Burl formation in mango (Mangifera indica): a neglected tumour disorder and the structure of its secondary xylem

Abstract

Mango burl is an important disease affecting many mango plantations in India and causes great loss in yield and decrease of vigour. We carried out a diagnostic survey for burl disease (sometimes also referred to as crown gall) in different varieties of mango (Mangifera indica L., Anacardiaceae) throughout India during 2015 and 2016. More than 500 mango genotypes were screened for disease susceptibility and more than twenty-three mango genotypes in different parts of India were found susceptible to this disease. Burls initiate as small tumorous growths and become more pronounced as warty outgrowths with the increasing age of the individual tree. Samples of burl were collected from all popular varieties from different parts of the country and subjected to histological investigations. The present study confirms that mango burl disease is caused by Agrobacterium tumefaciens using evidence from the “carrot assay” and molecular identification of the presumed causal organism. The latter was isolated and inoculated on carrot disks to induce typical symptoms. The xylem of the burl tissue was deformed and showed no specific orientation of the wood cells. Dimensional details and morphology of xylem cells vary at different positions within the burl. In a 10 cm diameter burl, the marginal portion showed xylem cells in circular arrangement. Vessel elements and fibres were very short while rays were relatively low and biseriate rays were observed rarely. Orientation of the xylem tissue was lost and all three plains (i.e. transverse, tangential and radial view) were observed in the same section. Cells from the middle portion of the burl were isodiametric, oval to circular, thick-walled and lignified, in morphology not dissimilar from callus tissue. Tyloses were common in all the cell types viz. fibres, ray cells, tracheids, axial parenchyma cells, and vessel elements.