Ganoderma versus mycorrhiza
AZIZAH HashimThis Basal stem rot (BSR) disease, which has long been recognized as the most annihilating disease of field palms in Southeast Asia, still reigns as the number one killer of oil palms and serves a disastrous blow to palm oil production. With the identification of the causal agent, i.e. Ganoderma boninense, several control measures have been recommended and implemented. However till today, none of the methods practiced has satisfactorily controlled or inhibited the spread or occurrence of this fatal disease. It is crucial to look for some other forms of control measures. The arbuscular mycorrhizal (AM) fungi offer a practical alternative that should be seriously considered and hopefully implemented.
What are AM fungi? The AM fungi are highly evolved, mutualistic associations between soil fungi and plant roots. Past and present AM research in Malaysia has concentrated on mineral nutrient acquisitions by these fungi front our highly weathered soils, hence designating AM’s role as a biofertilizer as well as a bioenhancer of plant growth, Recently, evidence obtained has proven further the fungal ability to secrete antibiotic substances and / or to produce secondary metabolites that could be utilized as a means of combating plant diseases, especially those of soil borne origin.
Several experiments were conducted to evaluate AM’s role as a biocontrol agent. The earliest work was on interaction of AM with one Ganodermaspecies, i.e. Ganoderma pseudoferum, which causes the red root disease of cocoa. Pre-inoculation of cocoa seedlings with AM significantly (pGanodermainfection as compared to over 10% mortality of seedlings in the absence of this symbiosis. An infectivity study done on 6-week-old oil palm plantlets showed the appearance of necrosis on the leaf five weeks after inoculation withGanoderma. A cross-section of the infected roots revealed breakdown of the cortical cells interspersed with the fungal hyphae, which penetrate right into the vascular regions. These symptoms were delayed in the mycorrhizal plants.
The promising results obtained prompted further research along this line. Two-leaf stage oil palm seedlings were subsequently inoculated with the AM fungi followed by inoculation with theGanoderma after six weeks of establishment with AM. The symbiosis successfully lengthened the incubation period of the pathogen to bring about infection or kill the young palms. All non-mycorrhizal palms succumbed to the disease nine months after exposure to the pathogen. Only 20% of the mycorrhizal palms showed the disease symptoms after nine months, with. only 10% mortality. Results from the pot trials were later confirmed in one preliminary field trial. A total of 120 palm seedlings, 60 mycorrhizal and 60 non-mycorrhizal, were planted in areas highly infested withGanoderma. After 10 months in the field, five of the uninoculated palms had died, while only two mycorrhizal palms had succumbed to the disease.
The mechanisms of defense as a result of mycorrhizal symbiosis could probably be strong competition between AM fungi and pathogen for space and host photosynthates. Healthier mycorrhizal plants directly or indirectly produce more vigorous seedlings with higher and stronger internal resistance to ward off disease, higher density roots produced in the Presence of mycorrhizal are able to compensate for loss of roots as a result of disease infections, significant Ca deposition in mycorrhizal cells, creates a physical barrier to advancement of the disease in the palm roots, higher deposition of secondary metabolites by mycorrhizal roots inhibits spread of the pathogen in the palm roots. This review paper aims to highlight these mechanisms and to determine how these benefits could be utilized or harnessed in the attempt to control this number one killer disease of oil palm, i.e. the BSR disease.
Tags: GANODERMA, BIOCONTROL, MPOB PUBLICATIONS, MYCORRHIZA, PLANT PROTECTION, oil palm, Basal stem rot
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