weeds in Australian winter crops. Dr. Gavin Ash had previously isolated rhizobacteria from the rhizosphere of annual ryegrass from several locations in New South Wales, Australia. Preliminary screening studies conducted by an undergraduate honors student demonstrated that several of these isolates were deleterious to annual ryegrass growth. For this project I conducted laboratory, greenhouse and field experiments to screen rhizobacteria for growth inhibition of annual ryegrass, canola and wheat. The isolates included several species and types of Pseudomonas, as well as Stenotrophomonas, Burkholderia, Serratia, Salmonella, Micrococcus, and Xenorhabdus. From petri dish bioassays, isolates were categorized as producing general non-selective growth inhibition, selective root inhibition of one or more of the bioassay species, or selective growth promotion of one or more of the bioassay species. Isolates were selected from each category for greenhouse and field tests. Although growth inhibition and promotion were observed in petri dish assays, none of the isolates caused growth inhibition or promotion in greenhouse or field studies.

Outcomes:

Although the ultimate goal of finding a biocontrol agent for annual ryegrass was not achieved in this series of experiments, we did demonstrate that both growth inhibitory and growth promoting rhizobacteria are present, but further research will be necessary to identify isolates that would be more effective under field conditions. Participation in this research improved my understanding of plant pathogenic bacteria and my skills in isolation, culture and evaluation of such pathogens as potential weed biocontrol agents. In particular I learned new skills in how to isolate, culture, store and formulate bacterial plant pathogens. The formulation methods used in these trials will be applicable for use with fungal pathogens as well. I also developed new understandings about some of the pitfalls of the methods. In particular, I discovered that the nutrient broth used to produce bacterial inoculum can be phytotoxic to bioassay plants. For this reason, additional “controls” must be placed into the experiments to ensure reliable results. These experiences and acquired skills will be directly applicable to my research program here at NC State in which I am evaluating plant pathogens for biocontrol of weeds in horticultural cropping systems. It is my hope to grow this aspect of my research program and develop biologically based weed management strategies for horticultural crop producers in the southeastern U.S. and beyond.

Understanding weed management systems in Australian nursery crop production:

Nursery crop producers in Australia have fewer herbicides available and fewer sources of information on weed management than their US counterparts. For example, they have no extension service to assist them and they have no weed scientists in the public sector working on weed management in nursery crops. From early July through the end of August I was hosted by was Dr. Ian Gordon, Center for Native Floriculture Crops at the University of Queensland, Gatton, Queensland. During this time I visited with horticulture faculty, visited nurseries, field cut flower producers and botanic gardens in eastern Australia in order to discuss nursery weeds and weed management strategies. The nursery industry in Australia produces plants predominantly in containers. A per-container levy is utilized to fund research and education projects relating to nursery crop production. Although there are no university-based extension specialists, each state has a Nursery Industry Development Officers (NDO’s). The NDO’s serve much the same role as an extension specialist. They work with the nursery industry to develop certification programs, solve production problems, and conduct some applied research. I had the opportunity to travel with two NDO’s and see how they work with nursery managers. They were also excellent sources of information on which weeds were important in nurseries and what management strategies were utilized.

It was remarkable to find that the majority of the weeds in container nurseries in Australia were the same as those in the US. The few weed species that were different had closely related relatives that are important in US nurseries. Nursery weed control strategies are similar to those employed in the US except that in Australia there are only 3 herbicides labeled for use in nursery crops. In the US we have over 30 herbicides registered. The challenges for Australian nursery crop producers is not “which herbicide to use”, but rather “how can I use this herbicide without damaging my crop”. The two most commonly used herbicides used in Australian nurseries will damage the foliage of many crop species. Growers have devised unique application devices to apply the herbicide to the surface of the potting substrate without contacting crop foliage. This is a labor-intensive process and would not be practical for very many crops in the US, but for some high-value crop species, it may be a viable option. Also, without many herbicide options, Australian growers rely heavily on sanitation practices to prevent weed introduction, spread and seeding.

I also visited several woody cut flower plantations in Queensland. Woody cut flower production is a significant export crop in Australia but few herbicides are labeled for use in these crops. Without very many weed management tools available, growers have relied upon pre-plant weed management, geotextile fabric mulches, sanitation, and directed applications of broad spectrum herbicides.

Outcomes:

I have emphasized the importance of sanitation in nursery weed IPM program training in the past; however, my observations and experiences in Australia have confirmed that sanitation must be a more dominant component of nursery weed management programs in the US. My future extension programs will have a greater emphasis on nursery sanitation for weed management. Practical examples from my experiences in Australia will support these efforts. One full-color bulletin on weeds of container nurseries has been published through the University of Queensland’s Center for Native Floriculture Crops. It has received positive reviews from Industry Development Officers and nursery crop producers, and will be utilized by the Nursery Industry Development Officers in nursery IPM training programs.

In the Southeastern US, interest in field production of woody cuts is increasing. My experiences in Australia will improve my ability to address growers’ needs in the future. I worked with the Center for Native Floriculture at the University of Queensland to produce a weed management guide for these crops. This full-color bulletin should be in print before the end of 2003. Though tailored to Australian crops, this document will serve as the template for a revised weed management chapter for the Southeast Outdoor Cut Flower Manual published by the North Carolina Commercial Cut Flower Growers Association.

Publications:

• Neal, J. C. and I. Gordon. 2003. Common weeds of container nurseries and their control. Center for Native Floriculture Crops, Univ. of Queensland, Gatton. 8 pp.
• Neal, J. C. and I. Gordon. 2003. Weed management in woody cut flower plantations. Center for Native Floriculture Crops, Univ. of Queensland, Gatton 8 pp.

Conferences:

I attended two international symposia on weed biocontrol and several nursery, landscape and turfgrass conferences. First, was the international workshop on the biocontrol of weeds with plant pathogens, conducted in association with the International Plant Pathology Society conference in Christchurch, New Zealand. Second, was the XI International Symposium on Biocontrol of Weed in Canberra, Australia. At the Canberra conference I presented results from my research evaluating the potential for off-target movement of Xanthomonas campestris pv. poannua (a selective bacterial pathogen) following application as a biocontrol agent for annual bluegrass (Poa annua). I also was invited to speak at the annual meetings of the National Garden Industries of Australia, Australian Turfgrass Conference, and International Plant Propagator’s Conference.

Outcomes:

Attending these conferences enabled me to network with the world’s leading researchers and educators in weed biocontrol. An immediate and direct benefit from this is a new collaborative research project with Dr. Karen Bailey of Agriculture Canada, Saskatoon, Saskatchewan. Dr. Bailey reported on a new fungal pathogen that appears to have the potential to work as a preemergence herbicide for winter annual broadleaf weeds. The fungus is widespread and endemic to the southeastern US, including North Carolina. I have applied to the USDA-APHIS for permits to test the fungal pathogen in North Carolina for winter annual weed control in container nursery crops. Assuming permits are issued, I will begin these experiments in late October 2003. Speaking at the Australian nursery, landscape and turfgrass industry conferences provided me with the opportunities to interact with growers and landscape maintenance personnel from all over Australia and to learn about their weeds and weed management strategies.

Two reviewed proceedings papers were published as a result of the sabbatical…

• Neal, J. C., N. D. Williams, and E. B. Nelson. 2003. Evaluating off-target
  movement of Xanthomonas campestris pv. poannua following application as a biocontrol agent for Poa annua on golf turf. Proc. XI International Symposium on Biological Control of Weeds. pp. 301-304 In: Cullen, J.M., Briese, D.T., Kriticos, D.J., Lonsdale, W.M., Morin, L. and Scott, J.K. (eds.) Proceedings of the XI International Symposium on Biological Control of Weeds. CSIRO Entomology, Canberra, Australia
• Neal, J. C. 2003. Biology and management of nursery weeds. Combined Proceedings of the International Plant Propagator’s Society 53:8-11

P.S. What do rhizobacter and Vegemite have in common?

They both inhibit root growth of annual ryegrass.