Basal stem rot of oil palm: Disease development in mineral and peat soils

Basal stem rot (BSR) of oil palm, caused by Ganoderma boninense, is a major disease of economic concern in Southeast Asia, especially in Indonesia and Malaysia. The disease causes palm death and the reduction in the oil palm stand, results in lower yields and serious economic losses. In Indonesia, incidence of BSR has been reported in both mineral and peat soils. To control BSR in peat and mineral soils, an understanding of disease development is necessary. Disease developmental studies were conducted at two oil palm plantations in North Sumatra, representing mineral and peat soils. Palms with mild BSR infection were recorded and disease progression monitored. After 37 months, critical symptoms of basal stem rotting were observed. The percentages of palms with advanced disease symptoms in mineral and peat soils were 62.5% and 90.6%, respectively. The observations indicated that BSR symptoms developed faster in oil palms planted in peat soil compared to those in mineral soils.


Introduction
Basal stem rot (BSR) caused by Ganoderma boninense continues to be the main threats to the South-East Asian oil palm industry, causing decline of the palm stand.Incidence of Ganoderma has been observed in mineral and peat soils in Indonesia and Malaysia [1].Basal stem rot (BSR) became a problem in the 1960s when the disease was detected in younger palms at 10-15 years old oil palm.In first generation of oil palm converted from coconut, high incidence observed with 35 -39% infection, while in first generation converted after rubber, 2-4% infection observed [2].Later, BSR increase in 4-5-year-old, continued at 12-24 months especially in the first generation of oil palm [3] or oil palm with coconut palm in under-planting system [4].In oil palm converted from jungle or from rubber, 25% incidence occurred after 25 years.However, in oil palm converted from coconut, 60% BSR incidence observed after 16 years.In addition, 33% Ganoderma incidence could be observed after it was converted to oil palm [3].
BSR associated with G. boninense causing significant crops decline with shorted economic oil palm period [8].In Malaysia, it was estimated that financial losses of BSR disease ranged from RM 225 million to RM 1.5 billion a year [9].In Indonesia, the financial losses for every 1% BSR incidence was estimated at US$ 256 million [10].On average, where 50% palm die caused by Ganoderma, 35% yield losses was reported [11].
Some crop protection control methods such as sanitation during replanting, cultural technique, fungicides, and biological control have been applied to mitigate BSR disease in the field [5,12].This includes planting with moderately tolerant materials [13].No single method completely controls the disease; therefore, integrated disease management is suggested [6].Observations of disease development based on disease severity of infected palms is important as prediction on the survival of the infected palms can be estimated, and thus potential yield.
Ganoderma disease is common in mineral soils [12].In peat soils it was thought that the disease was less problematic [1,14].However, serious diseases have been observed [15] and Ganoderma disease occurring in young palms [16].Today, BSR disease in peat soil is a major concern with high disease incidence observed [17].

Materials and methods
Observations of existing Ganoderma infected oil palms were carried out in two oil palm plantations in North Sumatra: Simalungun (mineral soil) and the South Labuhan Batu (peat soil).Observations were made over a 37-month period, starting in May 2019 until May 2021.The oil palm in mineral soil was planted in 2006 and this was a second-generation planting, with the first-generation of oil palm planted in 1979.One block was selected as it showed 42% BSR infection which was regarded as high disease incidence with many of the infected oil palms at Stage 2 (mild infection).
Oil palm in the peat soil was planted in 2006, and this was a first-generation planting.Four blocks were selected in which the disease incidence ranged from 0.4 -1.1%, which was considered high for first generation oil palm planted in peat soil.In the first disease incidence census (May 2019), eight BSR oil palms were selected per plot with four replicates in both mineral and peat soil area.The assessment of BSR disease development was adapted from Chen et al. [18] with some modifications as noted in Table 1.

No
Infection Stages Symptoms 1.
Stage 0 (Healthy palm) Green fronds with open spear leaves.

Stage 1 (suspected of BSR infection)
Pale young leaves, yellowish, unopened spear leaves, no sign of basidiocarps.

Stage 2 (mild BSR infection)
Fronds collapsed, pale and yellowing of the fronds and leaves, unopened spear leaves with basidiocarps starting to form at the base of the oil palm tree.

Stage 3 (severe BSR infection)
Fronds collapsed, pale and yellowing of leaves, drying of fronds (especially older fronds).Basidiocarps at the base of oil palm trunk.

Stage 4 (critical BSR infection)
Rotting of basal trunk area or upper part of the stem, die back at the tip of fronds, frond collapse and dead fronds.Basidiocarps appear, moribund palm or collapse of the trunk.

BSR infection in mineral soil
Observations of oil palm BSR in mineral soil started in May 2019 and mild BSR infection (Stage 2) was noted (Figure 1a).At 12 months of observation (April 2020), mild infection (Stage 2) decreased from 100% to 87.5%, and 12.5% of palms were at critical infection stage (Stage 4).Approximately 12.5% of oil palms with mild infection (Stage 2) developed directly to critical infection (Stage 4), palms with severe infection (Stage 3) were not observed.Symptoms of mild infection were like those observed at the beginning of observations.At 24 months of observation (April 2021), the percentage of infected palms at Stage 2 decreased from 87.5% to 46.9% (Figure 1a and Figure 2).The percentage of the palms at Stage 3 was 6.3%.Stage 4 palms increased from 12.5% (from the previous observation) to 46.9%.At 37 months of observation (May 2022), the percentage of infected palms at Stage 2 decreased from 100% at the beginning of observations to 34.4%.Infection at Stage 3 decreased to 3.1% whereas palms at Stage 4 increased to 62.5% (from 46.9% in previous observations).Twenty infected palms showed collapsing fronds with dying and dead trees and exhibited basidiocarps at the base of the stem (Figure 2).At 24 months of observation (April 2021), the infected palms at Stage 2 reduced (from 50% to 18.8%), but the percentage of palms at Stage 4 increased from 50% to 81.3%.Stage 3 infection was not observed.At 37 months of observation (May 2022), the percentage of Stage 4 palms increased to 93.8% (from 81.3% at 24 months of observation).Only 3.1% of the infected palms were at Stage 2 and 3.1% at Stage 3. Observations of BSR development of oil palm planted in mineral and peat soils started in May 2019 at which time mild infection (Stage 2) occurred in all palms in the selected blocks.Severe infection (Stage 3) with 3.1 -15.6% occurred after 14 months in mineral soils and after 5 months with a lower percentage of infected oil palm (3.1%) in peat soil.
Critical infection (Stage 4) occurred at 3 months in peat soil, which was earlier than mineral soil.Table 2 shows significant differences between BSR infection at Stages 2 and 4 for both infected palms in peat and mineral soils at 37 months of observations with P≤0.1.The results show that BSR infection (from mild infection, Stage 2) developed faster to critical infection (Stage 4) in palms planted in peat soil than mineral soil.The results show faster disease development in peat soil compared to soil (P≤0.1, table 2).Similar observations were reported by Wirianata et al. [19] in which G. boninense infection was higher in peat soil than mineral soils.The existing pathogen inoculum in second generation plantings also contributes to the epidemic of BSR infection.Organic matter in peat soil and plant residues are important sources of nutrients for pathogenic fungi including G. boninense to complete the disease cycle in the plantations [20].The BSR incidence in 2 nd and 3 rd generation plantings is also associated G. boninense inoculum present in the soil.The results show faster disease development in peat soil compared to mineral soil (P≤0.1, table 2).Similar observations were reported by Wirianata et al. [19] where G. boninense infection was also higher in peat soil than mineral soils.In addition to soil type, infection rate faster might be due to earlier disease inoculum in second planting generation.Furthermore, plant residues and peat soil organic matter are good nutrients for G. boninense as in completing the cycle as pathogenic fungi [20].Rakib et al. [21] also reported that nutrient deficiencies of Cu and Zn related to Ganoderma incidence.Cu and Zn deficiency are common in peat area.Cu has role in mechanism of plant defence against diseases, e.g., as peroxidase and laccase enzymes cofactor [18,22].

Conclusions
Based on 37 months of observation, BSR infection developed faster in oil palms planted in peat soil compared to mineral soil.The number of palms which developed from Stage 2 to Stage 4 was 62.5% in mineral soil and 93.8 % in peat soil showing clearly the faster rate of development in peat soil.

Figure 2 . 4 3. 2 .
Figure 2. Percentage of infected oil palms in mineral soil at Stages 2, 3 and 4 of infection over time.

Figure 4 .
Figure 4. Percentage of infected oil palms in peat soil at Stages 2, 3 and 4 of infection over time.

Table 2 .
Percentage of infected oil palm and BSR infection stages in mineral and peat soil.