A field experiment in Ghana on the tolerance of cocoa seedlings to cocoa swollen-shoot and cocoa mottle-leaf viruses

Seedings of Amelonado Cocoa and of two progenies obtained by crossing Iquitos (Upper Amazon) parents were infected with three strains of cocoa swollen-shoot virus (CSSV) and with cocoa mottle-leaf virus (CMLV). CSSV strain I A had the most severe effects on the growth and canopy condition of all varieties. All four virus isolates had more effect on the growth of Amelonado than on the Iquitos progenies, except that CSSV strain I W affected the growth of Amelonado and one of the Iquitos progenies equally, as did CMLV which caused the greatest decrease in the first crop of all varieties. The relative virulence of different CSSV strains in any one variety can be assessed from single criteria such as length of latent period or effect on growth; CMLV had a shorter latent period than CSSV Strain I A but less effect on growth.     

Field experiments on the resistance of cocoa to cocoa swollen-shoot virus

Progenies from crosses between different Upper Amazon cocoa types and between Upper Amazon and Amelonado were compared for their field resistance to infection with cocoa swollen-shoot virus (CSSV) virulent strain A. Among the intra-Amazon progenies, those from crosses of Scavina, Iquitos and Nanay groups showed most resistance. Progenies from crosses within these groups may have sufficient resistance to be of immediate practical value in reducing crop losses in areas where CSSV is widespread. Some Nanay progenies were more resistant than others and this provides scope for improvement by breeding. Progeny of crosses between Upper Amazon and Amelonado parents were less resistant than those from intra-Amazon crosses. The present results confirm those previously obtained in gauze-house tests on young plants.     

The virobacterial agglutination test as a rapid means of detecting cocoa swollen shoot virus

The virobacterial agglutination (VBA) test was developed as a means of detection of cocoa swollen shoot virus (CSSV). Identification of CSSV-infected Theobroma cacao in the field has only been possible by visual examination of symptoms, by virus transmission using mealybugs and by grafting to induce symptom expression in Amelonado cocoa seedlings. Detection of latent infection has not been possible even using enzyme-linked immunosorbent assays (ELISA). The VBA test successfully detected CSSV in infected sap diluted to 1/2560. Antisera to a range of mild and severe CSSV isolates were tested, and the results suggest a close relationship between seven isolates (1A, Bosomtwi, Bosomuoso, Nkrankwanta, Nsaba, Seidi-Nkawie and SS365B) while the mild isolate N1 appears to be less closely related. The VBA test was compared with both direct and indirect ELISA in the field. Only VBA detected all the cocoa trees which were known to be infected and additionally identified infection in many symp-tomless trees.     

Resistance of cocoa to swollen-shoot virus in Ghana. I. Field trials

In Ghana, field resistance to cocoa swollen-shoot virus (CSSV) in young, bearing trees was assessed by monitoring natural spread by mealybug vectors from adjacent graft-inoculated line sources. Apparent rates of virus spread were estimated by regressing proportions of infected trees on time from inoculation. In some hybrids between parents derived from Upper Amazonian material, spread was only 25–40% that in currently recommended varieties. This resistance is the most effective, feasible measure to reduce economic losses and the resistant hybrids could be made available quickly for large scale use. The results from trials in which virus spread most rapidly were the easiest to interpret. Spread of CSSV is erratic, necessitating large experiments, and in trials with equal replication the largest plots gave the greatest precision. Neighbouring plot adjustment proved unsatisfactory as it improved precision but appeared to reduce accuracy.     

Evaluation and use of a screening method to aid selection of cocoa (Theobroma cacao) with field resistance to cocoa swollen-shoot virus in Ghana

Seeds of cocoa progenies were inoculated with cocoa swollen-shoot virus manually or by vector transfer techniques to assess the relative resistance of the progenies to infection. Assessment of progeny included in both inoculation tests and in field trials were positively correlated, indicating that the inoculation technique is suitable for the selection of progenies for testing in the field. Pure Upper Amazon progenies were more resistant than selfed Amelonado, with hybrids between Amelonado and Upper Amazons usually intermediate. Among the main Upper Amazon populations, Iquitos and Nanay clones were better sources of resistance than Parinari and Scavina clones. A survey of these populations indicated that, within populations, resistance levels do not vary greatly. A range of progenies based on the Upper Amazon female parents in existing seed gardens was screened for resistance and some were consistently more resistant than the equivalent Amazon x Amelonado hybrid now being distributed to farmers.     

Impact of cocoa farming on vegetation in an agricultural landscape in Ghana

Cocoa production occurs almost wholly within areas identified as biodiversity hotspots in West Africa and it has been noted as a major contributor to deforestation at the forest‐agriculture interface. This study investigated the impact of cocoa farming on vegetation in relation to three land‐use types of increasing cocoa production intensity from remnant native forest through shaded to unshaded cocoa farmlands in Ghana. The study used transects and forty‐two 25 m × 25 m vegetation plots. The overall noncocoa plant species richness decreased significantly (95% CI) from the remnant native forest through shaded to the unshaded cocoa farmlands. Significant differences (P ≤ 0.05) were also found in the mean density and basal area of noncocoa plants per hectare with the remnant native forest recording the highest values and the unshaded cocoa farmlands the lowest. The relative density of about 44.7% out of the 41 most abundant plant species declined in cocoa farmlands. The results of this study showed that cocoa farming could result in a drastic forest plant species loss with subsequent recruitment of nonforest species, forest plant species population decline as well as changes in the structural characteristics of the vegetation. This impact increases with increasing cocoa production intensity.     

Morphological variation in Planococcoides njalensis occurring on cocoa in Ghana

A study was made to determine the extent of morphological variation in Planococcoides jalensis (Laing), the most important mealybug vector of cocoa swollen shoot virus (CSSV) in West Africa, and to resolve if more than one Planococcoides species is present. Fourteen populations of P. njalensis collected on cocoa from different localities in Ghana and one population each on Gliricidia sepium (Jacq.) Steud and Coffea canephora Pierre were compared by measured and meristic female characters using two types of morphometric analysis, Discriminant Functional Analysis and Principal Component Analysis. The analyses showed that P. njalensis exhibited a wide range of intraspecific morphological variation, particularly in meristic characters, but failed to identify new Planococcoides species. The results are discussed in relation to the control of the mealybug vectors of CSSV and to the suitability of Gliricidia as a shade tree for cocoa.     

Genetic aspects of resistance to cocoa swollen-shoot virus in Ghana

The inheritance of resistance in cocoa to infection with cocoa swollen-shoot virus was studied in seed inoculation experiments with progenies of Upper Amazon, Trinitario and West African Amelonado parents. In an eight parent diallel cross grown in two seasons and in six male × female factorial experiments, general combining abilities were much more important than specific combining abilities. The diallel cross revealed significant maternal effects but there was little evidence of specific reciprocal differences. Estimates of general combining abilities corresponded with expectation and were positively correlated with maternal effects, the latter indicating that tester parents should be used as males. The results from scaling tests with hybrids between selections from the River Nanay (a Peruvian tributary of the River Amazon) and Amelonado fitted a model with additive resistance; there was no evidence of dominance. Attempts to obtain stronger sources of resistance by hybridisation and selection were unsuccessful. In three crosses the resistance of individual seedlings was compared with that of the parents using test crosses but no evidence of transgressive segregation was obtained. In further experiments seedings of two hybrids between contrasting parents were heavily inoculated with virus so that few healthy survivors remained. Test crosses showed that these survivors and randomly selected seedlings of the same parentage did not differ as resistance sources.     

Tolerance trials with young cocoa trees infected with cocoa swollen-shoot virus

In three short-term field trials, 24 cocoa progenies were assessed for sensitivity to infection with a severe Ghanaian strain of cocoa swollen-shoot virus. Some of the progenies were derived from tolerant parents, others had themselves shown mild symptoms as young seedlings. Stem diameters, diameter increments, canopy scores and latent periods (time between inoculation and expression of symptoms) of plants infected when 12 to 22 months old were compared with similar measurements of healthy plants. During the period between 6 and 18 months after inoculation, infection reduced the stem diameter increment of the least sensitive progeny in each trial by 52, 25 and 7%, the last value increasing to 38% during the period between 24 and 30 months after inoculation. Within trials, the effects of virus infection on stem diameter increments and canopy scores were well correlated but latent periods were not significantly correlated with any other variate. In one trial, pod yield was more closely related to canopy condition at the time of cropping than to stem diameter increment 2 yr earlier. Weak correlations between pre- and post-bearing increases in diameters in these trials indicated that tolerance was not directly related to vigour. None of the progenies is considered suitable for direct use as tolerant varieties in Ghana.     

Inventory and abundance of mealybug species in immature and mature cocoa farms in Côte d'Ivoire

From 2013 to 2018, surveys were conducted in counties not previously surveyed in order to determine species of mealybugs present in the cocoa orchard in Côte d'Ivoire as well as their abundance according to the age of cocoa trees. Immature and mature cocoa trees were inspected to hand‐height in 5 and 29 counties infected with Cacao swollen shoot virus (CSSV). In each cocoa farm, mealybugs were searched for on fruits, leaves, flowers, twigs and trunks. Mealybug species were identified, and colonies were counted. Five mealybug species were identified on immature cocoa trees: Ferrisia virgata, Formicococcus njalensis, Planococcus citri, Planococcus kenyae and Pseudococcus longispinus. In addition to these species, four species, Dysmicoccus brevipes, Maconellicoccus hirsutus, Phenacoccus hargreavesi and Pseudococcus jackbeardsleyi were identified on mature cocoa trees. On immature cocoa trees, Fo. Njalensis, Pl. citri and Ps. longispinus comprised were, respectively, 35%, 33% and 19% of colonies, respectively. On mature cocoa trees, Fo. Njalensis and Pl. citri comprised 63.2% and 21.0%, and others species 15.8%. Nevertheless, the abundance of mealybug species varied according to the age of cocoa trees. The preferred organs of mealybugs were pods (74.1%) followed by twigs (13.4%) and flowers (7.4%). Previously, the mealybug Paracoccus burnerae (Brain) was found on Theobroma cacao, which is the first record for this species in Côte d'Ivoire and on this host‐plant.     

The resistance and tolerance of different cocoa varieties to cocoa swollen-shoot virus in Ghana

The resistance and tolerance to infection with cocoa swollen-shoot virus (virulent strain A) of forty-five and thirty-eight cocoa progenies respectively was estimated using a manual inoculation method. Resistance was estimated from the proportion of inoculated seedlings which developed symptoms, and tolerance from the severity of these symptoms during a 4-month period. Resistance was found among Nanay, Iquitos and Scavina, all Upper Amazon types; and an inter-Nanay cross was outstandingly resistant and offers scope for improvement by breeding. Some Amazon-Amazon and Trinitario-Amazon progenies snowed tolerance, especially those with Trinitario T9/21 as a parent. Some progenies seem sufficiently promising for field testing to select varieties suitable for planting in those areas in Ghana where swollen-shoot disease is epidemic.     

Evidence from the virobacterial agglutination test for the existence of eight serogroups of cocoa swollen shoot virus

Many isolates of cocoa swollen shoot virus (CSSV) have been found in Ghana. Relationships between these isolates have been based on symptom expression and limited serological information. This paper reports on the serological relationships between 44 accessions of CSSV using the virobacterial agglutination test. The CSSV group is differentiated into eight groups using seven antibody 'types'. The largest group comprising those isolates closely related to CSSV 1A is sub-divided into four further groups. These groupings are compared with previous results. Differences are seen between accessions of the same isolate which may be due to contamination of the source plants. These serological results can be used for studying mixed isolate infections as well as for determining the most closely related CSSV mild isolate for appropriate cross-protection against severe isolates.     

Cocoa agroforestry is less resilient to sub‐optimal and extreme climate than cocoa in full sun

Cocoa agroforestry is perceived as potential adaptation strategy to sub‐optimal or adverse environmental conditions such as drought. We tested this strategy over wet, dry and extremely dry periods comparing cocoa in full sun with agroforestry systems: shaded by (i) a leguminous tree species, Albizia ferruginea and (ii) Antiaris toxicaria, the most common shade tree species in the region. We monitored micro‐climate, sap flux density, throughfall, and soil water content from November 2014 to March 2016 at the forest‐savannah transition zone of Ghana with climate and drought events during the study period serving as proxy for projected future climatic conditions in marginal cocoa cultivation areas of West Africa. Combined transpiration of cocoa and shade trees was significantly higher than cocoa in full sun during wet and dry periods. During wet period, transpiration rate of cocoa plants shaded by A. ferruginea was significantly lower than cocoa under A. toxicaria and full sun. During the extreme drought of 2015/16, all cocoa plants under A. ferruginea died. Cocoa plants under A. toxicaria suffered 77% mortality and massive stress with significantly reduced sap flux density of 115 g cm^?2 day^?1, whereas cocoa in full sun maintained higher sap flux density of 170 g cm^?2 day^?1. Moreover, cocoa sap flux recovery after the extreme drought was significantly higher in full sun (163 g cm^?2 day^?1) than under A. toxicaria (37 g cm^?2 day^?1). Soil water content in full sun was higher than in shaded systems suggesting that cocoa mortality in the shaded systems was linked to strong competition for soil water. The present results have major implications for cocoa cultivation under climate change. Promoting shade cocoa agroforestry as drought resilient system especially under climate change needs to be carefully reconsidered as shade tree species such as the recommended leguminous A. ferruginea constitute major risk to cocoa functioning under extended severe drought.     

Improved methods for the purification and detection of cocoa swollen shoot virus

The extraction of cocoa swollen shoot virus (CSSV) from cocoa leaves with pectinase, purification from the concentrated extracts by filtration through Celite and Sepharose 2B, and concentration of the virus by isopycnic CsCl centrifugation are described. The pectinase and Celite treatments effectively removed mucilage and particulate host plant materials, and enhanced the release of virus particles, Isopycnic CsCl centrifugation resulted in 10–100-fold concentration of particles, but with apparent loss of infectivity. After extraction with pectinase, CSSV particles were regularly seen by electron microscopy in sap. Immunosorbent electron microscopy (ISEM) considerably enhanced the detection of CSSV in sap. The particles were bacilliform or bullet-shaped and of various lengths. The common dimensions were 142 × 27 nm.     

The host range and vector species of viruses from Cola chlamydantha K. Schum., Adansonia digitata L. and Theobroma cacao L

Seventeen species in the Tiliaceae, Sterculiaceae and Bombacaceae were tested for susceptibility to three virus isolates from naturally infected Cola chlamydantha trees and three from Adansonia digitata trees in Ghana. Seven species of Pseudococcidae were tested as vectors of the six isolates. These studies indicate that the Cola isolates should be classified with cocoa swollen shoot virus and those from Adansonia with cocoa mottle leaf virus.     

Studies on the possibility of increasing resistance to cocoa swollen-shoot virus by breeding

In tests using seed, the resistance of cocoa progenies to primary infection with cocoa swollen-shoot virus by mealybugs was stable to inoculum pressure and effective against several virus strains. The results suggested that the resistance would be effective throughout the cocoa growing areas of Ghana. There were indications that resistance factors from different cocoa populations could be accumulated to give progenies of higher resistance than presently available.     

Effects of host shading on consumption and growth of the geometrid Epirrita autumnata: interactive roles of water, primary and secondary compounds

Shading is assumed to reduce allocation to plant phenolics and to defense in general. We here report the results of experimental shading of individual branches or whole canopies in mountain birch on foliar chemistry and on the growth and consumption of a geometrid, Epirritaautumnata. Branch‐wide shading tended to have at least as strong effects on both leaf chemistry and herbivore performance as canopy‐wide shading, indicating local responses of the host to shading. Responses to shading varied among the key leaf traits. Leaf water content was higher and toughness lower in shaded than in non‐shaded leaves. Leaf sugars were lower and protein‐bound and free amino acids higher in shaded than in control leaves. Sucrose and galactose were at high levels in unshaded branches adjacent to shaded ones, suggesting that partial shading enhanced translocation of sugars within canopies. Total phenolics and soluble proanthocyanidins were low in both shading treatments. Of the other phenolic groups, concentrations of gallotannins and cell‐wall‐bound proanthocyanidins did not differ between shaded and non‐shaded leaves. Epirrita larvae grew better in both types of shading treatments compared to either unshaded control trees or to unshaded branches in the branch‐shading trees. By far the most important correlate of larval growth was the amount of water consumed with leaf mass (r=0.94). When variance in water intake was standardized (also largely eliminating parallel variation in proteins), fructose and glucose still had significant positive correlations and proanthocyanidins negative with larval growth on control but not on shade leaves. Concentrations of several phenolic compounds correlated negatively with intake of dry matter and especially water, and different phenolics were important in shaded (gallotannins) and in control (flavonoids) leaves. Our findings strongly suggest that the effects of putatively defensive leaf traits on insect consumption and growth interact with nutritive leaf traits, particularly with water.     

Methods for assessing the tolerance and resistance of different types of cocoa to cocoa swollen-shoot virus

Different types of cocoa were compared for their resistance to infection and tolerance of infection with the virulent strain A of cocoa swollen-shoot virus, inoculated either by mealybugs or manually. The method of infection did not affect the ranking of the cocoa types for resistance, except at high percentages of infection, or for tolerance. In comparison with other methods, manual inoculation simplifies the screening of cocoa types for resistance and tolerance to virus infection, and provides a more sensitive test for resistance.     

Resistance of cocoa to swollen-shoot virus in Ghana. II. Assessment of seed inoculation experiments

Differences in amounts of infection following seed inoculation, by the mealybug vector (Planococcoides njalensis (Laing)) of cocoa swollen-shoot virus were used as a short-term test of virus resistance in a range of cocoa progenies. The test results were highly correlated with virus resistance as measured in field trials. The best assessment of seed inoculation experiments allowed for progeny-dependent variation in the latent period of the virus. Experiments in which estimates of progeny means varied significantly with replicate sometimes gave poor agreement with known field performance.     

Potential for biological control of tent building species of ants associated with Phytophthora palmivora pod rot of cocoa in Papua New Guinea

The tents of different ant species collected from young cocoa trees in their first or second year of bearing and free from visible pod rot, were tested for the presence of Phytopthtora palmivora by inoculating wounded cocoa pods with tent material. Tents of all species harboured viable P. palmivora but those consisting mainly of soil were more frequently positive than plant debris-type tents. Although Anoplolepis longipes, a dominant, ground nesting, non-tent building species sometimes transported inocula in the laboratory, it did not significantly increase black pod infection in the field. Trees infested with the dominant debris tent building species, Technomyrmex albipes, however, had significantly more black pod than those infested with A. longipes or trees without ants. A. longipes forms dense populations and can exclude other dominant ants and some cocoa pests; its introduction may be a potentially economical method of reducing the transmission of P. palmivora in redeveloped cocoa in Papua New Guinea.