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Dactylaria higginsii, a Bioherbicide Agent to Control Purple Nutsedge (Cyperus rotundus) UF#1553 and UF#1762 A Synopsis

Concept: Purple nutsedge, Cyperus rotundus, has the distinction of being called the world's worst weed. It affects more crops in more countries than any other agriculturally important weed. The primary means of control of purple nutsedge, and the closely related yellow nutsedge, C. esculentus, is by soil fumigation with methyl bromide. Although several chemical herbicides are available in the market, no single chemical is suitable for nutsedge control in all crops and cropping situations. Currently companies are relying too heavily on sufonylurea herbicides to control sedges. However, sulfonylureas are prone to induce resistance development in weeds, including nutsedges. A bioherbicide agent (i.e., a pathogen registered for use as an inundative biocontrol agent) that can be used against purple nutsedge, yellow nutsedge, and other sedges is expected to find acceptance as an alternative to methyl bromide and by growers of minor crops, organic crops, and other specialty crops for which no registered herbicides are available or acceptable.

Pathogen: Dactylaria higginsii is a fungal pathogen of purple nutsedge that naturally occurs on this weed in the southeastern USA. We discovered an isolate of this pathogen in Gainesville, Florida and demonstrated its potential as a control agent for purple nutsedge. We have also determined the environmental conditions necessary for disease and epidemic development and shown that the pathogen is highly specific to plants in the sedge family, Cyperaceae. Finally, we have confirmed the efficacy of this pathogen in field trials. Even at moderate levels of disease, this pathogen is capable of improving crop yield by reducing competition from purple nutsedge.

Susceptible hosts: In trials conducted in a greenhouse, the following weedy species of Cyperus were susceptible to D. higginsii: purple nutsedge (C. rotundus), yellow nutsedge (C. esculentus), annual sedge (C. compressus), globe sedge (C. globulosus), rice flatsedge (C. iria), and green kyllinga (C. brevifolius [ = Kyllinga brevifolia]). All of these are weeds in different situations such as vegetable crops, turf and lawns, and rice. D. higginsii was not pathogenic to plants outside the sedge family, including the crops species tested. It is therefore considered a safe bioherbicide candidate.

Symptoms: D. higginsii causes small, dark-brown flecks on the purple nutsedge leaves, four days after the leaves are sprayed with a suspension of spores (conidia). The spots coalesce to form larger irregular spots and blotches, and the leaves die-back from the tips, partially killing most of the leaves. The fungus sporulates on infected leaves and initiate secondary infection cycles on the emerging leaves within 20 days after inoculation.

Environmental factors: The fungus requires a minimum dew period of 12 h and a temperature of 25ºC during the dew period to produce severe levels of disease. Plants at the 4- to 6-leaf-stage are more susceptible than older plants. Under these conditions, 75% disease (percent leaf area damaged) and excellent weed control (nearly 100% plant mortality) were achieved in greenhouse trials. At a dew-period duration of 24 h and dew-period temperature of 30ºC, the number of days to obtain 50% disease severity on 4- to 6-leaf-stage plants was significantly less (10 days) compared to older plants (16 days). To achieve effective control, D. higginsii should be applied early in the growing season when purple nutsedge plants are young and the temperature and dew-period requirements are not limiting. The need for a long dew period (> 12 h) for infection and disease development is a limiting factor in this pathosystem. This limitation could be overcome by using inoculum suitably formulated in moisture-retaining gels or other material.

The efficacy of different surfactants and gels in promoting infection, disease development, and nutsedge control was studied in a greenhouse experiment using 1 x 106 conidia per ml of water and various types of amendments including two surfactants (Silwet L-77 and Triton X-100), four polysaccharide-containing natural polymers (xanthan gum, Metamucil®, Natrosol®, and N-gel®), combinations of Silwet + Metamucil, Silwet + N-gel, and Silwet + xanthan gum, and control (water only). A fungus-free control was also included. Based on the ability of the amendment to promote spore germination, disease incidence, and disease severity, 0.02% Metamucil and 0.05% N-gel were rated the best materials to formulate D. higginsii spores for experimental (i.e., non-commercial) use. Figure 4 in the attached U.S. Patent, No. 5,698,491 illustrates the control obtained with formulations of conidia containing N-gel and Metamucil compared to a formulation of Silwet + conidia, a fungus-free control, and a control without any amendment.

Biocontrol efficacy: In greenhouse trials, D. higginsii produced significant reductions in shoot numbers, shoot dry weight, and tuber dry weight of greenhouse-grown purple nutsedge plants. The reductions were 72%, 73%, and 67%, respectively, compared to the fungus-free control treatments. In repeated field trials, three applications of the fungus, each at 106 conidia/ml, provided effective control of purple nutsedge compared to a single inoculation, as measured by shoot dry weight, tuber numbers, and tuber dry weight. Higher rates of disease progress and disease levels, defined by the area under the disease progress curve, occurred with three inoculations at 106 conidia/ml. Disease progress was slower and the level of weed control was lower at 105 conidia/ml compared to the higher inoculum level. Three applications of 106 conidia/ml provided > 90% nutsedge control.

Cultural characteristics: D. higginsii can be grown easily on common laboratory media such as V8-juice agar, tomato-paste agar, potato-dextrose agar, and several inexpensive substrates. The fungus can be grown in liquid culture to produce mycelial mass but not spores. We have tested several off-the-shelf microbial media as well as novel media and natural substrates to identify media and conditions for spore production. These results are provided in the attached Plant Disease publication by Wyss et al., 2001. Further scale-up work is necessary to develop a commercially feasible method.

Research and development needs: Studies on industrial inoculum production and formulation should be done, preferably in partnership with a company committed to commercial development and registration of this bioherbicide agent. Commercial prototype formulations can then be tested in growers' fields to develop efficacy and other data needed for EPA registration. We have identified a network of scientists willing to undertake cooperative, multi-location field trials.