Читать книгу Biotechnology of Fruit and Nut Crops - Группа авторов - Страница 130

The oil palm still faces formidable challenges in terms of its overall yield per hectare, its oil quality, and the perceived sustainability of some aspects of plantation development and management. Thanks to recent progress in crop breeding and genomics, together with emerging recognition of the importance of improved management practices across the industry, prospects are good to confront these challenges (Murphy, 2009).

Genetic improvement of the oil palm has definitely a role to play in sustainable plantation management. Indeed, the choice of the appropriate seed variety is the first step towards a sustainable cultivation system. There is an average turnover of 20 years for each planting cycle, and it is reasonable to expect at least a 20% increase in oil yield from the latest generation of tenera hybrid planting material, i.e. clones, biclonal or traditional hybrid seeds. With the rapid development of the smallholder sector which is being facilitated by governments and donors in many countries, oil palm breeders now must meet their specific requirements (Cochard et al., 2005). Farmers are paid based on the weight of FFB (fresh fruit bunch), and not on the quantity of extracted oil, and the quality of fruits entering the mill directly impacts the purchase price through a system of bonus/penalties rewarding optimal ripening stage at the time of harvesting.

Breeding work undertaken over the last 50 years by major plantation companies and research institutions has led to ongoing genetic progress which is now estimated at 1% per annum (Cochard et al., 2005). Yields are now expected to reach and even exceed 10 t of oil/ha/year in the near future (Corley and Tinker, 2016).

Oil palm estates in South-east Asia face substantial losses due to basal stem rot. Field observations have been carried out in North Sumatra, Indonesia, on planting materials of known origin (Durand-Gasselin et al., 2005). Differences in disease susceptibility have been detected within E. guineensis and E. oleifera. Within E. guineensis, material of Deli origin is highly susceptible compared to material of African origin. Differences also exist in the response to infection between parents and between crosses within a given origin. The variability of resistance to basal stem rot within the same cross is also illustrated by the diverse responses of clones derived from palms of the same origin. Such results are encouraging for the development of early selection tests for genetic resistance to basal stem rot (Mercière et al., 2015).

Early screening tests for genetic resistance to diseases at the nursery or pre-nursery stage are based on artificial inoculation and correlated with field observations. The success of nursery seedling artificial inoculation relies on parameters: (i) defining the inoculum potential, i.e. the aggressiveness of G. boninense isolates; (ii) the incubation period of pre-infected rubber wood blocks (RWBs); (iii) the ratio between the size of pre-infected RWBs and the volume of soil for infection; and (iv) the quality of nursery or pre-nursery shade as predisposing factors. When this set of parameters was optimized, disease symptoms are apparent 3 months after the inoculation of germinated seeds (Breton et al., 2006). This avoids the transfer of seedlings from the pre-nursery to the main nursery, minimizes transplanting shock and root damage, and also reduces the time taken for screening and the nursery area required. This early inoculation enables the discrimination of isolates according to their aggressiveness, expressed by the quantification of external and internal disease symptoms and by using a standardized scoring scale. Development of this method will lead to the selection of isolates that can be used in screening for resistance or tolerance to basal shoot rot. Preliminary results show that it is possible to distinguish the degree of susceptibility of progenies inoculated artificially at the germinated seed stage or when transferred to polybags. A good correlation was obtained between those two stages, suggesting that the level of resistance was conserved irrespective of the physiological stage used for screening in the nursery.