ecological toll discredits this model of agriculture and makes it unsustainable (Jepson et al., 2014; Sharma et al., 2019). The chemical plant protection products extensively used in conventional agriculture have drastic effects on non-target species and affect animal and plant biodiversity, as well as aquatic and terrestrial food webs (Tudi et al., 2021). The use of chemical plant protection products is thus of increasing concern for both agriculture itself and the environment as well as human and animal health (Warra & Prasad, 2020). Improvingagriculturalproductionsystemstowardnew modelsofsustainable,resilient,andenvironmentally- friendly agriculture is becoming an urgent need (Dornbush & von Haden, 2017). The agroecological strategy of relying on ecological pro- cesses of interest, within agroecosystems, to maximize pro- ductivityandsustainability,whilereducingtheuseof chemical inputs is gaining acceptance. Agroecosystems thus represent a preferred pathway to resilient and future agricul- tural systems (Bennett et al., 2021). Indeed, inspired by nat- ural ecosystems, which are the result of an adaptation of animal and plant communities in a given environment, and thus to specific biotic and abiotic constraints, these systems provide a model for sustainability and resilience (Ngo Bieng et al., 2013). Among these characteristics of natural ecosys- tems, species diversity is assumed to play a key role in the sustainability and multifunctionality of agroforestry sys- tems. Thus, an increase in plant diversity in agroecosystems is assumed to improve the provision of many ecosystem services, including pest regulation (Schreefel et al., 2020) Agroforestry is a dynamic and ecological resource man- agement system that, through the integration of trees into agricultural and rangelands, diversifies and sustains small- holder production for increased socio-economic and envi- ronmental benefits. In recent decades, various studies have described and characterized the structure of cocoa-based agroforestrysystems(CBAS)(AkoutouMvondoetal., 2020; Gidoin et al., 2014). In the Cameroonian rainforest areas, citrus trees are mainly produced in CBAS (Akoutou Mvondo et al., 2019; Ndo, 2011). Although various citrus species/varieties are grown in Cameroon, the main species found areC. sinensis, C. reticulata, C. paradisi, C. maxima andC. limon(Fig.1A). In agroforestry, the term shading is related to the microcli- matic alterations that occur in the understory due to the pres- ence of an upper vegetation canopy in the agroforestry system. The presence of trees associated with a crop buffers air and soil temperature, decreases wind speed and the quan- tity and quality of transmitted light ; and increases relative air humidity and soil moisture in the understory (Tscharntke et al., 2011). The effect of shading on pests is controversial (Schroth et al., 2000). While dense shade favours the spread of some pathogens, light shade or no shade may limit the development of other pathogens (Akoutou Mvondo et al., 2019). Several studies have shown the implication of shad- ing on the development and spread of various pathogens (Durand-Bessart et al., 2020). The effects of shade have been documented; and light, especially ultraviolet light has been shown to stimulate sporulation of many fungal species. In addition, microclimate can indirectly influence pathogen through its impact on the host plants (Pumari~no et al., 2015). Phytophthoraare important soil and waterborne patho- gens responsible for significant damage in agricultural and natural ecosystems. The genusPhytophthorarepresents a significantandemergingbiosecuritythreatworldwide (Puglisi et al., 2017). To nowadays, about 182Phytophthora species have been formally described, with an estimated total of 326 species covering 12 phylogenetic clades (Bose et al., 2021). Of thesePhytophthoraspecies, about 13 infest citrus (Asim et al., 2019).P. nicotiana(Syn.P. parasitica), P. palmivoraandP. citrophthoraare the most damaging species worldwide and are therefore the most economically important (Gade & Lad, 2019).Phytophthorafoot rot dis- ease (PFRD) is thus a major constraint and a global concern for the citrus industry (Ramallo et al., 2019;Chaudhary et al., 2020).Phytophthoralosses on citrus result from seed- ling dieback in the germplasm, root and crown rots in the nursery and infield. Foliage dieback resulting from root rot precipitates tree death and limits orchard life span. Pre- and post-harvest fruit rots also result in huge production losses (Graham & Feichtenberger, 2015). Among these symptoms, citrusPhytophthorafoot rot disease (PFRD) is the most severe, and causing the most damage in the major citrus pro- ducing areas, as well as in Cameroon (Fig. 1B and C) (Akoutou Mvondo et al., 2019, 2020). PFRD managing requires a complex and costly set of strategies (Gade & Lad, 2019). Of all these methods, the chemical control approach is the most widely used in the major citrus growing areas (Sawake et al., 2022). In Came- roon, however, most citrus production is provided by small- holders, for whom these methods are not feasible and too expensive. Production systems have remained traditional, and cultural practices empirical (Ndo, 2011). Citrus varieties used are not always tolerant to PFRD, and plots were estab- lished with ungrafted seedlings. In addition, ever-increasing concerns about environmental and soil health are driving the search for more environmentally friendly means of disease control (Panth et al., 2020). Thus, there is an urgent need to develop targeted PFRD management strategies tailored to local production systems and context. This paper aims to evaluate the effect offloristic diversity in CBAS and the resulting shade rate on PFRD intensity. The basic hypothesis is that microclimaticalterations in CBAS contribute to reducing the intensity of PFRD. Materials and methods Study area and citrus production basins Five citrus production basins (CPB) were selected from four different ecologies: A.M. Etienne et al. / Basic and Applied Ecology 64 (2022) 134146135
Bokito CPB (Latitude: 4° 33059.9900N; Longitude: 11° 06060.0000E), located in the forest-savanna transition zone. The climate is hot and humid, with average annual temperature of 25.5 °C, average rainfall of 831.7 mm with a bimodal pattern, and an average relative humidity of 75%. CBAS are usually established in previous cropping areas (Jagoret et al., 2012b, 2018). Given the scarcity of native forest trees, fruit trees are abundantly introduced to provide shade for cocoa trees and are considered as valuable source of income. Evodoula CPB (latitude: 011°140592 ''E; longitude: 04°060226 ''N), located in the degraded forest zone. The climate is hot and humid, tem- peratures around 23.0 °C, average rainfall of 1727 mm with a bimodal pattern, and an average relative humidity of 85%. This site is located in the Lekie Sub-division which is currently the largest cocoa-produc- ing area in Cameroon. Cocoa is produced in complex and highly biodi- verse CBAS. Fruit trees, including citrus, are an essential component of these CBAS; Bikok (Latitude: 3° 38031.31 ''N; Longitude: 11° 31054.88 ''E) and Boumnyebel (Latitude: 3° 52059.500N; Longitude: 10° 51000.800E) CPBs are located in the dense forest zone. The climate is hot and humid, temperatures around 25.7 °C, average rainfall of 1800 mm with a bimodal pattern, and an average relative humidity of 84%. These two CPBs are different from each other, particularly in terms of the crops grown by the local populations. In Bikok, cocoa farming is practiced in complex and highly biodiverse CBAS, whereas in the Boumnyebel, farmers practice oil palm production in addition to cocoa farming. Thus, in the latter CPB, it is very common tofind palm trees associated not only with cocoa trees, but also with fruit trees such as citrus; Muyuka CPB (Latitude: 4° 17023.2800N; Longitude: 9° 24037.0800E), located in the humid forest zone with monomodal rainfall. It is a very humid and hot dense forest area, with average annual temperatures ranging from 22 to 29 °C and relative humidity of 85 to 95%. The average annual rainfall varies from 2500 to 4000 mm and even 11,000 mm in some localities. It is characterized by a 3-month dry sea- son (December to February) and a 9-month rainy season (March to November). Muyuka represents one of the largest cocoa production basins in Cameroon (Mukete et al., 2018). Here, cocoa production is intensive and farms are managed in such a way as to optimize cocoa production. this is done in particular by reducing the number of associ- ated trees. It has been shown that in this locality CBAS tend to be bi- specific, associating cocoa and fruit trees, generally citrus (Akoutou Mvondo et al., 2019). Across all of the CPBs, 33 plots were established includ- ing eight in Bokito, seven each in Bikok and Evodoula, six in Muyuka, andfive in Boumnyebel. Each CPB represents a geographical area with fairly homogeneous climatic and edaphic conditions and can therefore guarantee homoge- neous citrus production. Within each CPB, study plots were at least 3 km apart. In each CPB, CBAS plots with (i) at least 12 citrus trees (including all citrus species) on a surface of 2500 m2, (ii) diverse forest trees and (iii) fruit trees other than citrus, were selected. Mapping of CBAS and tree characterization CBAS mapping essentially consisted of taking the x and y coordinates of perennial plants present in the sample area in order to reconstruct CBAS and quantify shade rate. Mapping Fig. 1.Illustration of (A) an orange tree (Citrus sinensis) planted in a complex cocoa-based agroforest; (B) an orange tree attacked byPhy- tophthorafoot rot disease; (C) and a mandarin tree (C. reticulata) completely declined following a severe attack of thePhytophthorafoot rot disease. 136A.M. Etienne et al. / Basic and Applied Ecology 64 (2022) 134146