Morphological, Biochemical and DNA Studies for the Development of Markers for the Selection of Theobroma Cacao L. Clones Resistant to Vascular Streak Dieback Disease
Vascular streak dieback (VSO) is a serious disease of Theobroma cacao L. (cocoa) plant and it is attributed to the fungal pathogen Oncobasidium theobromae. Control of the disease is largely dependent on the use of systemic fungicides. Resistance to the pathogen had been found in the cocoa plantin...
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| Format: | Thesis |
| Language: | English English |
| Published: |
1999
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| Subjects: | |
| Online Access: | http://psasir.upm.edu.my/id/eprint/8648/1/FSAS_1999_1_A.pdf |
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| Summary: | Vascular streak dieback (VSO) is a serious disease of Theobroma cacao
L. (cocoa) plant and it is attributed to the fungal pathogen Oncobasidium
theobromae. Control of the disease is largely dependent on the use of systemic
fungicides. Resistance to the pathogen had been found in the cocoa planting
materials but as yet there was no marker to select for the resistant clones. In
relation to this, this study was conducted to develop suitable markers for selection
of cocoa clones resistant to a VSO disease. These include morphological,
biochemical, enzyme activity, isozyme pattern, light and electron microscopy, and
DNA markers.
The cocoa clones chosen represented three levels of resistance to VSD
disease. The reported resistant clones were MHP80, SCA6, AMA 15-15, ICS95,
PA13, KKM25 and SCA9; moderately resistant clones were PBC140, KKM22
and MHP136, while the susceptible clones were MJS21, ICS84, NA32, NA33,
UA13, MHP14, MHP3?, PAY, IMC67 and EET 399.
The results showed that the morphological characteristics of leaf (shape,
color and size), pod (color, shape and surface) and bean (shape, size and
and cotyledon color) could not be used to distinguish between the cocoa clones resistant and susceptible to VSD disease.
The results on the biochemical and enzyme parameters indicated that the
activity of polyphenol oxidase and concentration of chitinase in cocoa leaf tissues
were related to resistance to VSD disease. The level of both enzymes were
significantly higher (P≤0.05) in the resistant cocoa clones compared to the
susceptible clones. Others biochemical parameters such as the concentration of
total soluble phenol and β-1,3-glucanase, activity of peroxidase, proteins and
isozyme patterns could not distinguish the resistance of cocoa clones to VSD
disease.
Studies on light microscopy showed that the growth of Oncobasidium
theobromae in the leaves of resistant clones resulted in less hyphae compared
to those from the susceptible clones. However, the results from the electron
microscope studies showed that the infected tissues of both resistant and
susceptible cocoa clones showed similar damage symptoms such as cell
shrinkage and plasmolysis.
The results on Amplified Fragment Length Polymorphism (AFLP)
technique to distinguish between cocoa clones that are resistant and susceptible
to a VSD disease using selected primer combinations showed polymorphisms
among cocoa clones. However, the similarity index and cluster analysis obtained
from the data could not classify the cocoa clones as being resistant or susceptible
to VSD disease. Meanwhile, a cloned unique fragment of resistant cocoa clone
obtained from an AFLP analysis was able to differentiate between resistant
(ICS95, KKM25, MHP80, A MA15-15) and susceptible (NA32, PA7, MHP14,
MHP37) cocoa clones. This DNA marker was named VSDr1. |
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