Oil palm (Elaeis guineensis Jacq.) is a perennial crop widely cultivated in tropical regions, with significant production in Southeast Asia, Africa, and South American. Native to the wild and semi-wild groves of tropical rainforests in West and Central Africa, as well as parts of South America, oil palm has been cultivated since the early 1500s (Corley and Tinker, 2015; Soh et al., 2009). Indonesia and Malaysia are the leading global producers of palm oil production, accounting for greater than 80% of total output (Murphy et al., 2021). In Africa and South America, oil palm production is primarily managed by smallholder farmers, with countries such as Ghana, Nigeria, Côte d'Ivoire, Cameroon, the Democratic Republic of Congo, Benin, Burundi, Angola, Tanzania, Colombia and Uruguay producing for local markets (Bakoumé et al., 2017; Corley and Tinker, 2015). Few plantations managed by companies are currently established in some of these African countries.

In Tanzania, the oil palm industry is undergoing revitalization, by achieving palm oil yield of 42,180 tons in 2018/19 derived from 26,190 hectares, primarily in the Kigoma, Mbeya and Pwani regions (Ministry of Agriculture, 2020). Oil palm cultivation is a key economic activity in these regions, offering employment opportunities across its value chain, particularly benefiting women and youth. Key activities within the crop value chain include the extraction and retail of crude palm oil, kernel palm oil and soap production, contributing to poverty alleviation (Walker et al., 2019). However, the area under oil palm cultivation has declined to 13,736 hectares, leading to reduced plant density and significant drop in palm oil yield, averaging to 24, 696 tons in 2019/20 (National Bureau of Statistics, 2021). This decline is closely associated with Basal stem rot (BSR) disease caused by Ganoderma spp., posing a major threat to oil palm plantations in Tanzania (Ibrahim et al., 2020; Idris et al., 2016; Wong et al., 2012).

BSR has also been persistent as a significant problem in other major oil palm-producing countries, including Malaysia and Indonesia (Ibrahim et al., 2020; Murphy et al., 2021; Zakaria, 2023). In Africa and South America, BSR has been reported in countries such as Cameroon, Côte d'Ivoire, Ghana, and Colombia (Abraham et al., 2023; Castillo et al., 2022; Lekete-Lawson et al., 2024; Mih and Kinge, 2015). The disease manifests through a complex external symptoms, including yellowing and wilting of the fronds, hanging dead fronds, accumulation of unopened spear leaves, the appearance of sporocarp on trunks, and eventual plant collapse (Chong et al., 2011; Corley and Tinker, 2015). The development of these BSR symptoms also are influenced much with soil characteristics, mainly soil chemical fertility which plays a crucial role in plant resistance to diseases (Ramdan et al., 2024).

Despite its severe economic impact, there is limited data on the prevalence and severity of BSR in Tanzanian plantations, creating a significant gap in disease knowledge. As the country seeks to expand its area under oil palm cultivation to boost palm oil production for self-sufficiency and export. these survey findings provide valuable guidance to farmers and other stakeholders. They indicate where new plantations can be established and advise against replanting in area with a high incidence of BSR, as remnants of previous infected palm trees serve as a primary source of Ganoderma inoculum-the causal agent of BSR in the plantations. Additionally, factors such as plant age and soil type are key step to understanding the disease's epidemiology and are critical steps toward developing effective disease management strategies for sustainable oil palm production in Tanzania.

BSR disease was observed in all oil palm plantations across the surveyed regions of Tanzania. Notably, BSR incidence and severity were lower in Mkuranga district compared to other regions, suggesting that factors beyond soil type and palm tree age may contribute to disease variation. Loam soil, which contains high decomposed organic material, retain more water, increasing soil acidity and promote Ganoderma spp. mycelial growth while limiting nutrient availability for plant physiological and biochemical functions (Peng et al., 2019). Additionally, oil palm trees aged 25 years and older are more prone to BSR disease due to its slow progression. A study conducted by Susanto (2012) revealed that BSR infection increases at rate of 1-2% in young palm trees, while an increment rate of 25% was observed in trees older than 25 years. Therefore, the typical external symptoms of BSR are more easily observed in aged oil palm trees, as the life cycle of Ganoderma spp. shifts to a necrotrophic phase, where ligninolytic enzymes degrade the palm tree's cell wall components including lignin and cellulose, once the plant's defense mechanisms are overcome (Ho and Tan, 2015). Other contributing factors include temperature and relative humidity. An in vitro study on ideal environmental conditions for Ganoderma mycelial growth revealed that temperature 25-32°C and relative humidity levels between 50-60% are the optimum for rapid mycelial growth (Peng et al., 2019). These conditions are similarly to the weather patterns in the study areas. Additionally, field management practices, such as sanitation and replanting, especially in wild and older plantations, influence the recorded BSR incidence and severity. Stumps and other palm trees remnants in the fields serves as the primary source of Ganoderma inoculum, facilitating new disease infestations during replating (Flood et al., 2005).

A positive correlation was found between palm trees age and BSR incidence, likely due to the expanded canopy architecture of mature trees, which creates higher humidity and soil temperature, facilitating the accumulation of active Ganoderma inoculum in the plantations (Rees et al., 2007). Furthermore, plantations with loam soil exhibited exceptionally high levels of BSR incidence and severity. The primary spread mechanism of BSR in plantation is roots contact between infected and healthy oil palm roots (Ho and Tan, 2015). Oil palm trees planted in loam soil tend to have abundant root biomass with a large horizontal spread, increasing root intersection and facilitating rapid BSR transmission within plantations (Corley and Tinker, 2015; Ho and Tan, 2015). The study findings revealed high incidences and severity of BSR in Kigoma, Kyela and Uvinza districts, primarily due to the presence of many wild oil palm plantations with aged trees. These findings align with those of Lekete-Lawson et al. (2024), who reported a 73% BSR incidence rate in oil palm plantations in Ghana. Similarly, Mih and Kinge (2015) reported BSR incidence rates ranging from 5.4% to 39.0% in 16-year-old oil palms in Cameroon.

Furthermore, naturally germinated palm trees from dropped seeds during harvesting may exacerbate the spread of BSR in surveyed plantations, contributing to the recorded disease incidence and severity in the surveyed districts. In contrast, Côte d'Ivoire recorded a significantly lower BSR incidence rate of 1.15% in Ehania at the early stage of disease infection (Abraham et al., 2023). The high BSR incidence in the surveyed districts can be attributed to the fact that most oil palm fields are over 30 years old, with some trees naturally germinated from seeds dropped during bunch ripening and harvesting.

Many oil palm plantations with severe BSR incidence had older palm trees, consistent with observations by Chen et al. (2017), who found that BSR incidence increased from 3.7% in 8-year-old oil palms to 42.2% after an additional 8 years. This slow disease progression may result from the long incubation period of external symptoms, which may only become apparent as the disease worsens (Naher et al., 2013). However, emerging reports from Malaysia and Indonesia indicate that BSR is beginning to affect young trees as well, raising concern about its impact on early-stage production (Zakaria, 2023).

High BSR incidence and severity were also recorded in oil palm plantations with loam soils compare to those with sandy loam soils. Soil characteristics significantly influence BSR progression. For instance, Malaysia reported varying BSR incidences across different soil types, with a low incidence of 8.72% in inland soils and high incidence of 27.7% in lateritic soils (Ibrahim et al., 2020). Similarly, Indonesia reported high BSR incidences of 90.6% in peat soils and 62.5% in mineral soils (Virdiana et al., 2024). These regional variations in soil composition particularly in terms of pH, magnesium (Mg), potassium (K), sodium (Na), and manganese (Mn) likely affect plant resistance to BSR and the pathogen's ability to thrive in these environments (Ramdan et al., 2024). Soil pH influences the availability and mobility of plant nutrients, and extreme acidity or alkalinity can inhibit nutrients uptake, leading to deficiencies in the plantations (Neina, 2019). Ganoderma mycelium requires a pH between 4 and 5 for rapid growth (Peng et al., 2019). Both plant and microbes need nutrients such as Mg, K, Na, and Mn for their resistance and aggressiveness (Peng et al., 2019). When Mg and K are adequate available, oil palms increase tissue resistance to degradation enzymes released by Ganoderma, as Mg enhances plant defense against BSR (Peng et al., 2019). Additionally, K inhibits the activity of ligninolytic enzymes and prevents Ganoderma from penetrating palm trees, leading to a lower incidence of BSR (Anothai et al., 2023; Ortel et al., 2024). Ligninolytic enzymes released by Ganoderma during the degradation of cell wall components requires Mn and Na to catalyze their reactions. This leads to a higher BSR incidence under condition of high Mn and Na availability (Schmidt and Husted, 2019).

Moreover, replanting practices in older plantations have likely contributed to the observed increase in BSR incidence and severity. Ongoing replanting in regions such as Malaysia and Indonesia have shown significant increases in BSR infestation rates. When this practice is carried out without complete removal of infected oil palm trunks and remnants in the field, it may inadvertently promote the spread of the disease at an early stage of the oil palm life cycle (Ibrahim et al., 2020; Priwiratama et al., 2020).

These findings provide essential guidance to farmers for managing BSR in their plantations, especially considered that current BSR management relies largely on cultural practices such as field sanitation and windrowing. Exclusion of plant tissues from previous plantings is a common practice in many infected plantations in Malaysia, Indonesia and Papua New Guinea. This practice ensures the removal of basidio-spores that might develop from previously infected plant remnants, thereby helping to minimize the spread of the disease to the next planting generation (Chong et al., 2017; Pilotti et al., 2018). The exposed distribution of the disease in the surveyed districts will assist farmers in selecting appropriate areas for establishing new plantations, helping to prevent the continued spread of BSR. The findings also serve as a work-up call to Tanzanian farmers to begin replanting once palm trees reach 30 years of age-the recommended thresh-old for commercial productivity. This approach helps limit the buildup of Ganoderma inoculum, compared to the current situation where palm trees may be left in plantations for over 50 years. In addition, the traditional oil palm production system in Tanzania-where palm trees are the part of productive landscape-remains a major limitation to effective BSR management. Therefore, management interventions, such as raising disease awareness among smallholder farmers and promoting proper field sanitation are crucial to reducing BSR risks, particularly in older plantations.

The survey highlights the widespread of BSR disease across the major oil palm-producing regions of Tanzania. Factors such as palm tree age and soil type significantly influence disease incidence. Without effective mitigation and management measures, BSR incidence is likely to continue to rise particularly, in newly established oil palm fields. The findings are particularly concerning for smallholder areas, where the continuous productivity of existing oil palm plantation is threatened, and effective management measures for BSR are not yet available. It is recommended that policymakers and oil palm stakeholders implement pro-active disease management measures to ensure sustainable oil palm production in the country.