Ecology of Basal Stem Rot Disease of Oil Palm (Elaeis guineensis Jacq.) in Cameroon

Basal stem rot (BSR) disease caused by species of Ganoderma is of immense importance in oil palm production. Although much is known on the occurrence of this devastating disease, fundamental studies on the ecology in oil palm in plantations are rather limited. This study sought to determine the incidence, severity, distribution and spread pattern of BSR disease in oil palm plantations and relate disease parameters to climatic and edaphic factors. Surveys were carried out for two years on two–hectare plots in each of five oil palm estates of the Cameroon Development Corporation. Data for disease incidence and severity in each estate were recorded. Disease spread patterns were generated from Arc GIS version 9.3 using GIS coordinates of diseased plants. A correlation between disease parameters and soil physicochemical properties and multivariate analyses were done. Typical BSR disease symptoms were observed including unopened spear leaves, skirt–like appearance of leaves, basidiocarp formation, bole creation and death of the palm. The disease incidence ranged from 5.4% in 16-year old palms at Bota to 39.0% in palms of the same age in Mungo were about 50% of infected plants had extreme severe symptoms. Although principal component analysis showed that six soil properties account for variation in BSR disease incidence and severity, only fine sand content was positively correlated (P≤0.05) with disease incidence and severity, while C/N ratio was negatively correlated. This study has established the occurrence and spread of basal stem rot disease in five oil palm plantations in South Western Cameroon.


Introduction
Oil palm, Elaeis guineensis Jacq is an important oil crop in Cameroon where over 230,000 tons of crude palm oil is produced in about 190,000ha [1]. In the past, production was based on the Dura type palm whose production was low, but with the development of plantation agriculture, the more productive Tenera hybrid is widely cultivated. This hybrid produces up to eight times the amount of oil produced by other vegetable oil seeds like soybean and sunflower [2]. The inflorescence is also tapped for the much cherished palm wine.
Generally, oil palm suffers from relatively few important diseases in each of the different environments where it has been planted commercially. In Southeast Asia, basal stem rot (BSR) disease caused by species of Ganoderma is the most important disease of oil palm [1]. Although in several African countries, vascular wilt caused by Fusarium oxysporum f.s. elaeidis was thought to be the only disease causing serious problems in some plantations, BSR disease has become one of the major diseases of oil palm, especially in Cameroon [4]. The disease was first recorded in Malaysia where it was initially considered a disease of older palms as it occurred in palms of over 25 years [5]. The BSR disease has also been recorded in Malaysia, Indonesia, Nigeria, Ghana, Zaire, Angola, Tanzania, North Mozambique, Papua New Guinea and Cameroon [5,6]. The causal agent of basal stem rot disease in Malaysia was first identified as G. lucidum (W. Curt.) Karst [5]. At least seven species of Ganoderma have been associated with BSR of oil palm in Malaysia, Indonesia, Papua New Guinea and Cameroon including G. boninense Pat., G. miniatocinctum Steyaert, G. chalceum (Cooke) Steyaert, G. tornatum (Pers.) Bers., G. zonatum Murill, G. xylonoides Steyaert, G. ryvardense Tonjock and Mih and G. lobenense [7,5,6,8,9]. BSR disease has been found to infect oil palms as young as 1 to 2 years of age, and is serious on palms aged 4 -5 years of age, particularly in replanted areas [10]. In new oil palm planted from jungle or old rubber plantations, BSR incidence of 25% has been recorded after 25 years while in that planted from old coconut plantations, an incidence of 60% occurred after 16 years [11], whereas oil palm to oil palm under planting has resulted in 33% infection after 15 years. The highest disease incidence is in coastal areas [5,12]. In Malaysian coastal areas, a 50% loss of yield was recorded from 80% disease incidence on 13 year old plantings [13]. A survey has also reported typical levels of disease incidence of 30% on 13 -year old palms in both inland and peat soils [14]. In North Sumatra (Indonesia), by the time of replanting (25 years) 40 -50% of palms are lost in some fields with the majority of standing palms showing disease symptoms. Where oil palm stumps were left in the ground at replanting then more serious palm losses due to Ganoderma have been observed in some fields with up to 25% incidence that occurred within 7 years [15].
The natural infection with Ganoderma in oil palm occurs as a result of contact between healthy roots and diseased tissues left buried in the soil [16]. Subsequent spread occurs by root to root contact once a few palms are infected [17].
In young palms, the external symptoms of basal stem rot normally comprise a one sided yellowing, or mottling of the lower fronds, followed by necrosis [10]. The newly unfolded leaves are shorter than normal and chlorotic, and additionally the tips maybe necrotic. As the disease progresses within the plant, the diseased palm may take on an overall pale appearance, with retarded growth and the spear leaves remain unopened [10]. Affected leaves die, necrosis sets in, beginning with the oldest leaves and extending progressively upwards through the crown. Dead dessicated fronds droop at the point of attachment to the trunk or fracture at some point along the rachis and hang down to form a skirt of dead leaves.
Although there have been sporadic reports of basal stem rot in Cameroon, there is no comprehensive study on the ecology of the disease that could guide its management. The objectives of this work was therefore to determine the incidence, severity, distribution and spread pattern of basal stem rot disease in south western Cameroon and see how the disease relates to climatic and edaphic factors.

Establishment of Sampling Plots
The ecology of basal stem rot was studied in five oil palm estates belonging to the Cameroon Development Corporation (CDC) in south western Cameroon (Fig. 1). The means of various environmental parameters are shown on Table 1.
In each estate, four 2ha plots were mapped out, each of the same age as shown on Table 2. Location of plot was done through a stratified random sampling technique.

Disease Scoring
Each plant in the sampling plots was observed for symptoms of BSR during the wet (June -August) and dry (October -December) seasons of 2010 and 2011 respectively, and scored for BSR severity on a scale of 0 -4 as shown on (Table 3) according to the method of Abdullah et al. [18]. Adapted from Abdullah et al. [18] The disease incidence was calculated as follows; .

100%
(1) Where I= Incidence To assess the severity of the BSR disease, the disease severity index (DSI) was calculated using the method of Abdullah et al. [18] thus: (2) Where: A -Disease class (0, 1, 2, 3 or 4) B -Number of plants showing that disease class per estate. The Geographical Positioning System (GPS) point of each symptomatic plant was recorded. The data were processed using arc GIS version 9.3, to generate the disease spread pattern map.

Soil Sampling and Analysis
For each of the estates surveyed, five core soil samples were collected at a depth of 0 -10 cm, bulked, mixed thoroughly, air dried and sieved through a 2mm sieve.

Results and Discussion
Both asymptomatic and symptomatic plants of various levels of severity were observed in the field. Symptoms observed in the field are shown on Fig. 2. These ranged from presence of multiple unopened spear leaves at the centre, to skirt-like appearance of the leaves. Generally, the symptomatic palms had a pale appearance. Other symptoms observed were, production of fruiting bodies, bole creation on the base of the trunk, and finally death of the palms. The symptoms were typical of those described for the disease [19]. These are different from symptoms of vascular wilt of oil palm in that there is dryness of the lower leaves, the breaking of the rachis at about one third the length from the trunk, the hanging of the dry leaves along the trunk for the typical or acute form, the narrowing of the trunk at the top taking a "pencil -point" appearance, and the cracking of the trunk resulting from deterioration of the vessels for the chronic form [20,4]. During the first year of observation, the incidence ranged from 4.6% in Beneo estate to 38.0% in 16 year old palms at the Mungo estate ( Table 4). The incidence was generally high in young palms (≤16years) when compared to old palms (>30years) which recorded an incidence range of 14.2 -20.7%. The second year of observation showed a general trend of increasing incidence from 6.8% in Bota to as high as 40.0% in Mungo. Considering the two years of observation, the average disease incidence was highest at Mungo in palms that are at about their peak production age. Except for Mungo, the incidence of basal stem rot was generally low when compared to reports from other parts of the world on palms of comparable age. For example, values of 85% have been reported in 25 year old palms in Malaysia [21] and 30% in 13 years old palms in Malaysia [14]. The basal stem rot severity index was highest in Mungo where the mean for the period of observation was 80%, rated as very severe (Fig. 3). This is supported by the fact that the proportion of symptomatic plants with a score of five was highest there (Fig. 4). The high severity indices are an indication of the threat of this disease to production. Other observations on small holder fields in the Mungo area have similarly been shown to be highly infected, resulting in death of palms [1]. Basal stem rot disease spread showed a cluster pattern of spatial dispersion with only Bota showing a sparse pattern (Fig. 5). The disease had a dense cluster pattern in Mungo, Mondoni, Beneo, and Idenau estates. The spread pattern of BSR disease in this study was typical of soil-borne diseases which occurred in patches [22].    They typically appear in clusters or patches. The clustered pattern of infected plants was recorded at a number of sites and this was analogous to the pattern of basal stem rot reported by Rao et al. [14]. The mode of survival of Ganoderma as hyphae in oil palm residues would favour a clustered pattern of infected plants since dispersion of the residues of each plant would tend to overlap with that of other plants and a continuum of infested residue may result as the incidence of infected plants increases. Thus, under environmental conditions which favour a high incidence of infected plants, the pattern of infected plants would become regular. Soil physicochemical properties varied across the estates ( Table  6). Of these properties, fine sand had a significant positive correlation with disease incidence and severity at 0.01 probability level, while C/N ratio had a significant negative correlation at 0.05% probability level. There was no significant correlation between any of the climatic factors and disease incidence. This does not preclude the indirect effect of climate through the vigour of the plant and soil. This observation is expected because the disease is soil borne and systemic. Results of the principal component analysis (PCA) showed that the first four principal components explained 100% of the total variation ( Table 7). The PC 1 is strongly associated with fine silt, pH in KCl, organic carbon and total organic matter. PC 2 is strongly associated with pH in H 2 O, base saturation and Sodium. PC 3 is strongly associated with total nitrogen, clay content, base saturation and fine sand, and PC 4 is strongly associated with coarse silt, moisture content, cation exchange capacity and water holding capacity ( Table 8). The climatic data for Mungo was comparable to those of other sites. However, the edaphic factors were unique. The soils have a very high sand content, low clay content and consequently, low water holding capacity. The soil factors may partly contribute to this high incidence, given that there was a strong positive correlation between fine sand and incidence. Also, it may be due to the low nutrient status since there was a significant negative correlation between C/N ratio and disease incidence and severity. The low percentage of clay content of soil in Mungo estate might have been more conducive to the development of the basal stem rot disease than those with high clay contents. Soils low in clay content have been shown to be favourable to some soil borne diseases as is the case for damping-off of tomato seedlings caused by Sclerotium rolfsii in the Nigerian Savanna [23]. The incidence of basal stem rot would be much higher in future than the values obtained in the present survey because detection was based only on symptomatology. Plants at the early stages of infection generally do not manifest any symptoms. More elaborate techniques may be required to detect such plants [24].

Fig. 2. Field symptoms of BSR disease of oil palm in plantations of the CDC. A) Asymptomatic plant, B) Unopened spear leaves at the centre, C) Skirt-like appearance, D) Basidiocarp formation, E) Bole creation, F) Death of palm.
Tengoua and Bakoume [4] recorded a 40% incidence level of basal stem rot in Mussaka palm plantation of the CDC planted between 1967 and 1969 but they actually projected a 60% incidence level of basal stem rot disease. However, these were very old palms and their result was not comparable to the present result in terms of percentage disease incidence but it shows that this disease had been present in oil palm plantations of the CDC.
The high disease severity indices observed in the palms was expected. Plants with severity scores 3 and 4 constituted over 50% of the plants, thus resulting in the high disease indices observed. There was no management strategy put in place in the oil palm estates surveyed. The effects of the disease could easily be mitigated in Cameroon with proper management efforts. A preliminary survey in plantations of SOCAPALM where a rigorous eradication scheme was supplemented with trenching to curb spread revealed an extremely low incidence of less than 1%. However, in Malaysia a basal stem rot incidence of 80% and more were recorded in areas with attempted management strategies [14].
Fine sand was primarily the edaphic factor that influenced basal stem rot disease, having a significant positive correlation. High fine sand content (45.86%) found in Mungo estate allows for easy growth of roots thus enhancing contact and subsequent spread of basal stem rot. A very strong significant negative correlation was observed between incidence of basal stem rot and C/N ratio. Soils with low C/N ratios tend to release nutrients fast [25]. These nutrients easily leach away in the low water holding capacity soils thus starving plants and predisposing them to attack. Unsuitable soil conditions for plant development generally arise from lack of organic matter content in the soil [26,27]. Higher content of easily decomposable organic matters might be associated with higher microbial activity and ultimately lead to the decline of Ganoderma species population. Thus higher population of Ganoderma may aggravate the disease situation in locations where soil reaction is more acidic and organic carbon content is less as in the case of Mungo estate. The lower disease incidence in other estates may be attributed to high organic carbon contents and weak acidity. Similar results were obtained by Sharma et al. [28] for ginger rhizome rot.

Conclusion
This study documents the first comprehensive study on the incidence, severity and associated factors of Ganoderma disease of oil palm in South Western Cameroon, after the initial report by Tengoua and Bakoume [4]. The incidence and severity of the disease shows that it is of increasing importance in oil palm production such that its management needs to be considered in the oil palm production plan, given that the disease is very devastating in Malaysia and other oil palm producing countries of the Indian subcontinent. The pattern of disease occurrence and distribution was attributed to variations in edaphic factors which are important in pathogen survival and host predisposition. Although there was no correlation between incidence and severity of BSR with climatic factors, of Oil Palm (Elaeis guineensis Jacq.) in Cameroon principal component analyses showed the indirect influence of climate on the disease. Although no resistant clones are known for the disease, seed producers in Cameroon should screen for resistance in different progeny crosses. Also, there is need to monitor the disease on a continuous basis.