J. Lawrie

Towards the biocontrol of bindweeds with a mycoherbicide

Within the framework of the European COST Action 816, afive-year collaboration between scientists from five Europeancountries has made an important contribution to biologicalcontrol of field and hedge bindweeds (Convolvulus arvensis andCalystegia sepium, respectively). A fungus Stagonosporaconvolvuli strain LA39, able to infect both field and hedgebindweed, was found in the UK and its biocontrol efficacyimproved by optimising mass production, formulation and storagetechniques. This fungus controlled bindweeds in both a cemeteryand in maize crops. Its use fits best in an integrated pestmanagement system where a green cover controls most of the weedsexcept the bindweeds. DNA marker analyses indicate that thefungus reproduces sexually, which could be used to furtherimprove this mycoherbicide. In addition, the insect Melanagromyzaalbocilia, which itself exhibits biocontrol potential againstbindweeds, may be used in combination with LA39 to improve theability of the fungus to penetrate the stem of bindweeds.Overall, the results suggest that S. convolvuli LA39 haspromising potential as a bioherbicide for control of field andhedge bindweed.

Biocontrol of Amaranthus spp. in Europe: state of the art.

Within European COST Action 816, a 5-year collaboration between scientists from 6 European countries has made an important contribution to the previously unstudied insect fauna associated with Amaranthus spp. in Europe. This provides a basis for future introductions of a non-native biocontrol agent into Europe. In addition, two promising microbial herbicides, based on the fungi Alternaria alternata and Trematophoma lignicola have been characterised. Further work on their use in integrated farming systems is required. The use of microbial herbicides in conjunction with new cropping systems, such as green cover crops or living mulch using Trifolium subterraneum is an approach which offers much potential.

Effects of the plant-pathogenic fungus Mycocentrospora acerina (Hartig) Deighton on growth and competition of Viola arvensis (Murr.) in spring wheat

Mycocentrospora acerina (Hartig) Deighton can kill or suppress the growth of Viola arvensis (Murr.) in spring wheat in pot-grown plants in both glasshouse and outdoor studies. The level of damage to V. arvensis and the effect on its competition with wheat were affected by inoculum density and environmental conditions. High inoculum density (105 macroconidia/ml) caused high mortality in V. arvensis and eliminated its competitive effect on wheat. At a lower density (104 macroconidia/ml), the V. arvensis fresh weight and its competition with the wheat were suppressed by up to 38 and 28% respectively, depending on environmental conditions.

Investigation of Spray Application of Microbial Herbicides Using Alternaria alternata on Amaranthus retroflexus

Inclusion of Alternaria alternata conidia in a spray formulation affected the distribution pattern on the target. The dry weight of Amaranthus retroflexus plants was reduced by more than 83% when A. alternata conidia (107 ml -1 ) were applied at 200 L ha -1 or greater and when given a 24 h dew period. At low application volumes (25 or 50 L ha -1 ) plant dry weight was reduced by only 29 or 54%. After 7-8 h dew period, conidial germination on the leaf surface was 11-19%. This increased to 62-91% after 24 h dew period. Counts of conidia on leaves indicated that up to 86% of the conidia sprayed were not retained on the target plant, or did not reach it. This is reflected in lesion numbers per unit area being only 3-5% of the calculated theoretical numbers. The results cast doubt on the suitability of A. alternata as a microbial herbicide for the control of Am. retroflexus .

Effects of Storage on Viability and Efficacy of Granular Formulations of the Microbial Herbicides Alternaria alternata and Trematophoma lignicola

Alternaria alternata Pesta (gluten matrix) granules stored at 12% relative humidity were still viable and infective, giving total control of Amaranthus retroflexus after 24 months storage. Viability of the propagule in the granule was an important factor of the evaluation test and not just the viability of the granule. The concentration of inoculum in the granule was important, with 10 5 to 10 6 conidia g -1 being the ideal, since lower concentrations may not be able to out-compete the soil micro-flora. Even with soil application, the effect of local humidity was still important. Trematophoma lignicola was also formulated as microbial herbicide granules but the conidia were severely damaged in the production process.

Studies of Spray Application of Microbial Herbicides in Relation to Conidial Propagule Content of Spray Droplets and Retention on Target

This paper addresses the hypothesis that the spray application system used will affect the retention of spray deposits of microbial herbicides on different plant structures. There was greater spray retention on the hypocotyl of 4 to 6-true leaf Amaranthus retroflexus plants (20-31%) sprayed with an aerosol-producing laboratory sprayer than with a hydraulic nozzle (3-5%). Spray deposition and retention from the hydraulic nozzle was increased by 28-42% when the nozzle was used horizontally. Spray droplets generally contained the expected conidial number for the conidial concentration but this was affected by increased inoculum density (size, concentration and mycelial contamination), causing a reduction in the actual number of conidia present, compared to the expected. Up to 40-50% of the conidia in the sprayed suspension could not be accounted for when conidium deposition was on a target. This loss could increase to 90-92% when the weed was sprayed under a crop canopy.

Outdoor Studies of the Efficacy of Alternaria alternata in Controlling Amaranthus retroflexus

Application of 250 L ha -1 containing 10 7 Alternaria alternata conidia ml -1 caused 61% reduction in dry weight and a 45% mortality in Amaranthus retroflexus plants growing in a wheat crop. However, only 10-22% of the applied conidia were retained on the leaf. In one experiment, conidia remained viable for 4 days on the leaf surface until conditions were favourable for germination. Competition from the wheat crop, as well as its creation of a moist micro-climate, improved the control of Am. retroflexus by A. alternata . Total control of Am. retroflexus may be difficult to achieve with A. alternata , but suppression to reduce or eliminate its competitive effect on crops is possible.

Trematophoma lignicola (Petrak): A Potential Microbial Herbicide for Control of Amaranthus retroflexus (L.)

Trematophoma lignicola (Petrak), sprayed on leaves at 106 conidia ml -1 in oil emulsion, kills Amaranthus retroflexus plants after a 16 h dew period. Application as a soil drench kills seedlings of this species.

A Proposed Mode of Action of Oil-Based Formulations of a Microbial Herbicide

Formulation of some microbial herbicides in emulsions of vegetable oil can significantly reduce their dependency on long dew periods to allow infection and control of the host weed. Examination of leaves of Viola arvensis , by a range of microscopy techniques, after spraying with an oilin-water emulsion formulation of the potential microbial herbicide Mycocentrospora acerina , suggests that the effect arises as a result of inversion of water into both oil droplets on the leaf surface and oil that penetrates into the leaf tissue. This oil probably mixes with aqueous components from the leaf tissue and, in doing so, forms an invert emulsion. Wesuggest that this supplies the water required by the fungus to initiate and sustain growth so that infection can result.

Efficacy of Trematophoma lignicola Petrak as a Biological Control Agent for Amaranthus retroflexus L.

The pathogenicity of Trematophoma lignicola on Amaranthus retroflexus was increased when its conidia were formulated in oilseed rape emulsion. However, this formulation did not reduce the dew period required to allow conidial germination and infection of the host, nor did it protect the conidia from desiccation before the onset of dew. The fungus gave effective control of A. retroflexus plants up to the 4-true-leaf stage. Plants with 5 to 6-true-leaves were infected and their dry weight reduced, but plants with more than 6-true-leaves were unaffected. Plants grown in warm conditions (18-21°C/12-15°C, day/night) were more likely to be successfully controlled than those grown in the cold (10-12°C/7-8°C; day/night). Significant dry weight loss of A. retroflexus plants were achieved at application volumes down to 100 l ha -1 at 5 2 10 6 conidia ml -1 . More effective control was achieved by application of conidia of T. lignicola than mycelial or pycnidial applications.