Cassava mosaic disease and its whitefly vector in Cameroon: Incidence, severity and whitefly numbers from field surveys.

Cassava plays a key role in the food security and economy of Cameroon, but its production is constrained by cassava mosaic disease (CMD). However, comprehensive surveys of CMD in Cameroon have been lacking. This study aimed at evaluating the current status of CMD and its whitefly vector. Field surveys were conducted in 2020 using a sampling, diagnostics and data storage protocol that has been harmonized across 10 West and Central African countries for ease of comparison. Thirty plants per field were assessed for CMD severity, whitefly abundance and source of infection. Surveys were conducted in 343 fields and confirmed the presence of CMD in all 10 regions of Cameroon. Among the 10,057 assessed plants, 33.07% were deemed healthy (asymptomatic). At the field level, only 6.7% fields were found to be healthy. The mean CMD incidence across the country was 66.93%, and the mean severity score was 2.28. The main mode of infection was likely through contaminated cuttings. The mean whitefly count per plant was 5.78. This study is the first countrywide survey of CMD in Cameroon and provides insights that can be useful for improving the country's CMD intervention and management strategies.

Potentials of cocoa pod husk-based compost on Phytophthora pod rot disease suppression, soil fertility, and Theobroma cacao L. growth.

Cocoa black pod disease caused by Phytophthora megakarya and reduced soil fertility are major constraints to cocoa production resulting in high yield losses. In the absence of effective control measures and constraints related to the use of chemical fungicides and fertilizers, there is a need to develop additional and sustainable disease and fertilization management strategies. With the lack of studies related to the use of compost in cocoa cultivation, the present study aims to evaluate the potential of cocoa pod husk (CPH)-based compost as a soil amendment to reduce the severity of cocoa black pod disease and enhance plant growth. In vitro antagonism test showed that compost water extracts (CWE) reduced mycelial growth with inhibition rate reaching 100% associated with microorganisms. Disease score of cocoa plantlets grown on compost-amended soils significantly reduced compared to plantlets grown on non-amended soil (control). All compost rates tested significantly increased populations of actinomycetes and fungi and biological activity in the soil. Compost application increased soil pH and majority of the essential elements but decreased Al content, which is toxic to cocoa growth in acidic soils. Soil application of compost at the dose of 20% (v/v) significantly increased stem length and number of leaves compared to the control. This study shows that CPH-based compost can not only improve soil fertility and cocoa growth but also reduce cocoa black pod disease severity by direct effects on inoculums level in the soil and by inducing resistance in the plant.

Somatic embryogenesis and plant regeneration of cassava (Manihot esculenta Crantz) landraces from Cameroon.

A procedure to regenerate cassava (Manihot esculenta Crantz) cultivars from Cameroon via somatic embryogenesis (SE) was developed. Shoot apical meristems and immature leaf lobes were used as explants on Murashige and Skoog (MS) basal medium containing 33 or 50 µM of the auxins Picloram (Pic), 2,4-Dichlorophenoxyacetic acid (2,4-D), Dicamba (Dic), and α-Naphthalene acetic acid. Cultivar performance was assessed using SE and number of somatic embryos produced. Overall, the frequency of primary somatic embryogenesis (PSE) and the mean number of somatic embryos produced varied considerably with genotype, type of auxin and concentration tested. For example, cultivar (cv.) Ngan Mbada showed the best performance on MS medium supplemented with 50 µM Pic with a SE frequency of 40 % and an average number of somatic embryos of 90. The second best performance was recorded in cv. Local Red on MS medium supplemented with 33 µM 2,4-D, where the SE frequency was 40 % and an average number of somatic embryos of 60.5. Cultivar Ekona Red recorded the best performance on medium supplemented with 50 µM Pic showing a SE frequency of 47 % and an average number of somatic embryos of 45. We further examined secondary and cyclic somatic embryogenesis (SSE, CSE) and both were also observed to vary with genotype, however, both exhibited significantly higher frequencies of SE compared with PSE. SE started to decline at the fourth cycle of embryogenesis. Examination of organogenesis showed that shoot bud induction from green cotyledons varied across cultivars and benzylaminopurine was shown to outperform Thidiazuron in the ability to induce organogenesis. Furthermore, the frequencies of bud induction were identical under light and dark conditions. Finally, regenerated plants grew easily in the greenhouse with 90-100 % survival rate and did not display detectable variation in morphology.