Biological control of grassy weeds with fungi as bioherbicides

• National Institute of Plant Protection, Vietnam

Grass weeds are a major problem in world agriculture. As some of the most important staple food crops are from the grass family (Poaceae) grass weeds are difficult to control with conventional herbicides and by hand weeding because of their resemblance to cereal crops.

Rice is a major crop in Vietnam, occupying 6.42 million ha, providing about 35% of the GDP and employing 70-80% of the population. Grass weeds in rice cause significant crop losses. One field trial in the Mekong Delta reported losses of 34%. In Australia, the cost of weeds in agriculture is approximately $3.55 billion, with one particular weed, Avena fatua estimated to cost $42 million annually. In addition, the development of herbicide resistant weed populations are expected to increase costs.

The Vietnamese government is looking for alternatives to hand weeding and conventional herbicides, thereby minimising contaminations of herbicides to paddy water used for fish farming and household use.

This project aims to determine the extent and severity of the grass weed problem in Vietnam. It will collect fungi which attack weed grasses in Australia and Vietnam and assess their potential for use as bioherbicides. Bioherbicides, applied like conventional herbicides, have as their active ingredients naturally occurring micro-organisms, usually fungi. Spores or fragments of the fungi are sprayed on the weeds at high densities. As they are selective, environmentally safe, and can be produced in the type of low-tech fermenters already present in the region, bioherbicides represent a promising method of weed control in Vietnam and Australia.

Diseased plants will be collected in the field in Australia and Vietnam at different times in the one season and dried between newspaper. Diseases causing stem lesions or crown rots will be targeted.

The plants to be examined in the Australian survey are Avena fatua, Lolium rigidum, Echinochloa crus-galli and Imperata cylindrica. In Vietnam, collections will be made of the grassy weeds considered most important in rice. Surveys of weed density and abundance will also be done over two years with detailed mapping of densities at sites in the Red River Delta and Mekong Delta.

The fungi will be isolated on agar plates from the infected plants. Reinfection of the host weeds will be attempted and fungi reisolated from infected plants. These reisolated fungi will be sent to the quarantine facility at the Biological and Chemical Research Institute, Rydalmere NSW for full identification.

Promising fungi will be tested on host plants. These tests include spore production on a range of growth media, and the response of host weeds to increasing spore concentrations. Fungi showing promise after these tests will be screened for host specificity, using crops, native plants and other weeds and finally for human allergic reactions and toxicity.