Impact of barriers on the onset of a Phytophthora megakarya epidemic in cocoa

Black pod rot of cocoa caused by Phytophthora megakarya causes significant losses in Cameroon. Normally (chemical) control efforts only begin when the first diseased pods appear. In theory, preventive control, focusing on reducing primary inoculum or barring it from infecting cocoa pods could alter the spatial and temporal development of a P. megakarya epidemic and reduce its impact. The principal source of primary inoculum is thought to originate from the soil. Therefore, the objective of this study was to study the effect of barriers between the soil and the cocoa tree, physical as well as chemical, on the onset of a P. megakarya epidemic. A completely randomized plot design was used, including five treatments with three replicate plots per treatment. Each plot consisted of 16 cocoa trees. Treatments were i) Control, no intervention, ii) Bare Soil, removal of the litter layer surrounding the cocoa trees, iii) Double Litter Layer, the removed organic matter (OM) from the bare soil treatment was added to this third treatment, iv) Ridomil, Ridomil was applied in three week intervals unto the OM surrounding the cocoa trees and v) Plastic Barrier, a plastic sheet was spread on top of the litter layer surrounding the cocoa trees. Data, the number of healthy and diseased pods below or above 3 meters above ground level, were recorded weekly. The cumulative number of diseased pods (CDP) as well as the pod rot rate (PRR) were calculated and plotted over time. The results showed that the bare soil treatment had the highest number of diseased pods as well as PRR, followed by the control, double litter layer and Ridomil treatments. The plastic barrier treatment had the lowest CDP and PRR, moreover, the first diseased pods appeared later in time compared with the other treatments. Pod rot rate and number of diseased pods was higher up to 3 meters than above 3 meters. The results of this study clearly demonstrate the importance of soil-born primary inoculum. Moreover, it demonstrates that it should be possible to delay the onset and subsequently reduce the severity of a P. megakarya epidemic, which could lead to less fungicide applications and higher yields. More attention should be given to the development of preventive control measures for P. megakarya pod rot. (Texte intégral

Dispersal and distribution of Phytophthora megakarya in young cacao plantations

The arrival and subsequent dispersal of Phytophthora megakarya within young cacao plantations, previously free of black pod and containing cacao germplasm that is evaluated for its resistance to P. megakarya, is the subject of an ongoing study in Cameroon. A total of 12 plantations, established in 2006 on lands free of cacao cultivation for at least 20 years have been closely monitored on a weekly basis since June 2009. Near the end of 2009, the first infections were observed in three plantations, with a slow but gradual increase over 2010 and 2011 in the number of plantations, trees and pods that were affected. The spatial as well as temporal distributions of infections seem to indicate that multiple P. megakarya introductions occurred and that dispersal by humans plays an important role. Moreover, spatial distribution is not only a function of dispersal mechanism(s) but also depends on other factors such as microclimate and cacao genotype. Understanding the factors that govern the establishment and subsequent dispersal of P. megakarya in newly established cacao plantations will help farmers to delay the onset of cacao black pod epidemics. (Résumé d'auteur

Trichoderma asperellum PR11 soil treatments for Phytophthora megakarya control

Many Phytophthora species have a soil borne phase in their natural life cycles even though disease expression often occurs on aerial plant parts. In Cameroon, black pod rot of cacao (Theobroma cacao L.), is caused by P. megakarya. Primary inoculum of P. megakarya is locate d in the soil and with the onset of the rainy season, through rain splash, cacao pods become infected. From there the repeated cycles of pod infection and sporulation are at the origin of losses that can reach up to 80% when no control measures are in place. Thus, if this soil borne inoculum could be eliminated or prevented from reaching cacao pods, losses from black pod could be reduced considerably. In Cameroon, successful biological control of black pod rot has been obtained with the use of Trichoderma a sperellum PR11 applications directed at cacao pods. The objective of this project therefore, was to investigate the possibility of using T. asperellum PR11 in soil applications in order to reduce black pod disease incidence due to P. megakarya. The study was undertaken in a farmer's field near Nkolbisson, Centre region of Cameroon. Trichoderma asperellum PR11 was applied on a three weeks basis over two consecutive production seasons. A water only and a fungicide, Ridomil Gold 66 WP (6 % metalaxyl-M & 60 % CuO) treatment were used as controls. Weekly data collection consisted of counting all healthy and diseased pods. For each treatment, six replicate plots each containing 16 cocoa trees were used. The absolute number of rotten pods and percentage pod rot was lower for T. asperellum treated plots when compared with the water control yet higher than the fungicide control, albeit these differences were not significant. There was however, a significant difference between treatments with regard to disease progression over time. Given that T. asperellum applications directed at cacao pods have shown efficiency in controlling black pod rot, the use of soil applications of T. asperellum, which slows disease progression, should therefore lead to additive effects when both control strategies are employed simultaneousl

The impact of Trichoderma asperellum soil treatments on Phytophthora megakarya, the causal agent of black pod rot of cacao in Cameroon

Many Phytophthora species have a soil borne phase in their natural life cycles even though disease expression often occurs on aerial plant parts. In Cameroon, black pod rot of cacao (Theobroma cacao L.), is caused by P. megakarya. Primary inoculum of P. megakarya is located in the soil and with the onset of the rainy season, through rain splash, cacao pods become infected. From there the repeated cycles of pod infection and sporulation are at the origin of losses that can reach up to 80% when no control measures are in place. Thus, if this soil borne inoculum could be eliminated or prevented from reaching cacao pods, losses from black pod could be reduced considerably. In Cameroon, successful biological control of black pod rot has been obtained with the use of Trichoderma asperellum PR11 applications directed at cacao pods. The objective of this project therefore, was to investigate the possibility of using T. asperellum PR11 in soil applications in order to reduce primary inoculum and/or reduce black pod disease incidence due to P. megakarya. The study was undertaken in a farmer's field near Nkolbisson, Centre region of Cameroon. Trichoderma asperellum PR11 was applied on a monthly basis over two consecutive production seasons. A water only and a fungicide, Ridomil Gold 66 WP (6 % metalaxyl-M & 60 % CuO) treatment were used as controls. Weekly data collection consisted of counting all healthy and diseased pods. Soil samples were taken at the onset and end of the experiment to determine primary inoculum levels in the soil. For each treatment, six replicate plots each containing 16 cocoa trees were used. The absolute number of rotten pods and percentage pod rot was lower for T. asperellum treated plots when compared with the water control yet higher than the fungicide control, albeit these differences were not significant. There was however, a significant difference between treatments with regard to disease progression over time. Moreover, T. asperellum treatments were very efficient in reducing the amount of primary inoculum in the soil, even more so than the fungicide treatment. Given that T. asperellum applications directed at cacao pods have shown efficiency in controlling black pod rot, the use of soil applications of T. asperellum, which slows disease progression and reduces soil inoculum, should therefore lead to additive effects when both control strategies are employed simultaneously. (Résumé d'auteur

Effet des traitements du sol sur le développement de l'épidémie de la pourriture brune (Phytophthora megakarya) du cacaoyer au Cameroun

La pourriture brune causée par Phytophthora spp. est responsable des pertes de l'ordre de 30% au niveau mondial. Au Cameroun, l'espèce Phytophthora megakarya est la seule responsable de cette maladie et les pertes peuvent atteindre 100% si aucune mesure de contrôle n'est prise. En général, on considère que l'inoculum primaire qui initie l'épidémie provient du sol. De nombreuses études ont porté sur le contrôle des symptômes visibles de la maladie (les cabosses infectées) en utilisant différentes méthodes de lutte: culturale, chimique, génétique ou biologique. Néanmoins, très peu d'études se sont intéressées au contrôle de l'inoculum primaire pour voir comment ces types d'interventions pourraient aider à la gestion de cette maladie. Par conséquent, l'objectif de ce travail était de voir si c'est possible d'intervenir au niveau du démarrage de l'épidémie en effectuant un contrôle sur l'inoculum primaire. Un essai en champ a été conduit sur deux années consécutives (2010-2011). Des applications de Ridomil et Trichoderma asperellum PR11 ont été effectué en absence ou en présence de litière sur des parcelles de 16 arbres chacune. Des traitements avec l'eau ont été utilisés comme témoin. Chaque traitement à été répété trois fois. Les applications d'eau, de Ridomil et de T. asperellum étaient faites mensuellement tandis que les observations hebdomadaires ont porté sur les fruits sains et malades. Les cabosses malades et mûres étaient enlevées de l'arbre. Une évaluation des traitements sur la présence des antagonistes fongiques de P. megakarya présent dans le sol a également été faite. Les résultats ont montrés que Trichoderma peut avoir un effet positif sur la réduction du taux de pourriture de presque 30% comparativement au contrôle avec l'eau. Cet effet était plus évident quand Trichoderma était appliqué sur terre nue.. Les parcelles témoins ont montrées une plus grande diversité d'antagonistes de P. megakarya comparativement aux parcelles traitées. Trichoderma était le plus abondant dans le sol des parcelles traitées avec T. asperellum PR11 alors que Clonostachys était omniprésent. Les implications de ces résultats pour l'amélioration de la lutte contre P. megakarya sont discutées. (Résumé d'auteur

Microsatellite markers for population studies of Phytophthora megakarya (Pythiaceae), a cacao pathogen in Africa

Premise of the study: Phytophthora megakarya is the agent of black pod disease of cacao and is the main pathogen of this crop in Africa. Population genetic studies are required to investigate how this pathogen emerged. To this end, we developed 12 novel polymorphic microsatellite markers for P. megakarya. o Methods and Results: Microsatellite sequences were obtained by pyrosequencing of multiplex-enriched libraries. Candidate loci with di- or trinucleotide motifs were selected, and primer pairs were tested with nine P. megakarya isolates. The 12 most polymorphic and unambiguous loci were selected to develop three multiplex PCR pools. The total number of alleles varied from two to nine, depending on loci, and higher than expected heterozygosity was observed. o Conclusions: These markers were used for population genetic studies of P. megakarya in Cameroon and for comparison with reference strains from West Africa. This is the first time that microsatellite markers have been developed for P. megakarya. (Résumé d'auteur