Detection, control and resistance expression in oil palm (elaeis guineensis) caused by F.oxysporum f.sp. elaeidis

Abstract

Vascular wilt disease caused by Fusarium oxysporum f. sp. elaeidis (Foe) causes a devastating disease of oil palm in West and Central Africa. However, this disease has not been reported in South East Asia, in spite of long term importation for breeding purposes of African seed and pollen, known to be often contaminated with Foe. Malaysia is the second largest palm oil producer in the world and Foe remains a major threat to this industry, especially as this study shows four current palm genotypes grown there are susceptible. This research was conducted in order to help Malaysia avoid and/or be prepared for this potential problem. Disease epidemiology was studied in plantations in Ghana. Statisticalanalysis showed the disease mainly occurred in clusters, implying root-root transmission rather than aerial spread by spores. Many Foe isolates were obtained for genetic analysis from diseased palms, including 10 per cent from 21 symptomless trees. This shows that visual disease surveys are flawed. The only practical, sustainable approach to controlling Fusarium is by breeding disease resistant palm lines. The success of this strategy depends on the variability of Foe isolates. Resistance should be stable because this analysis showed Foe isolates have a monophyletic origin. Moreover, this study also showed early responses to Foe infection of roots through induction of the defence-related gene chitinase. Molecular diagnostic tools were developed for (1) rapid detection and quantification of Foe in seed and pollen for quarantine purposes in order to prevent transcontinental spread of Foe, (2) to test efficacy of putative disease resistant or tolerant palm genotypes, and (3) to facilitate epidemiological studies involving palm tissues and soils. Primers were designed for detecting the species F. oxysporum, based on the translation elongation factor gene (TEF-1α), superior to the existing ones used currently at quarantine. The first Foe-specific primers to be developed were based on a virulence effector gene that excluded 70 other phylogenetically closely related Fusarium species from various hosts and origins. Treatment by fungicides is undesirable and largely unsuccessful for this disease. For that reason, the mycoparasitic fungus Trichoderma was evaluated. The most effective strains were selected based on discerning techniques such as competition in palm wood and survival in soil and on roots, Trichoderma isolate TPP4 was shown to exhibit potential biological control by delaying and suppressing Fusarium wilt symptoms and colonization. Confocal microscopy was used to investigate interactions on the root surface between Foe and Trichoderma, which had been transformed with red and green fluorescent proteins respectively. Disease progress/extent/symptoms was substantially delayed/reduced in two Malaysian soils compared to other growth media, highlighting the possibility that Foe-suppressive soils in Malaysia might explain the non-appearance of this vascular disease there. From this study other potential biocontrol agents may be revealed, for example endophytic fungi that showed antagonism to Foe were isolated from plant species grown in Malaysian soils.

Date of Award	21 Nov 2012
Original language	English
Awarding Institution	University of Bath
Supervisor	Richard Cooper (Supervisor)