Biocontrol in tea

Tea plants are subject to attack by a large number of insect pests and fungal diseases, also by bacteria, algae and viruses. Infections can be exacerbated by the pruning and plucking regimes practised in plantation monocultures. While these restrict growth to form a short, vegetative bush form, and make harvesting easier and yield higher, this method of management also facilitates transmission of pathogens and reduces the success of control attempts. Losses can be particularly high in countries like Sri Lanka where tea estates are situated close together.

Chemical pesticides have proved effective in the past, but greater consumer health-consciousness is driving demand for teas completely free of, or only minimally contaminated with, chemicals. This has led to increased research into biocontrol methods, which also offer benefits of greater biodiversity and cost savings to growers.

Biocontrol using fungus

Natural methods of controlling fungi and insects are being tested in India and Sri Lanka with increasing success. Worldwide, there are 380 fungal pathogens which attack tea. In Sri Lanka, scientists are focussing on antagonistic fungi and bacteria that parasitize pathogenic fungi and destroy the pathogen in one of several ways. According to Abhaya Balasuriya, a scientist at the Tea Research Institute (TRI) in Talawakelle, Sri Lanka, the first step in biocontrol using fungi involves searching for strains with potential for antagonism from areas where diseases are prevalent. "Isolates showing some promise are made into pure cultures before they are used for in vitro tests," he says. He notes that Trichoderma harzianum is a widely studied antagonist because it has the ability to out-compete pathogenic fungi, and to parasitize them by producing toxic compounds or enzymes that destroy pathogenic cells.

credit: Indian Institute of Horticultural Research

In the lab, Balasuriya and his colleagues have found some fungal biocontrol agents that perform comparably with chemical fungicides: T. harzianum was able to suppress the vegetative growth of black root disease by 85 per cent, compared to fungicides Pyraclostrobin and Propiconazole, both of which achieved 100 per cent control. When tested against red root disease and horse hair blight, Trichoderma harzianum Rifai showed better control than the systemic fungicide, Bitertanol. And in a field trial of horse hair blight control, Balasuriya found that a treatment containing T. harzianum was capable of bringing comparable control (about 50 per cent) to that of hydrated liming or the fungicide Propiconazole.

Bacterial biocontrol of fungal pathogens is also proving effective and may be more successful than fungi in some cases because bacteria grow rapidly, are easier to handle and are able to survive under adverse conditions. At the Tea Research Foundation in Valparai, India, scientists studying grey blight have isolated two indigenous bacterial strains that produce lytic enzymes which can break down the cell wall of the fungal pathogen, effectively suppressing spore germination.

Insect biocontrol

There are some 150 insect pests of tea, the most harmful being spider mites, live-wood termites, parasitic nematodes and shot-hole borer beetles. At the TRI, a broad research approach has been used to locate the 'weak' points in the life cycle of pests and their ability to infest the crop. Among techniques used are host-plant resistance breeding, methods of horticultural control, and biologically active insecticides such as hormone-mimics and chitin-synthesis inhibitors. Scientists are also using computer models to predict insect and mite outbreaks, which helps farmers to limit the use of chemical pesticides to these events.

While most biocontrol agents are native to the country of origin, Balasuriya notes that in Sri Lanka "there is a classic example of a parasitoid wasp (Macrosentus homonae), imported from Java in 1936 for the control of Homona coffearia (a leaf-eating caterpillar also known as the tea tortrix), which is still working." The TRI states that, "This success is regarded in scientific literature as the most outstanding example of classical biological control ever achieved in the world on a perennial crop."

Natural pesticides

Pesticides derived from natural substances are less harsh and can at least partly replace synthetic pesticides. TRI scientists have discovered non-chemical alternatives to methyl bromide, an ozone-depleting substance being phased out across the world, which was used to eradicate nematode species that attack tea plants. Substituting nematode-infested soil with agricultural wastes, which have pesticidal properties (such as refuse tea, coir dust and paddy husk), reduces numbers of nematodes, as does drying nursery soil in the sun and planting nematicidal plants like wild sunflower. Meanwhile, in India, the Hindustan Lever company, using a bio-pesticide mixture made from sulphur and an extract of the montanova plant, has reduced total pesticide use on their plantations by 16 per cent from 2001 to 2003.

Article written by Treena Hein