Phytophthora Megakarya Research Articles

Mining GATA Transcription Factor Encoding Genes in The Cocoa Tree (<i>Theobroma cacao</i> L.) Suggests Their Potential Roles in Embryo Development and Biotic Stress Response

GATA transcription factors (TFs) are widely recognized as significant regulators, characterized by a DNA-binding domain that consists of a type IV zinc finger motif. This TF family has been widely investigated in numerous higher plant species. The purpose of the present work was to comprehensively analyze the GATA TF in cocoa plant (Theobroma cacao L.) by using various bioinformatics tools. As a result, a total of 24 members of the GATA TFs have been identified and annotated in the assembly of the cocoa plant. According to phylogenetic analysis, these TcGATA proteins were classified into four distinct groups, including groups I (10 members), II (seven members), III (five members), and IV (two members). Next, our investigation indicated that the TcGATA proteins in different groups exhibited a high variation in their physic-chemical features due to their different protein lengths, gene structures, and conserved motif distributions, whereas the TcGATA proteins in the same clade might share the common conserved motifs. Additionally, the gene duplication of the TcGATA genes in the cocoa plant was also investigated. Of our interest, the relative expression levels of the TcGATA genes were investigated according to available transcriptome databases. The results exhibited differential expression patterns of all TcGATA genes in various developmental stages of zygotic and somatic embryogenesis, indicating that these TcGATA genes divergently function during various developmental stages of the zygotic and somatic embryos. Moreover, TcGATA genes were differently expressed under Phytophthora megakarya treatment across different points of treatment and cocoa varieties. To sum up, our findings could provide a basis for a further deep understanding of the GATAs in the cocoa plant.

Bioactivity of Some Endophytic Fungi from Cameroonian Medicinal Plants against Verticillium albo-atrum, Rhizoctonia carotae and Phytophthora megakarya

Bioactivity of Some Endophytic Fungi from Cameroonian Medicinal Plants against Verticillium albo-atrum, Rhizoctonia carotae and Phytophthora megakarya

Optimal control of Cocoa Black pod disease: A multi-pronged approach

This study presents a novel approach to controlling cocoa black pod disease, caused by Phytophthora megakarya. Unlike previous methods, we employ a mathematical modeling framework based on Pontryagin’s Maximum Principle to optimize control strategies that minimize disease impact. The model incorporates the dynamics of healthy and infected pods while considering the combined effects of three key interventions: Infected pod removal, Targeted fungicide application and Promoting a healthy growing environment. Through numerical simulations, we identify the optimal timing and intensity for each intervention to minimize infected pods over time. This study highlights the following novelties: Synergistic effect of combined control which demonstrates that combining all three strategies is significantly more effective than relying on individual methods, Optimal strategies which involves dynamically adjusting control measures throughout the growing season to adapt to disease progression, Prompt action after disease detection proves crucial for successful control. These findings offer valuable data-driven recommendations for cocoa farmers and disease management professionals. By strategically implementing a combination of infected pod removal, targeted fungicide use, and environmental management, farmers can significantly reduce disease severity, enhance cocoa production, and promote a more sustainable cocoa industry.

Antagonistic Activity of Cellulase-Producing Yeasts Isolated from Cocoa Fermentation Against Pathogenic Molds Collected from Damaged Cocoa Fruits

The presence of pathogenic molds in post-harvested cocoa beans poses a significant problem to most Indonesian farmers. Pathogenic molds secrete phytopathogenic toxins which cause rotting, seriously damaging the cocoa fruits. The common pathogenic molds found in damaged cocoa beans are Phytophthora palmivora and Phytophthora megakarya. These molds are so far controlled by using chemical pesticides, whose long-term use is potentially harmful to human health and the environment. Thus, a healthier and safer alternative is needed, especially using biological agents such as yeasts. Some yeasts produce cellulose-degrading enzymes that can possibly break down the fungal cell wall which is composed mostly of the polysaccharide’s chitin and glucan. In this study, yeasts were isolated during the cocoa fermentation process, followed by screening for cellulolytic activity using direct planting and paste method, and subsequent antagonistic testing by double culture technique. Out of 128 yeast isolates, 77 possessed cellulase activity, with 6 of them having the highest activity index. Antagonistic activity test of these 6 isolates (C1.0.4, C1.1.3, C2.3.10, C2.3.14, C3.5.11, and C3.3.1) against the pathogenic molds resulted in isolate C3.3.1 showing the highest inhibition percentage from the 2

Assessing the Tolerance of Cocoa (Theobroma cacao L.) Progenies to the Black Pod Disease Caused by Phytophthora megakarya Bras. and Griff.

The cocoa (Theobroma cacao L.) is a cash crop of great economic importance to some cocoa-growing countries. However, diseases and pests, including black pod disease caused by Phytophthora megakarya, can cause yield losses of up to 100% if no phytosanitary treatment is applied. Thus, the aim of the present study is to evaluate the tolerance of five cocoa hybrids developed by IRAD (PA107*SNK614, IMC67*SNK109, SCA12*SNK16, IMC67*SNK64 and T79/501*SNK64) against black pod disease in relation to the parent clones. Mature pods (approximately 5 months old) were harvested and used to assess black pod tolerance using the Iwaro Detached Pod Test (DPT-SM). Means were separated using the general linear model (GLM) and ranked using the Student Newman-Keuls multiple comparison test (SNK). ANOVA showed a significant difference at P=0.01 on tolerance between hybrids and P=0.001 on tolerance between hybrids and clonal groups. 40% of the hybrids tested were more tolerant than all the clonal groups. The IMC clonal group was the most tolerant of the six clonal groups tested and ranked third overall between hybrids and clonal groups tested. Although the tolerance score varied between hybrids of clones PA107*SNK614 was the most tolerant (0.79) and T75/501*SNK 64 being least tolerant (2.00), all of these hybrids of clones possesses real potential to the tolerant to the black pod disease.

The potential of Trichoderma asperellum organic extract and its emulsion to inhibit cocoa ( Theobroma cacao L.) black pod disease and induce biochemical defenses against Phytophthora megakarya

This work aimed to evaluate the potential of Trichoderma asperellum organic extract and its emulsion to control cocoa black pod disease caused by Phytophthora megakarya. Organic extract was obtained after fermentation of T. asperellum and its emulsion prepared by emulsification. The in vitro antimicrobial assays of organic extract and its emulsion were evaluated and the in situ tests were carried out on detached cocoa pods. T. asperellum inhibited the mycelia growth of P. megakarya at the rates of 52% and 100%, respectively, on dual culture and the cellophane plate. This antagonist produced lytic enzymes such as cellulase, amylase, lipase and protease. The organic extract contained alkaloid, flavonoid and phenol compounds. The emulsion obtained was stable. At 100 μg · ml -1, the extract and its formulation completely inhibited the mycelial growth of P. megakarya. Similarly, when infected detached cocoa pods were sprayed with extract or emulsion, there was a significant reduction of necrosis both for healing and prevention with the latter being the most efficient. For the preventive tests, the total inhibition was recorded at 3000 μg · ml -1 and 1000 μg · ml -1, respectively, with crude organic extract and its emulsion· For curative tests, total inhibition was obtained at 4000 μg · ml -1 and 3000 μg · ml -1, respectively, for preventive and curative tests. There was a significant and positive correlation between the content of biochemical markers and the reduction of necrosis on cocoa pods after treatment with the extract or its formulation. T. asperellum organic extract emulsion could be used as an alternative in the bio- protection of cocoa black pods disease.

Phytophthora : taxonomic and phylogenetic revision of the genus.

Many members of the Oomycota genus Phytophthora cause economic and environmental impact diseases in nurseries, horticulture, forest, and natural ecosystems and many are of regulatory concern around the world. At present, there are 223 described species, including eight unculturable and three lost species. Twenty-eight species need to be redescribed or validated. A lectotype, epitype or neotype was selected for 20 species, and a redescription based on the morphological/molecular characters and phylogenetic placement is provided. In addition, the names of five species are validated: P. cajani, P. honggalleglyana (Synonym: P. hydropathica), P. megakarya, P. pisi and P. pseudopolonica for which morphology and phylogeny are given. Two species, P. ×multiformis and P. uniformis are presented as new combinations. Phytophthora palmivora is treated with a representative strain as both lecto- and epitypification are pending. This manuscript provides the updated multigene phylogeny and molecular toolbox with seven genes (ITS rDNA, β-tub, COI, EF1α, HSP90, L10, and YPT1) generated from the type specimens of 212 validly published, and culturable species (including nine hybrid taxa). The genome information of 23 types published to date is also included. Several aspects of the taxonomic revision and phylogenetic re-evaluation of the genus including species concepts, concept and position of the phylogenetic clades recognized within Phytophthora are discussed. Some of the contents of this manuscript, including factsheets for the 212 species, are associated with the "IDphy: molecular and morphological identification of Phytophthora based on the types" online resource (https://idtools.org/tools/1056/index.cfm). The first version of the IDphy online resource released to the public in September 2019 contained 161 species. In conjunction with this publication, we are updating the IDphy online resource to version 2 to include the 51 species recently described. The current status of the 223 described species is provided along with information on type specimens with details of the host (substrate), location, year of collection and publications. Additional information is provided regarding the ex-type culture(s) for the 212 valid culturable species and the diagnostic molecular toolbox with seven genes that includes the two metabarcoding genes (ITS and COI) that are important for Sanger sequencing and also very valuable Molecular Operational Taxonomic Units (MOTU) for second and third generation metabarcoding High-throughput sequencing (HTS) technologies. The IDphy online resource will continue to be updated annually to include new descriptions. This manuscript in conjunction with IDphy represents a monographic study and the most updated revision of the taxonomy and phylogeny of Phytophthora, widely considered one of the most important genera of plant pathogens. Taxonomic novelties: New species: Phytophthora cajani K.S. Amin, Baldev & F.J. Williams ex Abad, Phytophthora honggalleglyana Abad, Phytophthora megakarya Brasier & M.J. Griffin ex Abad, Phytophthora pisi Heyman ex Abad, Phytophthora pseudopolonica W.W. Li, W.X. Huai & W.X. Zhao ex Abad & Kasiborski; New combinations: Phytophthora ×multiformis (Brasier & S.A. Kirk) Abad, Phytophthora uniformis (Brasier & S.A. Kirk) Abad; Epitypifications (basionyms): Peronospora cactorum Lebert & Cohn, Pythiacystis citrophthora R.E. Sm. & E.H. Sm., Phytophthora colocasiae Racib., Phytophthora drechsleri Tucker, Phytophthora erythroseptica Pethybr., Phytophthora fragariae Hickman, Phytophthora hibernalis Carne, Phytophthora ilicis Buddenh. & Roy A. Young, Phytophthora inundata Brasier et al., Phytophthora megasperma Drechsler, Phytophthora mexicana Hotson & Hartge, Phytophthora nicotianae Breda de Haan, Phytophthora phaseoli Thaxt., Phytophthora porri Foister, Phytophthora primulae J.A. Toml., Phytophthora sojae Kaufm. & Gerd., Phytophthora vignae Purss, Pythiomorpha gonapodyides H.E. Petersen; Lectotypifications (basionym): Peronospora cactorum Lebert & Cohn, Pythiacystis citrophthora R.E. Sm. & E.H. Sm., Phytophthora colocasiae Racib., Phytophthora drechsleri Tucker, Phytophthora erythroseptica Pethybr., Phytophthora fragariae Hickman, Phytophthora hibernalis Carne, Phytophthora ilicis Buddenh. & Roy A. Young, Phytophthora megasperma Drechsler, Phytophthora mexicana Hotson & Hartge, Phytophthora nicotianae Breda de Haan, Phytophthora phaseoli Thaxt., Phytophthora porri Foister, Phytophthora primulae J.A. Toml., Phytophthora sojae Kaufm. & Gerd., Phytophthora vignae Purss, Pythiomorpha gonapodyides H.E. Petersen; Neotypifications (basionym): Phloeophthora syringae Kleb., Phytophthora meadii McRae Citation: Abad ZG, Burgess TI, Bourret T, Bensch K, Cacciola S, Scanu B, Mathew R, Kasiborski B, Srivastava S, Kageyama K, Bienapfl JC, Verkleij G, Broders K, Schena L, Redford AJ (2023). Phytophthora: taxonomic and phylogenetic revision of the genus. Studies in Mycology 106: 259-348. doi: 10.3114/sim.2023.106.05.

Evaluation of Cocoa Full-Sib Progenies in Challenging Conditions in Cameroon

The improved vegetal material cultivated by cocoa farmers in Cameroon consists in full-sib progenies issued from pods harvested in bi-clonal seed gardens set up during the 1970s. Interviews of cocoa farmers revealed their general satisfaction with the yield level obtained from these progenies but also their disappointment with the high level of susceptibility to black pod disease, a disease caused by Phytophthora megakarya, which is a serious challenge for cocoa production in this country. In order to select new future varieties that would combine high yield and resistance to black pod disease, new progeny trial plots were set up from 2005, in research stations and on farm.
 23 cocoa full-sib progenies were assessed and compared to 3 control progenies. in a trial plot set up in 2005 in a research station located in the south western region of Cameroon. The cocoa trees were assessed during eight consecutive years of cocoa production. A large level of variation was observed among the progenies for mortality rate (ranging between 6 and 52%), annual potential yield (ranging between 151 and 1,808 kg/ha), annual actual yield (ranging between 114 and 1,159 kg/ha) and black pod disease incidence (ranging between 19 and 39,8%) caused by Phytophthora megakarya. Nine of the assessed full-sib progenies were identified as promising and the authors suggest how to confirm their performances before their release to farmers.

Genetic variation among cocoa (Theobroma cacao L.) families for resistance to black pod disease under field and laboratory conditions

The laboratory detached black pod test method has been suggested as the most economical and effective screening procedure to complement field black disease resistance selection. General combining ability and association between black pod disease incidence under field and laboratory conditions among 24 cocoa F1 families were investigated over a 7-year period. The 24 F1 families were derived by crossing three seed garden parental clones with eight GU clones as males in a 3 x 8 North Carolina II (NC II) mating design. The 24 families together with four standard varieties planted in a randomized complete-block design with four replicates of 25 trees per plot were monitored for percentage black pod disease incidence (PBDI). Selfed pods from the 24 NC families and four standard varieties were arranged in a completely randomized design with four replicates of five pods each in the laboratory and subsequently inoculated with Phytophthora megakarya isolates. Percentage detached pod disease incidence (PDDI) was determined after seven days of incubation. Family effects was significant (p < .01) for PBDI and PDDI, and the association between them was positively and moderately correlated (r = 0.71; p < .01). Broad-sense heritability was high but narrow-sense heritability was low for both traits. General combining ability effects of eight of the 11 parental clones and specific combining ability effects of 17 of the 24 crosses were consistent (positive or negative) for PDDI and PBDI. Results from the study indicate that there is a considerable association between both traits, and clones GU 125/C and GU 255/V and their corresponding crosses PA 150 × GU 255/V, T60/887 × GU 125/C, and PA7/808 × GU 255/V had favorable effects for resistance to black pod disease expression. It is concluded that the detached black pod test represents a suitable method to support the field selection of black pod-resistant cocoa varieties.

Screening of Trichoderma Isolates and Potential of Their Organic Extract to Control Phytophthora megakarya, the Causative Agent of Cocoa Black Pod Disease

The aim of this work was to isolate Trichoderma agents of cocoa trees and to select the best antagonists able to reduce cocoa black pod disease caused by Phytophthora megakarya. Sampling was carried out in the Western region of Cameroon, locality of Bafang. Trichoderma and P. megakarya isolates were made on PDA medium in a Petri dish. In vitro antagonist activity of Trichoderma was evaluated by dual culture and antibiosis tests, and the best isolates were selected. The production of hydrolytic enzymes by the best isolates was done in specific solid media. The antimicrobial effect of the crude organic extract extracted from the selected Trichoderma isolates was performed in vitro by poisoned method. Results showed that 12 Trichoderma spp were isolated. Based on the rating scale of Bell, 4 isolates were selected, namely IS4, IS7, IS9 and IS11. These isolates were found to produce non-volatile and volatiles compounds as well as hydrolytic enzymes (cellulase, amylase, lipase and protease); Their organic crude extract significantly inhibited the growth of mycelial of P. megakarya with IC7 as the best isolates. IS7 Trichoderma isolate significantly produced cellulase, amylase, lipase and protease and at 100μg/ml, its organic crude extract total inhibited the mycelial growth of P. megakarya. It could be suggested that, Trichoderma IS7 isolate should be further investigated as promising biocontrol agent to manage P. megakarya infection.

Application of heat-treated oyster shell powder to induce priming of Theobroma cocoa seedlings plant defense system against Phytophthora megakarya attack

Priming with natural compounds is an interesting alternative for sustainable agriculture to explore the molecular mechanisms associated with the management of diseases caused by soil borne pathogens. The aim of this work was to evaluate the effect of the soil amendment with bio-oyster shell on the stimulation of the cocoa plant defense systems against P. megakarya in the nursery. For this purpose, the potential of bio-oyster shell to stimulate cocoa plant defense mechanisms was evaluated by soil amendment at 1% W/W under greenhouse conditions. After soil amendment, agronomic growth parameters and soil parameters were analyzed. The degree of expression of phytoalexins and defense genes was carried out by UPLC-DAD- HRESI–MS (+) and qRT-PCR analyses, respectively. The results obtained show that soil treatment with these composites improves the sanitary quality of cocoa seedlings in controlled conditions through the stimulation of agronomic growth parameters such as plant height, number of leaves, leaf surface area, and fresh weight of shoot and root. The latter leads to an increase in soil pH by about 32 % as well as the improvement of the nutritional status of the soil by 90%. Moreover, bio-oyster shell soil amendment induces up-regulation of TcPer-1, TcGlu1, TcChiB, and TcMYBPA genes and accumulation of primed metabolites such as flavonols, brevifolin, and isoflavone compounds before and after infection. This stimulation based on biological substances make it possible to pre-immunize cocoa plants against pathogen attacks and would be a good candidate to improve the quality of cocoa seeds in Cameroon.

Selecting Endophytes for Rhizome Production, Curcumin Content, Biocontrol Potential, and Antioxidant Activities of Turmeric (Curcuma longa).

Beneficial endophytes may enhance plant growth and stress tolerance. Yet, the plant health benefits of endophytes can be altered by biotic and abiotic factors and, thus, favour the inhibition of turmeric growth and curcumin production. The double petri dish method and greenhouse pot experiments were conducted to assess the biocontrol potential and impact of endophytes on the output, curcumin levels, and antioxidant activities of turmeric (Curcuma longa L.). The results showed that endophytes could control some disease-causing plant pathogens: 52% of all isolates have an antagonistic action against Fusarium oxysporum, 43% against Pythium myriotylum, 35% against Phytophthora megakarya, and 56% against Ralstonia solanacearum in vitro. Eight months after sowing, most endophyte isolates can increase the yield of turmeric rhizomes on a sterile substrate after inoculation, with yields ranging from 42 to 105% higher than the control and 3 to 50% higher than the urea treatment. In addition, 52% endophytes isolate significantly raised curcumin levels after 8 months of culture (from 2.1 to 3.1%) compared to control (1.7%) and urea treatment (1.8%). These endophytes promote an increase in the levels of reduced glutathione (22%), total thiols (26%), and carotenoids (91%) in turmeric. The study concludes that, in general, the endophytes-turmeric association can stimulate turmeric rhizome production, curcumin, and the antioxidant activities of the plant. They can also be used as biocontrol agents for plant pathogens.

Flavan-3-ol and flavonol analysis in healthy and infected parents and progenies of cocoa leaves (Theobroma cacao L.) with Phytophthora megakarya Bras. and Grif

Flavan-3-ol and flavonol analysis in healthy and infected parents and progenies of cocoa leaves (Theobroma cacao L.) with Phytophthora megakarya Bras. and Grif

Genome-Wide Identification and Analysis of Heat Shock Protein 70 Family in Theobroma cacao.

&lt;b&gt;Background and Objective:&lt;/b&gt; In plants, the 70 kDa heat shock proteins (Hsp70-s) play important roles during growth and development and in response to stresses. This study aimed to provide comprehensive information on the &lt;i&gt;Hsp70 &lt;/i&gt;family in cocoa (&lt;i&gt;Theobroma cacao&lt;/i&gt;), one of the most important tropical perennial cash crops worldwide. &lt;b&gt;Materials and Methods:&lt;/b&gt; A basic local alignment search tool (TBLASTN) against the cocoa genome was performed using &lt;i&gt;Arabidopsis&lt;/i&gt; Hsp70-s as queries to detect TcHsp70-s. Sequence analyses were carried out by various bioinformatics tools. &lt;b&gt;Results:&lt;/b&gt; A total of 18 members of the &lt;i&gt;Hsp70&lt;/i&gt; family has been detected and characterized in the cocoa genome. All general properties, such as physic-chemical parameters, gene structure, phylogenetic tree and sub-cellular localization, were determined using a variety of bioinformatics tools. The expression patterns of the &lt;i&gt;TcHsp70&lt;/i&gt; genes in different stages of the zygotic and somatic embryos were investigated. In addition, expression profiles of the &lt;i&gt;TcHsp70&lt;/i&gt; genes under &lt;i&gt;Phytophthora megakarya&lt;/i&gt; inoculation were analyzed. &lt;b&gt;Conclusion:&lt;/b&gt; The results of this study revealed the features and expression analyses of &lt;i&gt;Hsp70&lt;/i&gt; genes in cocoa. These findings could provide a strong foundation for further research of the &lt;i&gt;TcHsp70&lt;/i&gt; family, which could aid in the development and stress tolerance of cocoa species.

Effects of a Powder Formulation of Streptomyces cameroonensis on Growth and Resistance of Two Cocoa Hybrids from Cameroon against Phytophthora megakarya (Causal Agent of Black Pod Disease).

In the present study we evaluated the efficacy of a bioformulation of Streptomyces cameroonensis for control of black pod disease in cocoa and enhancement of seedling growth. The formulation developed using talc powder and cassava starch as carriers showed high shelf-life of 1.07 × 106 CFU/g after six months storage at 4°C. The formulation was tested for inhibition of spore germination in Phytophthora megakarya and showed 100% inhibition at 10% (w/v) of formulation. To determine the efficacy of the formulation, we performed an in planta assay in the greenhouse on two hybrids of cocoa seedlings, the tolerant SNK413 × (♂) T79/467 and the susceptible UPA 134× (♂) SCA 12. Detached leaf assay showed a significant reduction in the disease severity index of about 67% for the tolerant hybrid and 55% for the susceptible hybrid compared to non-treated plants. A significant enhancement in stem length, leaf surface area and root weight was observed. Analysis of biochemical markers of defense showed a significant increase in total polyphenol, flavonoid, and total protein contents. There was also significant upregulation of PR-proteins such as chitinases, peroxidases and β-1, 3-glucanases following treatment of both tolerant and susceptible hybrids, though with a higher level of synthesis in the tolerant hybrids. A significant increase was also observed in polyphenol oxidase activities in plants treated with the formulation. This work demonstrated the stability and effectiveness of the S. cameroonensis powder formulation in suppressing black pod disease in cocoa and subsequently enhancing the growth of seedlings.

The BAHD Gene Family in Cacao (Theobroma cacao, Malvaceae): Genome-Wide Identification and Expression Analysis

The benzyl alcohol O-acetyl transferase, anthocyanin O-hydroxycinnamoyl transferase, N-hydroxycinnamoyl anthranilate benzoyl transferase, and deacetylvindoline 4-O-acetyltransferase (BAHD) enzymes play a critical role in regulating plant metabolites and affecting cell stability. In the present study, members of the BAHD gene family were recognized in the genome of Theobroma cacao and characterized using various bioinformatics tools. We found 27 non-redundant putative tcBAHD genes in cacao for the first time. Our findings indicate that tcBAHD genes are diverse based on sequence structure, physiochemical properties, and function. When analyzed with BAHDs of Gossypium raimondii and Corchorus capsularis clustered into four main groups. According to phylogenetic analysis, BAHD genes probably evolved drastically after their divergence. The divergence time of duplication events with purifying selection pressure was predicted to range from 1.82 to 15.50 MYA. Pocket analysis revealed that serine amino acid is more common in the binding site than other residuals, reflecting its key role in regulating the activity of tcBAHDs. Furthermore, cis-acting elements related to the responsiveness of stress and hormone, particularly ABA and MeJA, were frequently observed in the promoter region of tcBAHD genes. RNA-seq analysis further illustrated that tcBAHD13 and tcBAHD26 are involved in response to Phytophthora megakarya fungi. In conclusion, it is likely that evolutionary processes, such as duplication events, have caused high diversity in the structure and function of tcBAHD genes.

Characterization of endophytic Streptomyces strains from roots of cocoyam (Xanthosoma sagittifolium L. Schott) in the South West Region of Cameroon, their in vitro plant growth promoting abilities and biocontrol efficacy against Pythium myriotylum

The focus of this study was to isolate, evaluate and characterize endophytic actinobacteria from cocoyam roots collected in the localities around Mount Cameroon for their potential antagonistic activities against Pythium myriotylum (the major pathogen that limits growth and productivity of this crop in Cameroon) and their ability to promote plant growth. A total of 171 actinobacteria were isolated from healthy cocoyams and were screened for their antagonistic activities against P. myriotylum, Phytophthora megakarya, Fusarium oxysporum and Colletotrichum gloeosporioides. Four isolates (PERM1, PERM2, PERM4 and PERM5) were selected based on of their strongest antagonistic activity in vitro against P. myriotylum. These four isolates were subsequently identified using phenotypic and molecular tests, characterized for extracellular hydrolytic enzymes, plant growth-promoting (PGP) traits, and their effects were evaluated on tissue culture-derived cocoyam plantlets. The sequencing of the 16S rRNA genes and alignment with sequences retrieved through the EzTaxon program indicated that PERM2, PERM4 and PERM5 have low similarity percentages with S. samsunensis EU077190 (92.4% sequence similarity), S. roietensis KX394336 (81% sequence similarity), S. flavoviridis AB184842 (91% sequence similarity) respectively. The comparison of some physiological and biochemical properties of PERM2,PERM4 and PERM5 with the most closely related Streptomyces species showed that these strains differ phenotypically. PERM2, PERM4 and PERM5 might represent new species. All selected Streptomyces were able to produce siderophores, indole-3-acetic acid (IAA), aminocyclopropane-1-carboxylate (ACC) deaminase, and solubilized tricalcium phosphate. With 20.30 μg/ml of IAA, the PERM2 isolate significantly increased the length of roots as compared to the positive control. These results suggest the possibility of using endophytic Streptomyces strains, especially isolate PERM2 in the protection of cocoyams against root rot disease and the promotion of root growth.

Parameter Estimation in a PDE Model for the Spatial Spread of Cocoa Black Pod Disease.

In this paper, we develop an epidemiological model with both environmental (primary infection from the environmental spores reservoir) and direct transmission (secondary infection from an infected host to a susceptible pod). This model simulates the spatiotemporal evolution of cocoa black pod disease caused by Phytophthora megakarya. Since reliable parameter estimation is a central issue for modeling realistic biological systems, we used a mechanistic-statistical approach to estimate model parameters from real observations of a specific cocoa plot. In addition, to refine numerical simulations of the pathosystem, data describing the shade intensity all over the plot were exploited and led to increased model predictions accuracy and also highlighted a higher number of infected pods located in areas of the plot with higher shading intensity. Recommendations in terms of promoting cocoa farming in systems with low shading intensity may be evident if these results are confirmed. Our results also highlight the importance of the environmental spore reservoir in black pod disease dynamics.

A simple method for the extension of shelf life of cultures of Phytophthora species causing black pod disease of cacao (Theobroma cacao L.)

Black pod disease of cacao caused by Phytophthora palmivora and Phytophthora megakarya in Ghana take heavy toll of cacao production in the field. Intensive research has been carried out worldwide on these pathogens. However, viability of the cultures during prolonged storage has remained a major challenge in the research. This paper reports findings of assessment of six storage media viz sterilised distilled water (SDW), sterilised and unsterilized soil suspension (SSS and USS), vegetable 8 juice broth (V8JB), Oat Meal Agar slant under mineral oil (at 4°C) and empty tube. Viability of the cultures was assessed on V8JA and in tetrazolium chloride test. Ability of zoospores of the cultures to infect cacao leaf discs was used to assess growth vigour and pathogenicity. Phytophthora cultures stored in SDW (26 ± 2°C; alternating day light and night) were preserved for 60 days (5 years). Both P. palmivora and P. megakarya performed better on SDW and SSS than on USS due to removal of staling substances in the soil medium by the sterilisation. Vigour of growth and pathogenicity of the stored cultures required re-inoculation of host tissue (cacao pod) in order to maintain potency to continually infect host.

Spatial and Temporal Analysis of Phytophthora megakarya Epidemic in Newly Established Cacao Plantations.

Studying spatial and temporal plant disease dynamics helps us to understand pathogen dispersal processes and improve disease control recommendations. In this study, three cacao plots devoid of primary inoculum of Phytophthora megakarya (causal agent of cacao black pod rot disease) upon establishment in 2006 were monitored for presence of disease on a weekly basis from 2009 to 2016. Ripley's K(r) function, join count statistics, and Fisher's Exact test were used to analyze spatial and temporal disease dynamics. Disease distribution maps showed aggregated disease patterns in all plots; however, for the years of disease onset, exogenous primary infections were mostly randomly distributed. The K(r) function confirmed these results indicating that inoculum generally disperses only over short distances. Moreover, significant positive spatial autocorrelations showed that diseased trees were often clustered up to a distance of 3 to 9 m. Temporal disease progression was low, meaning that endogenous inoculum failed to establish itself, which is partly explained by rigorous phytosanitation and partly by unfavorable microclimatic conditions for disease development. Because P. megakarya had difficulty establishing itself in the plots, proximity to already infected cacao plantations drove infection dynamics. Thus, isolation of newly established cacao plantations from infected ones and rigorous phytosanitation as a preventive strategy appears to be an effective approach to control cacao black pod rot disease for newly established cacao plantations.