The study was conducted from late December 2021 to late May 2022 and late December 2022 to late May 2023 in different agroecological conditions in three landscape units located in South-Eastern regions of Tanzania (Mtwara and Lindi) including coastal plains (Msimbati-Mtwara rural, S 10^°22´03.6´´, E 040^°23´46.4´´ 14 m above sea level), Makonde plateau (Chipite-Masasi, S 10^°22´´10.8´´, E 039^°12´57.7´´ 194 m above sea level) and Inland plains (Ungongolo-Liwale, S 09^°47´51.7´´, E 038^°02´03.1´´ 453 m above sea level). The climate of regions varies from coastal to inland areas; the mean annual temperature is 26°C in the coastal area and 24°C in the inland areas (Nene et al., 2017). The rainfall trend of the regions is characterized by a unimodal pattern commencing from late November and lasting until late April and the annual rainfall ranges from 810 to 1,090 mm (Dondeyne et al., 2003). Generally, the study areas are highly dominated by sand and sand-loam kind of soils.

Planting materials/seeds used for cover cropping practice were obtained from Tanzania Agricultural Research Institute-Naliendele center (TARI-Nalien-dele) located at Mtwara region, South-Eastern Tanzania.

The experiment included four different cover crops, specifically cowpeas (Vigna unguiculata), green gram (Vigna radiata), mucuna (Mucuna pruriens) and soybean (Glycine max) were used. These cover crops are good source of soil nutrients due to nitrogen fixation and the addition of organic matter. Their roots enhance soil structure by creating pores, which improve aeration and water infiltration. This fosters a more favorable environment for soil organisms, helps suppress weeds, and aids in pest and disease management. Additionally, they serve as a valuable food source in Tanzania.

In the experiment, five cashew trees exhibiting wilting symptoms with over 80% disease severity (equation i) were selected from a half-acre area (2,000 m²) in each cashew field. The experiment was designed as a randomized complete block, with each study site serving as a replication (block). Treatments were applied to single-tree plots, with test trees spaced 12 m apart from each other and from dead trees affected by Fusarium wilt. The experiment included five treatments: four cover crops and an untreated control (non-planted plot), each replicated three times per site. Each cover crop was planted randomly under the canopies of the infected cashew trees, from the edge of the canopies towards the stem. Planting spacing for each cover crop was 50×20 cm, except for mucuna, which was 50×30 cm due to its vegetative growth behavior. Each test cashew tree had eight rows of each cover crop. At the vegetative stage (45-60 days after sowing), four rows of each cover crop were chopped and incorporated into the soil, while the remaining four rows were left unincorporated until the harvesting stage.

Sampling points were identified by measuring 2 m from the stem towards the canopy end using a 5-m tape measure. Soils were taken around the cashew tree following compass direction (N, S, W, E) at the depth between 0-15 cm. Composite soils were collected from each test cashew tree, both non-planted and planted with cover crops.

The effect of cover crops was assessed on soil physiochemical characteristics before and after its planting. The physiochemical analysis was done for both non-incorporated and incorporated cover crop experimental plots. The composite rhizosphere soils were analysed for pH, electronic conductivity (EC in μS/cm), soil total nitrogen (TN in %) and soil total phosphorus (TP in mg/kg). EC and pH were determined with a soil to water ratio of 1:2.5 using electro conductivity and pH meters. TN content was analysed by digestion in concentrated sulfuric acid using Kjeldahl method (total kjeldahl nitrogen). TP content was analysed using Bray I & Olsen method while soil potassium (K in Cmol/kg), zinc (Zn in mg/Kg) and iron (Fe in mg/Kg) were determined using flame photometer and atomic absorption spectrophotometer respectively.

The isolation of Fusarium oxysporum f.sp. anacardi from the soils was done before and after cover cropping whereby a gram of soil was used for 10-fold serial dilution before culturing. Potato dextrose agar (PDA) was used for isolation of the fungal pathogen.

The effect of cover cropping practice on severity of the Fusarium wilt of cashew was evaluated on the cover crops planted experimental plots where non-planted experimental plots were used as control checks. The evaluation of the severity of Fusarium wilt was done monthly, starting at three months after planting. The evaluation was done for each cropping cycle (2021/2022 and 2022/2023) using a well-established leaf symptom expression scale (Lilai et al., 2021; Mbasa et al., 2021) as illustrated in equation (i).

Where ‘CF’ stands for cumulative frequency, which denotes the frequency of appearance (number of counts) of each rated degree of severity (score) under its respective scale. The percent midpoint range of the scale (PMRS) denotes the median of the two numbers under its respective scale, e.g., midpoint of 1-20% is 10.5 which follows under scale 1. Total number of cashew canopy sides used for disease rating denotes the four (4) equally divided cashew canopy sides as per compass direction (N, S, W, and E) using a 1×1 m² quadrat. “∑” Stands for summation which denotes the sum of the product of CF and PMRS.

Data analysis was done using GenStat Discovery Software (version 15.1 release PL 23.1; VSN International [VSNi], Hemel Hempstead, UK). A one-way ANOVA with equal replications (site-wise replication) was performed to test significance effects of cover crops on disease severity and soil-inoculum load at (P<0.05). A Tukey's Multiple Comparison Test was performed for treatments comparison and means separation. Spearman correlation was conducted to evaluate the correlation between severity of Fusarium wilt of cashew and soil physiochemical properties after cover cropping practice.

The results in Fig. 1 show the influence of cover crops (Fig. 2) on physiochemical properties of soils from three studied sites in two consecutive seasons (2021/2022 and 2022/2023). In both cropping cycles (2021/2022 and 2022/2023), soil pH, TN, TP, K and Zn increased after cover crops’ incorporation into the soils with exception for Fe which increased in both non-incorporated and non-planted experimental plots. The Fe content level in the soil was high for both non-incorporated (between 30.98 to 37.10 mg/kg) and non-planted (between 35.68 to 37.43 mg/kg) experimental plots compared to incorporated experimental plots (24.71 to 33.45 mg/kg). Generally, the Fe content in the soils for non-planted experimental plots (without cover crops) surpassed experimental plots with cover crops (both incorporated and non-incorporated plots). In both cropping cycles, TP decreased after incorporating mucuna and cowpea compared to non-incorporated plots which exhibited a slight increase of phosphorus (P) content in the soils. The levels of P content in the soil for the incorporated plots were between 5.79 to 6.26 mg/kg compared to non-incorporated plots (6.13 to 6.26 mg/kg).

The results showing the effect of cover crops on the population density of F. oxysporum f.sp. anacardi in the rhizosphere are presented in Table 1. Significant variation in the population density of F. oxysporum in the soil was observed between plots treated with cover crops and those left untreated (P≤0.05). Soil inoculum load was reduced in all plots where cover crops were incorporated, compared to the non-planted plots.

Likewise, plots without incorporated cover crops exhibited a significant reduction in soil inoculum load compared to those that were not planted (P≤0.05). Significant variation among the treatments was also noted for non-incorporated as shown in Table 1. Generally, both cover cropping practices reduced soil inoculum load compared to non-planted plots despite insignificance variations.

Fig. 3 presents the results demonstrating the suppressive effects of cover crops planted in infected cashew fields. All cover crops reduced severity of Fusarium wilt of cashew (P≤0.05). The level of disease reduction varied among the cover crops which ranged from 9.0% to 39.42% in first cropping cycle (2021/2022) compared to non-planted plots (57.12%) used as control check. In second cropping cycle (2022/2023), the severity of Fusarium wilt ranged from 7.0% to 33.50% compared to control check which recorded 62.50%. Mucuna was the most effective cover crop in both cropping cycles, which recorded lower severity of Fusarium wilt of 9.0% and 7.0% in 2021/2022 and 2022/2023 respectively followed closely by cowpea and green gram recording severity of 9.78% and 11.30% as well as 14.37% and 10.20% in the same seasons. Generally, all four cover crops displayed effectiveness on the reduction of Fusarium wilt of cashew despite insignificance variations among the treatments.

The present study displayed the correlation of soil physiochemical properties and severity of Fusarium wilt of cashew after application of cover crops in two cropping cycles (Table 2). The analysis unveiled both positive and negative correlations between the severity of Fusarium wilt and the soil physiochemical properties in cover-cropped plots (Table 2). Positive correlation for disease severity was found between Fe which portrayed by correlation coefficient of ρ=0.52, while negative correlations were revealed between K, P, TN, Zn and soil pH with ρ=-0.515, -0.310, -0.667, -0.619 and -0.333 respectively. Higher severity of Fusarium wilt of cashew (62.50%) was observed in non-planted experimental plots with higher level of Fe content, implying a positive correlation. The upsurge of soil mineral nutrients after application of cover crops revealed the reduced severity of Fusarium wilt of cashew ranging between 7.0-33.50% compared to non-planted plots (62.50%), implying negative correlation. Besides, negative correlations were also observed among soil physiochemical analysis. For example, negative correlations were observed between Fe (ρ=-0.395) and K (ρ=-0.515), Fe (ρ=-0.452) and soil pH (ρ=-0.333).

Generally, all four cover crops (green gram, soybean, mucuna and cowpea) influenced the increase of soil physiochemical properties compared to non-planted plots (Fig. 1). Most of the cover crops applied in infected cashew fields serve as economically important sources of nitrogen, P, K, and Zn, regardless of whether they are incorporated into the soil or not. For example, Muramoto et al. (2011) reported that a legume cover crop, such as cowpea is an important source of nitrogen into the soils, in which 90% of nitrogen is found in the crowns and the remaining 10% is found in the roots and stubble. Similarly, mucuna has revealed distinct characteristics on nodule formation. For example, a study conducted by Ennin et al. (2009) has shown a higher potential for nitrogen release from its plant biomass compared to many cover crops. Likewise, our current study demonstrated higher level of nitrogen release when both incorporated and non-incorporated. The soil mineral nutrients content slightly increased when cover crops incorporated in the soil compared to non-incorporated cover crops. Incorporating cover crops as green manure is widely conducted to enhance soil fertility through decomposition (Wen et al., 2017). The Fe content in the soil was observed higher in non-planted experimental plots than in planted plots (Fig. 1). High level of Fe content in the soil is induced by low pH in the soil (Fang et al., 2012; Florentín et al., 2010). The nature of soil where experiments were conducted are mostly sand or sand-loam, which experience low pH (Fig. 1). Correspondingly, a study conducted by Lilai et al. (2021) emphasized the role of soil pH on reduction of severity of Fusarium wilt of cashew. In addition to the previously mentioned benefits, cover crops are known to enhance soil structure by increasing soil organic matter and promoting the formation of soil aggregates. This improvement leads to greater soil porosity, allowing air and water to move more freely through the soil (Abdallah et al., 2021; Araya et al., 2022). Enhanced porosity facilitates root growth and improves root penetration and nutrient uptake. Furthermore, cover crops aid in retaining soil moisture by reducing evaporation and increasing water infiltration (Abdallah et al., 2021; Smith, 2018). The organic matter from decomposing cover crops boosts the soil's water retention capacity, making it more available for the main crop during dry periods (Mujdeci et al., 2019).

Cover crops enhance the development of fertile soils, promoting the attraction and growth of beneficial rhizosphere microorganisms (Teixeira et al., 2022). The presence of rich microbiome can also foster the development of pathogen-free soils (disease suppressive soils), hence lowering the population of the inoculum load due to natural occurring rhizosphere competition (Fang et al., 2012). For example, the presence of soil mineral nutrients such as P, K, and Fe enriched by planted cover crops have been observed to reduce the soil inoculum load compared to non-planted plots used as control check (Table 1). Similarly, studies conducted by Lilai et al. (2021), Orr and Nelson (2018), and Pérez (2014) emphasized the role of soil mineral nutrients such as P, K, soil pH and nitrogen on reduction of soil inoculum load. Moreover, high P and K content in the soil reduce severity of Fusarium wilt (Domínguez-Hernández et al., 2010; Novara et al., 2020; Woltz and Jones, 1981). On contrary, low level of Fe content increases soil suppression and reduce chlamydospore germination (Peng et al., 1999). The enriched soils contain potential biocontrol microorganisms that produce Fe-chelating compounds (siderophores). These compounds bind to Fe in the soil making it unavailable to pathogenic microorganisms, since Fe is critical for the growth and virulence of many pathogens (Abo-Zaid et al., 2023). Likewise, our current study found that high level of Fe content and low level of K, Zn as well as P led to high severity of Fusarium wilt in cashew.

Effectiveness of cover crops on suppression of soilborne diseases has been well documented in various studies (Dawadi et al., 2019; Fageria et al., 2017; Wen et al., 2017). Cover crops are capable of exhibiting natural disease suppression through provision of conducive rhizospheric conditions including soil mineral nutrients, temperature and physiochemical properties, hence enriching soil microbial populations, potential for biological control (Qi et al., 2020). Cover crops can protect plants from soilborne infection and lessening infection by destroying life cycles of soilborne pathogens, leading to low population of pathogens (Acharya et al., 2020; Berlanas et al., 2018; Qi et al., 2020). For instance, beneficial soil microbes, such as growth-promoting rhizobacteria (PGPR) induced by cover crops, are thought to enhance plant protection through various inhibitory mechanisms, including production of antibiotics against soilborne phytopathogens and competition for nutrients, space, and colonization sites (Dawadi et al., 2019; Larkin et al., 2010; Panth et al., 2020). These microbes also trigger Induced Systemic Resistance (ISR), a plant defense mechanism activated by beneficial microbes like Bacillus subtilis, Pseudomonas fluorescens, Rhizobium leguminosarum, Trichoderma harzianum, and Arbuscular Mycorrhizal Fungi (AMF) (Satish and Mehta, 2023). ISR involves the activation of signaling pathways regulated by jasmonic acid (JA) and ethylene (ET), which are key to plant defense (Kamle et al., 2020). Additionally, they produce various defensive compounds, including phenolics, pathogenesis-related proteins, and secondary metabolites, in response to ISR (Misra et al., 2023). Moreover, cover crops when used as green manure have been most effective in suppression of most soilborne pathogens (Larkin, 2014). Despite insignificance variation, mucuna and cowpea were far better than other cover crops in reducing disease severity (Fig. 3). The suppression of disease might have been due to diversity of the soil microbiota that occurs in the plant rhizosphere after enrichment of soils. Thus, the existence of microbial diversity leads to increased competition with soilborne pathogens for nutrients and space. In addition to the known benefits for soil health and disease suppression, cover crops can directly benefit the plants they are grown with. These benefits may include enhanced plant immunity by stimulating the plant's immune system, making them more resistant to diseases and pests, improved nutrient availability by fixing nitrogen or mobilizing other nutrients, eventually providing essential nutrients to the main crop and promoting healthier growth (Scavo et al., 2022). Cover crops have also been observed to release fungitoxic compounds that inhibit pathogens, enhance soil moisture retention, and ultimately improve soil structure and organic matter content, leading to better water retention and availability for the main crop (Everts, 2016; Quintarelli et al., 2022). Additionally, they reduce weed pressure by suppressing weeds, thereby decreasing competition for nutrients and benefiting the main crop (Dissanayaka et al., 2024).

As a result, these findings can be widely applied in cashew-growing countries as a sustainable strategy for disease management. Implementing soil amendments fosters sustainable agriculture by diminishing the reliance on chemical fungicides and protecting environmental health. Additionally, improved soil health and reduced disease prevalence can enhance cashew yields, thereby benefiting farmers and economies dependent on cashew production.

Several studies have displayed the relationship between soil physiochemical properties and severity of Fusarium wilt (Huang et al., 2012; Lilai et al., 2021; Orr and Nelson, 2018; Vukicevich et al., 2016). This aligns with our current study, which demonstrates positive correlations between Fe levels and disease severity, as well as negative correlations with soil pH. For example, our current study has noted increased severity of Fusarium wilt in non-cover cropped plots and reduced severity observed in cover cropped plots. Conversely, while the soil exhibited high Fe content, the soil pH was observed to decrease. As soil pH increases, the reduced form of Fe, ferrous Fe2+, transforms into less soluble or insoluble hydroxides or oxides (Abdelaziz et al., 2023). Thus, induced high soil pH might have contributed to suppression of Fusarium wilt of cashew (Burle et al., 1997). Cover crops have been also reported to attract potential soil microorganisms which portray bioactivities against the invading pathogens (Wen et al., 2017). For instance, Bacillus and Trichoderma genera are the most frequently isolated microorganisms in soil (Sayyed et al., 2012). Members of these genera act as siderophore-producing plant growth-PGPR or rhizofungi (Siderophore-PGPRs) (Abo-Zaid et al., 2023). These Siderophore-PGPRs serve as biological control agents by depriving pathogens of Fe nutrition (Sayyed et al., 2013). Additionally, they rapidly colonize plant roots and secrete a variety of antifungal metabolites, including antibiotics (such as phenazine-1-carboxylic acid, pyoluteorin, and pyrrolnitrin), volatile organic compounds (VOCs), and lytic enzymes (Sayyed et al., 2013). The Fe acquisition strategies in plants and pathogens is also well described by (Sun et al., 2023) in what they described as “the tug-of-war on Fe between plant and pathogen”. Negative correlations were observed between K, P, TN, and Zn with severity of the disease. This, implies that the decreased content of the soil mineral nutrients leads to increased severity of the disease and vice versa is true. Studies conducted by Gupta et al. (2010), Vukicevich et al. (2016), and Lilai et al. (2021) emphasized that nutrients levels in the soil determine the severity of Fusarium wilt on diverse of crops.

The study demonstrated that cover crops significantly improved soil physiochemical properties and reduced Fusarium oxysporum load and severity of Fusarium wilt in cashew trees compared to non-planted plots. Mucuna was found to be the most effective cover crop. These results emphasize the importance of integrating cover crops for sustainable disease management and soil nutrient enhancement. Further research is necessary to explore the mechanisms underlying pathogen suppression, host-pathogen interactions and the long-term impact of cover crop cycles on soil properties and disease management strategies.