Survey Of The Mango Tree Decline Disease, Characterisation Of The Causal Agent And Some Aspects Of The Biocontrol Of The Pathogen In Ghana

ABSTRACT

Mango (Mangifera indica L.) production in Ghana has the potential of a being an economically viable venture capable of replacing cocoa as a major export earner. However, the crop is bedeviled by a miscellany of disease of the plant itself as well as the fruits. Recently a disease described as the ‘mango decline disease syndrome’ has been found in Ghana gradually taking a heavy toll of the crop in the field and a survey was carried out to substantiate this and prescribe a remedy. In this present study, a survey was carried out in five regions (Ashanti, Brong-Ahafo, Eastern, Greater-Accra, and Northern Regions) of Ghana with the view to documenting quantitatively the incidence and severity of the disease. It was also envisaged to authenticate and characterise the causal agent and to prescribe a biological control method in order to complement the current control methods using chemical sterilants. The survey was carried out with structured questionnaires using farmers as respondents. The percentage incident and severity index were determined by Standard Conventional method and the disease symptoms recorded in the field included: wilting of upper tips of the plants (tip dieback), shoot dieback, drying of the leaves and rolling of their margins, discolouration of vascular tissues, gummosis, cracking of bark, entire death of the plant on a 5 point hedonic scale. Cultural, morphological, physiological and colour characteristics were studied in vitro in Petri plates and using the conventional dry weight measurement technique. Molecular characterisation was done by polymerase chain reaction (PCR) and sequences and phylogenetic analysis of the ITS region. All the symptoms associated with mango decline disease (wilting of upper tips of the plants (tip dieback), shoot dieback, drying of the leaves and rolling of their margins, discolouration of vascular tissues, gummosis, cracking of bark, entire death of the plant) were observed on the diseased trees. The disease incidence on the local mango variety varied from 62.5% (Greater-Accra) to 70% in the Ashanti region but was severer (p≤0.05) (62.5-70.0%) on the local mango variety than the exotic Kent mango variety. The disease severity followed almost the same trend as the percentage incidence. The disease occurred in all the surveyed agro-ecological zones (Coastal savanna, Semi-deciduous forest, Transitional, and Guinea savanna zones) and was again higher on the local mango variety (60.0-74.0%) as compared to 13.33-20.0% in the exotic mango. The causative agent was recovered from 95% of the symptomatic mango plant tissue. Artificial inoculations confirmed the pathogenicity of the isolated pathogen on both local variety and Kent

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(exotic) variety and induced similar disease symptoms which was severer (p≤0.05) on the local mango (Incidence 71.0%; Severity index 2.0) than the exotic Kent variety (Incidence 57.0%; Severity index 1.5). The fungi grew well on all the test media (natural and synthetic) and produced unique morphological, cultural and colour characteristics akin to the pathogen. Aqueous, ethyl acetate and petroleum ether extracts of local mango bark, local mango leaves, Kent (exotic) mango bark, Kent mango leaves, pine needles (Psuedotsuga spp.) and Egyptian date palm seeds (Phoenix sp.) supported vegetative growth of the pathogen to different extents but was best in the aqueous extract. Continuous light (75 lux intensity), continuous darkness, alternating 12 h light/12 h dark regimes produce nearly the same effect on the vegetative growth and colour of the culture which changed from white, to grey to black in 12-21 days. The fruit juice extract from local mango, exotic Kent mango fruit extract and soil extract also supported good vegetative growth of the pathogen showing that all the plant parts tested could support the survival of the pathogen. The mycelium was hyaline after 2 days and remained non-septate with a diameter of 2.0±1.3 μm and thereafter increased in diameter 2.0-15.0±1.5 μm. Septation occurred after 2 days. Rounding off of mycelium to form “chlamydospore-like” structures after 21 days was observed on some media but in the case of local mango bark, large “turbinate-like” cells with thickened walls were formed. The different light regimes employed did not influence the formation of these structures. The natural media, local mango fruit juice, Kent (exotic) mango fruit juice did not support conidia formation by the mycelium of the pathogen identified on the basis of cultural, morphological and physiological characteristics as Lasiodiplodia theobromae Griff and Manubl. But Soil extract agar, Potato Dextrose Agar and OGYE produced conidia of dimension 11.0-12.0±3.5 μm wide and 20.0-32.0±4.2 μm long which compared favourably with that reported in the literature for L. theobromae. PCR using universal primer pair (ITS1/ITS4) and sequence and phylogenetic analysis of the amplified ITS region showed all isolates from Ghana clustering together with the type strain of L. theobromae and other L. theobromae isolates which confirms that the pathogen from Ghana is L. theobromae. Aqueous extract of the leaf of Chromolaena odorata, Azadirachta indica and seeds of Carica papaya significantly depressed vegetative growth of L. theobromae at high concentration (undiluted-1:1 v/v) in the order of C. odorata = A. indica < Carica papaya. Practical implications of these findings are discussed and future studies suggested.