An evaluation of cross-infection of Botryosphaeria isolates among tree species used for agroforestry and plantations in Uganda

Abstract

Fungi in the Botryosphaeriaceae family are known to cause disease on many plant species. Evidence of cross-infection of tree species by this group of fungi is limited. The main aim of the study was to evaluate cross-infection by Botryosphaeriaceae species among tree species used in plantation forestry and agroforestry in Uganda. First, the pathogenicity of Neofusicoccum parvum isolated from Eucalyptus grandis on different tree species was assessed in a greenhouse using randomized block design (RBD). For this, a total of 300 seedlings of 10 tree species were used. Half of these seedlings were inoculated with the fungus and the other half were controls. The second part of the study examined the pathogenicity of 13 Botryosphaeria isolates from different tree species on Melia volkensii and Grevillea robusta using RBD in a green house. A total of 420 seedlings, 210 for G. robusta and M. volkensii were either inoculated by one of the isolates or used as controls (not inoculated). In studies, the stem lesions, shoot and whole seedling mortality, oozing, stem swelling, leaf necrosis, dieback, Botryosphaeria fruiting bodies and lesion length were measured. In addition re-isolation was done in order to confirm Botryosphaeria as the causal agent of the observed disease symptoms. The results of this study show that a number of multipurpose tree species used for plantation and agroforestry in the semi-arid areas of Uganda are susceptible to infection by N. parvum and that tree species differ in their susceptibility to this fungus. Neofusicoccum parvum was confirmed as an endophyte in M. lutea and M. excelsa, and Albizia coriaria was not infected by N. parvum during the experimental period. Neofusicoccum parvum was isolated from control seedlings of K. senegalensis and E. grandis. Pathogenicity studies showed that Botryosphaeria isolates obtained from native and exotic trees cross-infect both M. volkensii and G. robusta. A number of multi-purpose agroforestry and plantation tree species promoted in the semi-arid areas of Uganda are susceptible to infection by N. parvum, these susceptible tree species could be considered as a potential threat to other tree species promoted for both agroforestry and plantation forestry in the semi-arid areas because they can act as source of fungal inoculums. Botryosphaeria isolates from different tree species can cross-infect Grevillea robusta and Melia volkensii. It is recommended that agroforestry systems that bring together the susceptible tree species with non- hosts should be carefully designed to reduce possibilities of further infection. Pathogenicity studies should be conducted at field level to evaluate whether similar disease symptoms can be produced in various tree species.