Panama Race 1 isn’t supposed to infect Cavendish banana plants. So why is it killing them in an Indian growing district? Dr Thangavelu (Thanga) Raman and Jeff Daniells report on Race 1 causing widespread disease in Cavendish and research into biological controls.

It was in 2008 that Panama disease symptoms were first discovered on farms growing Cavendish Grande Naine bananas in Theni district of southern India.

The symptoms were typical of Panama – yellowing and drooping leaves around the pseudostem which also had longitudinal splitting. When the plants were cut there was brown vascular discolouration in both the pseudostem and corm.

An investigation began. It included a look at the growing conditions in the field where the sick plants were found. There was nothing unusual – the soil type was a red sandy loam with a pH of 7.3 and the plants were grown under drip irrigation.

Samples of both pseudostem vascular strands and infested corm tissues were collected and sent to the National Research Centre for Banana where the fungus responsible was isolated and purified.

Panama disease in bananas is a vascular wilt caused by the type of Fusarium known as Fusarium oxysporum f. sp. cubense or Foc.

Immunity lost

Testing for Panama involves Vegetative Compatibility Group (VCG) analysis to see if the sample being tested matches one of the identified Foc strains. Each strain has a different four- or five-digit code. In bananas, some strains are associated with Race 1, some with Race 2 and others with Subtropical Race 4 and Tropical Race 4.

There were ten representative samples collected from different banana-growing villages in the Theni district. To get a result, a VCG analysis was conducted using what are known as Nitrate non-utilising (nit) mutants derived from the Foc in the samples and a reference collection of nit-M testers. These particular testers were supplied by the Queensland Department of Agriculture and Fisheries.

In a VCG analysis, the testers are put on a Petri dish along with the nit mutants from the plant sample to be tested to see if they are compatible (the same strain). In this case, six of the ten samples were identified as known VCG types – three were VCG 0124 and three were VCG 0125. The four other samples were an unknown type.

Both VCG 0124 and 0125 are recognised as Race 1 strains and are not supposed to affect Cavendish.

The findings were confirmed and announced in 2010. Banana scientists are still trying to determine why some Cavendish in India are no longer immune to Race 1.

Tissue culture

Some seven years after the symptoms were first noted, an intensive survey of the banana industry in Theni was conducted. It found that Panama disease incidence ranged from 5 to 25 per cent.

During the survey, stem splitting was noted and mostly found to be due to Panama disease. A problem with banana stem weevil (Odoiporus longicollis) was also observed and may have been aggravating Panama disease severity.

Moreover, only ten days after planting, vascular discoloration associated with Panama disease was observed in the corm in certain plants in one block surveyed.

Where the disease was found in blocks that had not previously grown bananas it is suspected the pathogen may have been introduced with the tissue culture planting materials. Unlike Australia, there is no quality assurance scheme for banana plants, such as Australia’s Quality Banana Approved Nursery (QBAN) scheme, that ensures tissue cultured banana plants are certified free from Panama disease.

The plant pathologist who is responsible for investigating Panama disease race 1, Dr Thangavelu (Thanga) Raman, had observed vascular discolouration in tissue culture plants even before the Panama symptoms were first noted in Theni in 2008. In 2006, during a visit to a tissue culture company at Coimbatore in north west India he observed vascular discoloration in the second hardening phase of tissue cultured plants.

Biological controls

In India, no effective management system is yet in place for banana farmers struck by Panama disease.

Affected Cavendish growers are switching over to Red banana (Red Dacca) and Nendran (French Plantain), which have shown resistance to the Race 1 affecting Cavendish, and also to other crops like onion, grapes and beetroot.

However, biological control methods, particularly the use of native endophytic and rhizospheric microbes appear to hold the best prospects.

This is based on the greenhouse and field trials so far completed. The greenhouse investigations evaluated 79 fungal and 179 bacterial microbes as well as 33 botanical extracts. Following the successful pot trials, the best of these were evaluated in the field at a Panama disease hot spot.

Almost complete suppression of the disease was recorded during field trials from 2011 to 2013 where plants were treated with applications of bio-agents and botanicals during the first four months following planting.

The treatments were a combined application of fungal endophytic Penicillium pinophilum Bc2 + rhizospheric Trichoderma sp. NRCB3 and liquid formulation containing endophytic Trichoderma asperellum Prr2 + Bacillus flexus Tvpr1 and Zimmu (Allium cepa x Allium sativum – a hybrid of onion and garlic) leaf extract.

Treatment with these bio-agents and botanicals resulted in 100 per cent harvest of good bunches compared to only 35 per cent harvest in the control plants that received no applications.

A third trial is now in progress.

While these positive field trial results are encouraging, the eventual sustainable management of Panama disease affecting Cavendish bananas in India will very much depend on an integrated management strategy encompassing quarantine, cultural and biological methods.

Indian Cavs still pick of the bunch

India is the world’s largest banana producer and, although it grows more than 20 cultivars, it relies heavily on Cavendish writes Dr Thangavelu (Thanga) Raman.

India grows 28 million tonnes annually or 19 per cent of global production of bananas and plantains. The country has 800,000 hectares under banana production.

India is a recognised primary centre of diversity for banana with hundreds of indigenous cultivars and numerous wild species.

More than 20 cultivars are produced commercially in different parts of India. They include Red Banana (AAA Red = Red Dacca), Rasthali (AAB Silk, = Sugar), Poovan (AAB Mysore, ~Pisang Ceylan), Ney Poovan (AB), Thella Chakkarakeli and Karpuravalli (ABB Pisang Awak, = Ducasse), Nendran (French Plantain), Hill Banana (AAB Pome = Lady Finger) and Monthan (ABB related to the cooking banana Bluggoe).

However, the Indian banana trade mainly depends on Cavendish cultivars going by various names in different regions including Basrai, Robusta, Harichal, Grande Naine, Shrimanthi, Bhusaval and Pedda Pacha Arati.

The Cavendish cultivars occupy 52 per cent of the total area under banana cultivation and contribute 64 per cent of the total production.

Banana exports

The state of Tamil Nadu lies in the south easternmost part of the Indian Peninsula. Its most important banana growing district is Theni which is situated about 10° N (see map below left), about the same distance from the equator as the Torres Strait.

The average maximum monthly temperatures range from about 29-34°C and minimums from 20-25°C. Average annual rainfall is about 830 mm.

Bananas are mostly grown on mediumto heavy-textured red soils of moderate to high permeability with pH ranging from 6.5 to 8.5. A range of fertilisers plus farmyard manures are applied with typical total rates being 380 N: 150 P: 340 K in kg/ha/crop.

Crop density is high at 2,900 plants per hectare and so is yield, averaging between 75 to 90 tonnes per hectare.

The Theni district on average exports 24,000 tonnes annually of Grande Naine to countries in the Persian Gulf. This represents about 45 per cent of India’s banana exports.