Distribution, molecular identification and the effect of biological control of phytophthora cinnamomi on macadamia in the Limpopo and Mpumalanga provinces, South Africa.
Kunene, Nontokozo Sibahle.
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Macadamia is a nut-bearing tree that belongs to the family Proteaceae that consists of evergreen woody plants (Augstburger et al., 2002). The aboriginal tribes used the nuts as a staple food and as a base for medicines and cosmetics in Australia, where the nut originated from (de Villiers, 2003). Two different species of macadamia, namely Macadamia integrifolia (Maiden and Betche) (also M. ternifolia), and Macadamia tetraphylla (L.A.S. Johnson), together with their hybrids, are of importance in commercial cultivation (Augstburger et al., 2002). Macadamia production is limited to the tropical and subtropical regions of the world, but the nut is exported worldwide. In South Africa, macadamia production is mainly confined to the Limpopo, Mpumalanga, and KwaZulu-Natal provinces; and to a lesser extent to the Eastern and Western Cape provinces (DAFF, 2015). Plant pathogens (fungi, stramenopiles, bacteria, viruses, nematodes) are a major threat to plant production since they result in quality and quantity reduction of commercial crops worldwide (Bailey, 2010). There are major losses in agriculturally essential crops due to these pathogens, and they, therefore, remain important constraints in agricultural production (Bailey, 2010). A number of factors affect macadamia production, but those of great economic importance are the disease-causing and quality-reducing pathogens. The main and most severe diseases in macadamia are caused by Phytophthora spp. These are capable of reducing vigour, production, and may cause complete trunk death (Akinsanmi and Drenth, 2010). Phytophthora cinnamomi (Rands) has been the chief limiting cause to successful macadamia production in countries such as Hawaii (Ko, 2009), Australia (Rosengarten, 2004), California (Zentmyer, 1980), Kenya (Sikinyi, 1993) and South Africa (Manicom, 2003). It causes stem canker, root rot and quick decline in macadamia worldwide (Serfontein et al., 2007). Root rot and trunk canker are major diseases that could cause 60% yield losses and an estimated 10% of the annual gross value of macadamia (Muthoka et al., 2005). P. cinnamomi is the most important and destructive oomycete of not only macadamia worldwide, but over 1000 plant types (Zentmyer, 1980), including avocados, eucalyptus, kiwi fruit, chestnut, peach, pineapple, pear, and many native Australian and South Africa plants (Pegg et al., 2002). The pathogen is of specific importance to the avocado (Persea americana Mill.) and macadamia industries because of its potential to destroy avocado and macadamia orchards in a short time frame. It infects and kills trees of all ages, from nursery trees to large fruit-bearing trees through the destruction of feeder roots (Bekker, 2007). P. cinnamomi is responsible for the widespread damage of macadamia trees worldwide. It infects the feeder roots. Infection occurring through the fine feeder roots results in root rots whereas infection occuring through wounded trunks of mature trees results in the development of trunk cankers (Mbaka, 2011). Infected trees die in three to five years; this, however, depends on the management of the orchard.