Disease indices of incidence of white root rot of six
The rubber tree is subject to a plethora of economically important pathological problems, mainly of fungal origin (the basisdiomycetes) (Igeleke, 1998). In Nigeria, the most serious diseases of rubber seedlings and budded plants in the nursery are leaf diseases (Begho, 1990), while In mature plantation, the most devastating leaf disease is the South American Leaf Blight (SALB), and
Rubber tree exhibits natural resistance to invading root pathogens. Resistance often breaks down due to effect of pathogens that colonize living tissues of the tree to obtain nutrients as a result of the damaging and weakening of the plant with toxins or by preventing the plants defense mechanism (Jayasuriya 2004). A number of certain defense mechanisms in
The process of pre- infection involves pathogen breaking down the host cuticle and cell wall. Plants respond to infection process by producing anti-microbial compounds of low molecular weight (phytoalexins) (Darvill et al 1984).
The growth and spread of infective fungal pathogens from existing population have been on the increase with great virulency and inflicting damages even to resistant genotypes. The impact of fungal pathogens results in crop losses. The production of phenylalanine ammonia-lyase (PAL) is implicated as key enzyme in the plant phenyl propanoid pathway to catalyse the synthesis of phenyl lignin and phytoalexin from L-phenylalanine (Jones 1984). The synthesis of these anti-microbial compounds and the subsequent increase in PAL concentration are often useful resistance indicator in the host (Nicholson and Hammerschmidt 1992). Also, oxidases and peroxidases are known to be actively involved in polymerization of phenolic compounds in the lignin formation. In resistance mechanism action of peroxidase is related to initiation of hypersensitive cell collapse (Simons and Rose 1971).
Pathogenesis-related proteins(PR) is yet another defense response of rubber against pathogen infection (Narasimhan et al 2000) The PR protein induced by polyacrylic, acetyl salicylic (aspirin) and salicylic acids are known to increase resistance to pathogens (Gianinazzi 1984).
There is variation among high yielding genotypes in disease tolerance level. In this regard, some clones are resistant to virtually most of the diseases but are susceptible to few diseases. However, certain clones exhibit tolerance to few diseases but some others are susceptible to many diseases. There are few clones which tolerate characteristics of pathogens, and as a result of abiotic factors, produce new strains, and these strains can be more aggressive against rubber clones. Mutation is seen to be responsible for variability in pathogens, which involves changing sequence bases in the nuclear DNA, either by way of substitution or addition or deletion of one or many base pairs.
2. Mechanism of disease infection cycles
The host-parasite interactions involve attacks by
Analysis of root tissues (Geiger et al 1986) indicated that some enzymes – (CM – cellulose, pectinase, laccase) are present only in parasite tissues. It is explained that these enzymes are biosynthesized by the parasite and not by the host. However, it is yet to be shown that the fungi are able to perform biosyntheses of those enzymes.
3. Degrading enzymes of cell wall effects in pathogenicity
Involvement of CWDE goes through certain criteria. The synthesis of CWDE is not a proof of involvement in disease. Pathogens with low potentials for the production of CWDE, the ability of cells to reduce the viscosity of a polysaccharide or to grow on a polysaccharide implies synthesis of the relevant polysaccharide, and lack of growth can sometimes reflect inability to metabolize the end product.
Studies of transmission electron microscopy show valuable indications to the participation of CWDE. Microcopic alterations of walls of the infected tissue is displacement of wall fibrils indicating mechanical penetration, whereas extensive wall dissolution signifies freely diffusible extracellular CWDE. Loss of the middle Lamella implies action by pectic enzymes (Baker
In wood degradation by enzymes of white root rot fungi the structural elements cellulose, hemicelluloses and lignin synthesized and deposited in the plant cell walls reinforce the mechanical strength and rigidity of the stems of higher plants.
In the host specificity of wood rotters (Tuor
4. Pathogenicity of
rigidoporus lignosus and mechanism infection cycle
Infected trees show a general foliage discoloration, proceeded sometimes by premature flowering and fruiting. Affected tree branches die back until the whole canopy is destroyed and the tree eventually dies. In Nigeria, the foliage symptoms appear only when the tree is beyond treatment and recovery. The pathogen
When roots of infected are exposed, profusely branched white rhizomorphs are readily seen. The rhizomorphs are flattened mycelia strands of 1-2 mm thick that grow firmly attached to the surface of infected roots (Fig. 2).
The rhizomorphs grow rapidily and ahead of the rot and extend many meters through the soil freely hindrances from woody substrate. The internal progression of the development of the root rot pathogen is rather an ectotrophic growth characteristic. At infection point, the parasite penetrates the taproot down the soil. Nandris et al. (1987) explained for infection to take place, the rhizomorphs are subjected to morphogenetically state into infectious hyphae
exhibiting degrading extracellular enzymes capable of wood rotting. The authors further stated that this mechanism is strictly regulated by partial anoxia conditions in the soil. Following root infection, colonization within the taproot progresses towards the collar region and other parts of the root. Newly killed wood is brownish thereafter turns cream and soft. This shows fading of coloration along a gradient from the progression front of the parasite toward the tissues that were colonized before now. The effect of
Foliage of infected trees shows general discoloration, often preceded by premature flowering and fruiting. Branches of infected tree die back until the whole canopy is destroyed and the tree eventually dies. In Nigeria, usually the foliage symptoms appear only when the tree is no longer treatable. Exposure of roots around the collar infected trees is carried out by searching the collars with wooden like spades for the presence of rhizomorph filaments. The stick-trapping method is also a useful tool in detecting the growth of
Generally, two phases characterize the spatial spread of root rot disease in
5. Assessment of effect of white root rot disease on quantity and quality of rubber production
The level of incidence of white root rot disease and its effect on
0, 1, 2, and 3 are infection categories
a, b, c, and d are plants that fall into the infection categories
x is the maximum disease category which is 4
Disease indices recorded showed highest susceptibility score in PR 107 (30.55) and lowest score in GT 1 (14.71). Intermediate scores among other clones were equally high compared with PR 107. The lowest susceptibility in GT 1 indicated that GT 1 showed a significant level of resistance to the white root rot disease.
The result of the white root rot incidence in six clones are shown in Table 1.
Based on the susceptibility status recorded, the GT 1 Lowest susceptibility status was assessed for the volume of latex and dry weight of cup lumps from effects at different infection categories of white root rot incidence (Table 2). The ‘O’ infection category had the highest volume of latex (228.93 cm3), however, showed no significant difference with the rest clones. The highest infectivity category gave the highest dry weight of cup lumps (118.88g), followed by ‘0’ ‘1’ and ‘2’ categories but no significant difference among the clones was obtained. This may be attributed to increase in DRC in response to effect of the white root rot disease.
|Infection category||Volume of latex (cm3 )||Dry weight of cup lumps (g)|
6. White root rot observations of
Observations recorded at Rubber Research Institute of Nigeria (RRIN), Iyanomo, on the roots of some
|Clone||No. Planted Out||No. 9 Survival||No. with White Root Rot||No. of Dead From Root Rot||% Infection||% Survival|
|Clone||No. Planted Out||No. 9 Survival||No. with White Root Rot||No. of Dead From Root Rot||% Infection||% Survival|
7. Fungicide curative approaches
Reduction of root diseases is performed through some control approaches. Proper clearing of land is recommended especially mechanically clearing is the most effective clearing method. In the recent past in Nigeria, treatment of infected roots with root rot diseases involved excavating the soil around the roots of infected trees, and removal of surface rhizomorphs and roots sections penetrated by the pathogen. A long-lasting collar protectant dressing fungicides such as pentachloronitrobenzene (PCNB) is then smeared around the collar, tap root and basal portion of the main laterals of the tree, before replacing the soil. The same treatment is also given to immediate tree neighbors, especially those along the same row. This approach is rather labor intensive and quite difficult to apply on a large scale plantation.
Results of investigation on the effectiveness of calixin fungicide (a.i. tridemorph) at the RRIN mainstation, Iyanomo (Table 5) revealed significant reduction of white root rot disease of
|Clones||Dead/ No. Planted||No of Survivors||Pecentage of Plants cured|
8. Preventive measures against incidence of
Foliage discoloration of
Clearing methods such as uprooting and poisoning of old trees are found to reduce root disease incidence in replanting. The stumps from felled trees should be poisoned and the cut surface painted with creosote. This will prevent the root fungi of food sources.
Planting of legume cover crops helps to reduce root disease since they cause roots and stumps to rot faster (Malaysian Rubber Board 2000). It also asserted that about 30g of powdered sulphur should be amended into the planting hole. Sulphur is known to promote fungal growth antagonistic to incidence of fungal root diseases.
The use of collar protectant to paint the collar part of the tree is said to prevent root disease infection for few years (Malaysian Rubber Board 2000). Suitable fungicidal wound dressing should be applied to any damaged part of the tree. This prevents infective spores from gaining entry into the plant tissues. The digging of isolation trenches of one meter between infected and healthy trees prevents the spread of disease from infected plants to health trees. This method is rather difficult and may not be satisfactorily effective.
9. Treatment measures
On exposure of infected root system, all dead roots are removed, bulked and burnt. Soil particles attached to the roots are shaken off. After exposure, root surface are dried and suitable long lasting collar protect ant fungicide pentachloronitrobenzene (PCNB) is painted over the exposed infected root system, including collar, tap root and basal portion of the main laterals of the tree, before replacing the soil. The same treatment is carried out for the immediate tree neighbors, especially those along the same row. This method of exposing the root system and subsequent application of a collar protect ant fungicide is quite labour intensive exercise and uneconomical. The PCNB use is hazardous, being carcinogenic and has been banned (Rajalakshmy and Jayarathnam 2000). This method has been abandoned in most rubber growing regions of the world.
A less laborious approach is digging a slight furrow around the base of the infected tree and drenching the collar region with two liters of 0.5% calixin (a.i.tridemorph). Infected trees as well as two direct neighbors in the same row are treated every six months. Other suitable fungicides for the treatment of white root rot disease is the use of Bayleton (a.i. triadimefon), Bayfidan (a.i. tridimenol), Anvil (a.i. hexaconazole), Folicur (a.i. tebuconazole), Contaf (a.i. hexaconazole) and Daconil (a.i. chlorothaconil) (Jayaratne et al 2001). The use of these chemical fungicides as described above involving loosening the soil and forming funnel – like furrow around the base of the trunk, and then drench the chemicals into the furrow along the trunk of the tree with about 10-20 ml at every four to six months is highly effective.
The purpose of controlling root diseases is to remove their sources of infection or inocula at the very young stages of the trees in order to prevent the rest trees from being infected as they mature. Due to possible sources of infection it is necessary to put in place adequate management measures. In embarking on preparations for the management, the role of tapers is necessary since tapers know the locations of the diseased trees in the plantations and this however, simplifies task of locating the diseased trees. This explains the need to train tapers on how to recognize root diseases and other