Sri Lanka, the Philippines and Australia are significant producers of tropical fruit with good prospects for market development. However with current control measures, field and postharvest disease losses can affect productivity and hamper market access. The shelf-life of most tropical and subtropical fruit crops is limited by their high susceptibility to postharvest diseases caused by fungi - examples are the diseases anthracnose and stem-end rot - with losses of 20 per cent common. In mango, anthracnose also blights flowers and can cause complete crop loss before harvest, particularly if rain occurs at flowering. Field application of fungicides (e.g. copper compounds, mancozeb), and postharvest treatment with hot water and fungicides, currently form the basis for control of such pathogens, however due to the inadequacy of current options for field and postharvest disease control, alternative strategies need to be developed.
Until recently, little attention has been given to the fact that plants have evolved their own powerful defence mechanisms to limit and prevent disease on developing fruit. These include biochemical (e.g. pathogenesis-related proteins, phytoalexins) and physical (e.g. lignification) barriers to pathogen invasion, and may be constitutive (preformed) or inducible in nature. The chemical defences, involving preformed or induced chemicals, cause infections to remain localised and quiescent (with colonisation restricted). As climacteric fruit ripen, the defence mechanisms begin to break down (antifungal compound levels drop) and disease begins to develop. Some cultivars have naturally higher levels of the constitutive antifungals (for example the cultivar Hass avocado) and so disease development in ripening fruit is delayed, allowing more fruit to be marketed and consumed before disease develops. Furthermore, constitutive mechanisms may be up-regulated (induced) by a range of elicitors, to enhance host defences (and delay disease development).
The project aimed to improve control options for diseases of tropical fruit, both in the field and postharvest, by exploring the powerful defence mechanisms that plants have evolved to limit and prevent disease on developing fruit. By so doing, the project also aimed to reduce reliance on synthetic fungicides.
This project characterised natural disease resistance mechanisms in selected commercially important fruit crops from Sri Lanka (mango, banana), and Australia (mango, avocado). Strategies were developed for their enhancement, using defence-boosting treatments and improved grower practice, to reduce field losses and suppress postharvest disease development (Sri Lanka and the Philippines - mango; Australia - mango and avocado).
The Sri Lanka-Australia component of the project undertook a more detailed understanding of the nature and composition of natural defence responses of mango, banana and avocado fruit to pathogen invasion during fruit development and postharvest storage. The Sri Lankan and Australian teams sought to define the physicochemical nature of these host defence systems in terms of genotypic (species, variety or cultivar) and phenotypic (growth rate, production environment) influences.
Because harvested fruit becomes less able to suppress pathogen invasion as it ripens, relationships between levels of natural defence systems and fruit physiology (maturity and ripening-related changes in composition) were also examined. The impact of cultural practices on defence systems was also studied as part of formulating recommendations for the field management of postharvest diseases.
In other studies the research team investigated the physicochemical host defence systems or each of the host-pathogen systems under study, their induction and maintenance for natural defence. As well, the effectiveness of chemical (e.g. salicylic acid) and/or biological (e.g. non-pathogenic mutant) treatments was studied against the background knowledge of genotype and phenotype influences on host defence systems and their interaction with fruit physiology. The success of the initial work on defence boosting treatments for mango (1/7/2002-30/5/2004) in the Sri Lankan-Australian collaboration led to the expansion of this component to the Philippines.
The Philippine-Australian component sought to evaluate and demonstrate sustainable alternative approaches to improve control of field and postharvest diseases of mangoes. This entailed participatory evaluation of superior treatments at farmer field sites and research stations at several locations.
The project team tested activators that were known resistance-inducing agents, including acibenzolar-S-methyl (Bion), and elicitors derived from fungal pathogens (in banana). In another component they characterised some key biochemical defences contributing to the resistance, and identified treatments, varietal properties or other agronomic practices which may influence their relative concentrations.
Mango was the common crop studied in Australia, Sri Lanka and the Philippines. In field trials, Bion was the consistently effective activator of resistance to anthracnose disease, when applied as a foliar spray or as a soil drench 3-5 times throughout the fruiting period. There is clearly the potential for reducing the number of fungicides applied in a given season if Bion is applied. Ultraviolet (UV-C) treatment to harvested mango activated biochemical defences and reduced anthracnose. Another key finding from field trials in Australia (cv. Keitt) and Sri Lanka (cv. Karuthacolomban) was that increasing nitrogen fertilisation enhances anthracnose levels in fruit, which was correlated with high skin nitrogen and lower levels of preformed alk(en)ylresorcinols in skin tissue. (The capacity to analyse the alk(en)ylresorcinol compounds was made possible after establishing collaboration with an expert in Poland.)
Some mango cultivars (and rootstocks) consistently showed high levels of resistance to anthracnose, e.g. 'Keitt' in Australia and 'Gira' and 'Karuthacolomban' in Sri Lanka. There was some correlation between resistance amongst varieties and levels of constitutive defences. In Sri Lanka the galloyltannin class of compounds was identified as a major component contributing to antifungal activity in mango peel extracts.
All banana work was conducted in Sri Lanka. The existence and partial characterisation of several phenylphenalone-type phytoalexins accumulating in response to infection with Phyllosticta musarum, the pathogen causing freckle disease, was confirmed. Freckle infection also induced other biochemical defences, like pathogenesis-related proteins, phenolics and other structural defences. A banana leaf bioassay system was developed for assessing resistance-inducing capacity of elicitors derived from the banana freckle and anthracnose pathogens.
In field trials, preharvest treatment with Bion and salicylic acid reduced anthracnose and crown rot, and stalk-end rot was also reduced by salicylic acid. Fertiliser field trials demonstrated that increased nitrogen enhances anthracnose, while application of potassium reduced anthracnose and finger-end rot, particularly in soils with low initial levels of potassium. As with mango, cultivar differences in the resistance (or susceptibility) to anthracnose were demonstrated.
It is recommended that further field trials focus on the incorporation of Bion into field disease management programs, but its registration and adoption remains the decision of Syngenta. Other defence activators should be assessed as they become available. Postharvest UV-C treatment should be assessed under commercial packing-line conditions, and it is hoped that this will have application to disease management in mango in the near future.
The information on nitrogen fertilisation in banana and mango could have immediate impact if made widely available to growers and other agricultural/extension staff. The short-term impact of the variety work is that growers/industry could choose more disease-resistant varieties. In the longer term, the selection and adoption of more resistant rootstocks (mango) is feasible and the work on biochemical defences could lead to assays for screening germplasm for resistance as part of a breeding program.
The global knowledge of natural plant defence and what affects it has been significantly enhanced in this project, and the capacity of all project teams to conduct such research has been elevated. Some information is available immediately to industry and has been disseminated via workshops and field days.
Links:
[1] http://www.aciar.gov.au/country/Philippines
[2] http://www.aciar.gov.au/country/Sri Lanka
[3] http://www.aciar.gov.au/programarea/Horticulture