December 12, 2019 10:08 am Published by KirstyH

Manipulation of plant nutrition to beat banana disease.

Increasing volumes of innovative and interesting trials data and circumstantial evidence suggest a plant nutrition dimension can help manage Black Sigatoka disease through enhanced host plant resilience. Dr Terry Mabbett reports.

Who would have thought that banana, the biggest of all bulk-fruit crops grown in the tropics, could be under a real threat of ‘commercial extinction’ from the disease? But be in no doubt because this could easily happen, especially for dessert bananas. Musa species (banana) is inherently and desperately short on genetic variability required for disease resilience, specifically the dessert banana export trade, which is almost entirely based on the Cavendish variety. What’s more, the genetics of the Musa genus is complex and sufficiently so to make the development of disease-resistant and disease-tolerant banana varieties extremely difficult.

The first fungal plant pathogen to hit banana with a disease of survival-threatening capability was Mycosphaerella fijiensis. Black Sigatoka was first identified in 1963 and soon proved sufficiently aggressive to supplant its close cousin Mycosphaerella muscicola (yellow Sigatoka) as the major foliar disease in the banana growing regions of the world. Since then, banana plantations and estates have been sprayed with almost every conceivable fungicide to kill the pathogen and control the disease. Including sophisticated systemic curative fungicides with their site-specific (single-site) action and discovered, developed and marketed over the last half century or more.

Farmers find that bunch sprays of Omex Calmax and Omex DP98 alleviate the pressure on banana leaves from infection and disease development by Black Sigatoka to deliver bigger bunches, heavier hands and fuller fingers of banana fruit.

However, site-specific fungicidal action has turned out to be an Achilles’ heel for systemic curative fungicides, leading in large part to their downfall caused by a propensity to select out strains of fungi resistant to their intrinsic chemistry. Banana growers faced with such problems are essentially wasting money spraying against a disease which is beyond the control of the fungicide chemistry being used. Ironically the much older and generally less costly contact protectant fungicides with an inherent broad-spectrum activity, and shielding them from resistance development, are currently enjoying a renaissance and especially the ‘oldest of the lot’ which are the copper fungicides discovered 140 years ago. Over 60 years have passed since fixed copper fungicides like cuprous oxide were first used to control the Sigatoka diseases on commercial dessert banana crops. They were used in ‘stand-alone’ applications and later alongside systemic singlesite action fungicides to ‘cover’ the latter against fungicide resistance development. 

Farmers in The Philippines who regularly use Omex nutrient products inspect thriving banana plants on an estate previously condemned due to Panama Disease TR4.

The plant nutrition dimension.

The divalent copper ion (Cu2+) which is the active principle in all copper-containing fungicides also has a vital role in plant metabolism as an essential micronutrient to present a ‘double irony,’ because ‘manipulation of plant nutrition’ now appears to be a novel pathway for Sigatoka disease management.

Increasing volumes of innovative and interesting trials data and circumstantial evidence suggest a plant nutrition dimension can help manage Black Sigatoka disease through enhanced host plant resilience. And to maintain fruit yields in face of an otherwise allconsuming foliar disease which reduces the photosynthetic capability and capacity of the foliage, leaving growers with smaller and lighter bunches of banana fruit.

Commercial dessert banana production is practised throughout the tropics in Central/South America, Africa and Asia. The Philippines is Asia’s biggest exporter with 2.85 million metric tonnes out of a production total of 9.36 million metric tonnes in 2018. Cavendish cultivars account for 52 per cent of production and an even greater proportion of exports. The Philippines is just the sort of major banana producer/exporter where a plant nutrition dimension is needed to protect and preserve the industry and perhaps even save the country’s export trade in dessert banana.

Specialising in leading-edge plant nutrition is OMEX Agrifluids, a research and development-based UK (United Kingdom) company with unrivalled breadth and depth in its product portfolio, and as such, well placed to assist banana growers fighting black Sigatoka.

To discover what kind of work Omex is doing in cooperation with banana growers in The Philippines, I spoke with managing director Peter Prentis, whose remit for research/development and marketing covers the whole of Asia. Peter describes the fascinating situation of a relatively small banana grower in the Philippines having secured his land from a multi-national banana producer, and now successfully growing and producing good banana yields while others all around are increasingly constrained by Black Sigatoka disease.

“Standard practice in The Philippines is to cut out the diseased and necrotic areas of typically large banana leaves to curtail the spread of pathogen and disease,” said Peter, “and clearly last resort to limit spread and preserve the photosynthetic capacity of as yet unaffected areas of the leaves. This particular grower is using OMEX Calmax and OMEX DP98 combined in a foliar spray and applied to banana bunches to harvest heavier and larger bunches of fruit. We know this because boxes are being filled with just five hands of fruit whereas previously it required eight hands of fruit to fill a box,” noted Peter.

Ability to manage plant disease through crop nutrition is universally important and especially for the banana in which the profit margin for growers is so low. Calculations using dessert bananas sold in the UK supermarkets show growers receiving just over 20 per cent of the retail price, the same proportion received by the importer/ripener/retailer. Unlike chemical fungicides for which the only function and effect is a potential for disease control, positive effects of nutrient products on banana disease management are just a small part of their overall input into securing best plant growth and development and crop yield and quality.

How is the OMEX DP98/OMEX Calmax ‘combo’ working?

OMEX Calmax is high in soluble calcium (22.50 per cent w/v) and already well established for enhancing structural resilience of plant tissue and susceptibility to disease in a wide range of harvested fruit and vegetables. Omex DP98 is high (37.50 per cent) in phosphorous but as the highly soluble phosphite nutrient as opposed to more usual and traditional phosphate.

Calcium is a vital element and plant nutrient with a structural role in calcium pectate which cements plant cell walls together to enhance overall strength and integrity of plant tissue, and thereby plant resilience to disease progression. However, calcium ions suffer from poor mobility to present difficulty in access and absorption by the roots even when there are ample calcium supplies in the soil, and also the movement of calcium ions inside the plant.

The water-soluble phosphite nutrient has biostimulant activity to assist entry of the inherently ‘lethargic’ calcium ions into the leaves and to help ‘ferry’ these divalent cations around the plant for efficient utilisation in a tissue-structure strengthening role. Evidence suggests this is exactly what could be happening in banana, with the combined foliar application of Omex Calmax and Omex DP98 conferring banana plant resilience to Black Sigatoka disease.

However, Black Sigatoka is not the only ‘survival-threatening’ disease confronting banana. Increasingly on the agenda to present an even worse pathogen/disease scenario is Tropical Race 4 (TR4) of the fungus Fusarium oxysporum f.sp. cube nse, the cause of Panama disease of banana. This is even worse than Black Sigatoka because the systemic nature of this disease within the banana plant means it is effectively beyond the ‘reach’ and control of proprietary fungicides.

A frenzied investigation into the management of Panama TR4 is underway and including within the plant nutrition arena. Research findings at ‘cellular level’ have already indicated that the nature of banana root cell walls – structure, composition etc – can act as a barrier to the fungus which needs to pass this barrier to interact with living plant cells. Coordinated chemical responses within cell walls in response to an incursion by this pathogen have been demonstrated.

The plant cell wall material is composed of complexpolysaccharides such as celluloses and hemicelluloses together with associated proteins and aromatic compounds. Simple but essential to overall strength and resilience of plant tissues and plant organs is pectate. Pectate ‘cements’ walls of adjoining cells together and is known to play a pivotal role in stalling microbial infection and disease development within plants in general. The type and form of pectate performing this cell-cementing function is Calcium Pectate. A potential role for the calcium ion and nutrient (contained in Omex Calmax at 22.50 per cent w/v) warrants consideration and investigation.