Incidence and Level of Mistletoe Infestation in Tree Species at Botswana University of Agriculture and Natural Resources' Sebele Content Farm Campus
Abstract
A survey was conducted at Botswana University of Agriculture and natural Resources at Sebele content farm to determine the intensity and level of mistletoe infestation in tree species. The intensity and the total number of trees per tree species infected by the parasitic weed were determined. Acacia eriobola, Acacia mellifera, Terminalia sericea, Ziziphus mucronata, Schinus molle, Acacia tortilis, Acacia erubesens and Acacia flekii hosted the highest number of mistletoe. Results revealed significant variation in level of mistletoe infestation between tree species. A 100% infestation was recorded in some tree species such as Acacia eriobola and Ziziphus mucronata. Severely infested indigenous tree species were dying, whereas most of the exotic tree species were either not infested or have very little number of mistletoe on them. The variation observed could be due to the fact that indigenous species the frequently visited by dispensers looking for food and shelter than exotic species. In addition, there is a possibility that the mistletoe species co-evolved with the indigenous tree species and the vector may be well established on the host tree species than on exotic species.
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Introduction
Mistletoe is a name originally referring only to the European species Viscum album, but is now used as a general term for woody shoot parasites in several plant families especially Loranthaceae and Viscaceae which belong to the order Santalales.
Loranthaceae (Parker and Riches, 1993). There are about 400 species in several genera of Viscaceae, of which at least five can be of economic importance (Barlow, 1993). In Botswana the common mistletoes are Viscum album, Tapinanthus oleifolius, Plicosepalus kalachariensis and Erianthemum ngamicom and all these species are well represented on Botswana University of Agriculture and natural Resources Sebele content farm where this study was conducted.
Mistletoes (excluding Arceuthobium spp.) are leafy and forms spherical, bush like growths reaching as far as 1.0 m and are particularly eye catching in winter when exposed by the loss of leaves of their host tree (Butin, 1995). The seeds are distributed by birds and germinate if they reach the bark of a suitable host plant, each initially forming a sticky adhesive disc at the tip of the radicle (Parker and Riches, 1993). Most species of Loranthaceae and Viscaceae have sound established photosynthetic capacity from an early stage and should not need to rely on the host as a source of carbon, however, completely dependent on the host for both water and minerals (Liddy, 1993). Mistletoes tend to grow on isolated trees, on the edge of the forest and in the higher branches of the trees (Parker and Riches, 1993;Buen et al., 2002).
Like other parasitic organisms, mistletoes show specialization on host species due to a number of factors. Some mistletoe species are specialized in living on different host due to frequent encounters between mistletoe seeds and commonest plants (Fadini, 2011) and in others, non-random perch preferences of seed dispensers play an important role in determining host specificity (Monteiro et al., 1992). Host tree preference (Fadini, 2011) and host tree height (Rahmad et al., 2014) are also reported to influence the concentration of mistletoe seeds on a host.
It has been observed that several trees species were dying at Botswana University of Agriculture and Natural Resources, Sebele content farm and in different localities across the country because of mistletoe infection. However, there has never been a comprehensive attempt to establish an inventory of the tree host species of mistletoes and the level of mistletoe infestation in the country. The aim of the present study is therefore to establish a baseline inventory of the known host tree species and the level of susceptibility of each host tomistletoes. It is hoped that this paper will stimulate a systematic documentation of the host species of these parasitic plants and the level of susceptibility of each host species to mistletoe infestation.
Conclusion
The study identified that indigenous tree species are preferred hosts of mistletoe and carry higher level of infestation compared to exotic species. The frequency of infection and density of mistletoe in each host results from interspecific differences in growth habit and abundance. The process of colonization of a tree likely depends on visit by dispersers, mistletoe seed deposition and establishment. Although the study touched upon only two factors that impinge on parasitism frequency, it highlighted a significant aspect of the interaction between mistletoes, hosts, and avian-seed dispersers. Further detailed research on the tri-trophic interaction and the effect on the ecosystem is recommended.