Smilja Lambert

Vascular Streak Dieback of cacao in Southeast Asia and Melanesia: in planta detection of the pathogen and a new taxonomy

Vascular Streak Dieback (VSD) disease of cacao (Theobroma cacao) in Southeast Asia and Melanesia is caused by a basidiomycete (Ceratobasidiales) fungus Oncobasidium theobromae (syn. =Thanatephorus theobromae). The most characteristic symptoms of the disease are green-spotted leaf chlorosis or, commonly since about 2004, necrotic blotches, followed by senescence of leaves beginning on the second or third flush behind the shoot apex, and blackening of infected xylem in the vascular traces at the leaf scars resulting from the abscission of infected leaves. Eventually the shoot apex is killed and infected branches die. In susceptible cacao the fungus may grow through the xylem down into the main stem and kill a mature cacao tree. Infections in the stem of young plants prior to the formation of the first 3-4 lateral branches usually kill the plant. Basidiospores released from corticioid basidiomata developed on leaf scars or along cracks in the main vein of infected leaves infect young leaves. The pathogen commonly infects cacao but there are rare reports from avocado. As both crops are introduced to the region, the pathogen is suspected to occur asymptomatically in native vegetation. The pathogen is readily isolated but cultures cannot be maintained. In this study, DNA was extracted from pure cultures of O. theobromae obtained from infected cacao plants sampled from Indonesia. The internal transcribed spacer region (ITS), consisting of ITS1, 5.8S ribosomal RNA and ITS2, and a portion of nuclear large subunit (LSU) were sequenced. Phylogenetic analysis of ITS sequences placed O. theobromae sister to Ceratobasidium anastomosis groups AG-A, AG-Bo, and AG-K with high posterior probability. Therefore the new combination Ceratobasidium theobromae is proposed. A PCR-based protocol was developed to detect and identify C. theobromae in plant tissue of cacao enabling early detection of the pathogen in plants. A second species of Ceratobasidium, Ceratobasidium ramicola, identified through ITS sequence analysis, was isolated from VSD-affected cacao plants in Java, and is widespread in diseased cacao collected from Indonesia.

Conopomorpha cramerella (Lepidoptera: Gracillariidae) in the Malay Archipelago: Genetic Signature of a Bottlenecked Population?

Abstract Conopomorpha cramerella (Snellen) (Lepidoptera: Gracillariidae) is a devastating pest of cacao, Theobroma cacao L. (Sterculiaceae), in Southeast Asia, particularly in the Malay Archipelago. We surveyed genetic variation at two unlinked loci, mitochondrial cytochrome oxidase I (COI) and nuclear elongation factor-1α (EF-1α), in C. cramerella from throughout most of their known geographic range. Given the enormous area sampled, COI variation is extremely low; EF-1α variation may be low as well, but this is more difficult to assess due to the lack of appropriate data sets for comparison. Our results strongly suggest that sampled C. cramerella populations have experienced at least one bottleneck in their recent past, although the possibility that COI variation has been reduced by a selective sweep cannot be excluded based on available data. We suggest that one or more bottlenecks likely occurred when C. cramerella from an as yet unknown source population, either within or outside the Malay Archipelago, became established on cacao, which is not endemic to this region (Conopomorpha is an Old World genus and cacao originated in the New World). Identification of the source of this pest could be important in efforts to identify natural enemies for biological control.

Activity Evaluation of Cocoa Pod Borer Sex Pheromone in Cacao Fields

Abstract The previously identified female sex pheromone of cocoa pod borer, Conopomorpha cramerella, was re-evaluated for its attractive activity in different field conditions. It was found that lures containing 100-μg of synthetic sex pheromone blend, (E,Z,Z)- and (E,E,Z)-4,6,10-hexadecatrienyl acetates, and the corresponding alcohols in a ratio of 40:60:4:6 in a polyethylene vial attracted male C. cramerella moths in Sabah and peninsular Malaysia and in Sumatra and Sulawesi, Indonesia, suggesting that the same pheromone strain existed in a wide stretch of the Indo-Malayan archipelago. Of the three kinds of trap designs tested, the Delta traps were more effective than Pherocon V scale traps. Male captures were not significantly different among traps baited with 100-, 300-, or 1,000-μg doses of sex pheromone. A release rate study of pheromone formulation conducted in the laboratory showed that volatile active ingredients were desorbed from polyethylene vials following first-order kinetics, which indicates a satisfactory “half-life time” of a 100-μg loading is ≈6 wk under laboratory conditions. A satisfactory attractiveness of the lure with a 100-μg loading was ≈1–2 mo in the fields.

Phosphonate applied by trunk injection controls stem canker and decreases Phytophthora pod rot (black pod) incidence in cocoa in Sulawesi.

Stem canker and Phytophthora pod rot (PPR) or black pod caused by Phytophthora palmivora are serious diseases of cocoa (Theobroma cacao L.) in Sulawesi, Indonesia, causing high yield losses for smallholders, possibly exceeded only by losses due to the cocoa pod borer (CPB), Conopomorpha cramerella. Potassium phosphonate (phosphite) applied by trunk injection has been demonstrated to effectively control canker and PPR in Papua New Guinea. The method was tested in a field trial in south-east Sulawesi. Fifty trees were injected with phosphonate, 50 with water and 50 were left untreated. Phosphonate was applied at a rate of ∼16 g active ingredient per tree per year, depending on the size of each tree. Trees were evaluated each month for canker severity, for PPR incidence and for CPB incidence and severity. From 4 months after the initial injection, trees treated with phosphonate had negligible levels of canker. Over a 2.5-year period, phosphonate significantly decreased PPR incidence. Cycles of PPR infection occurred in the wet season with PPR incidence fluctuating from less than30% to greater than 75%. These fluctuations might have been due to variations in rainfall causing natural cycles of sporulation and infection. CPB incidence did not differ significantly between treatments. Since trunk injection of phosphonate effectively controls stem canker and decreases PPR in the long term it provides a valuable option for the management of these diseases for cocoa smallholders.

On-farm selection for quality and resistance to pest/diseases of cocoa in Sulawesi: (i) performance of selections against cocoa pod borer, Conopomorpha cramerella

In Sulawesi, Indonesia, cocoa smallholdings are seriously affected by the cocoa pod borer (Conopomorpha cramerella, CPB). CPB is detrimental to both cocoa production and bean quality, and is a major concern to cocoa smallholders, processors, exporters and the international market. An Australian Centre for International Agricultural Research (ACIAR)-funded project was initiated to develop a locally applicable, farmer-participatory methodology for selecting and testing promising cocoa genotypes on farms. In a trial established on a farm in South-East Sulawesi, local Indonesian and international cocoa selections were propagated clonally by side-grafting onto mature trees of mixed genotype and evaluated for 2 years for pod value, quality and resistance to pest/diseases. Local selections were based on the observations of farmers, extension officers or researchers and included a number of clones specifically selected for resistance (or susceptibility, as checks) to CPB. Our results on the CPB-infestation of 34 clones in the trial indicate the potential of employing on-farm selection and testing to improve cocoa farms. Severity of infestation was determined in ripe pods according to whether the proportion of beans damaged by CPB larvae was light (less than 10%), moderate (10–50%) or severe (over 50%). Total CPB incidence in ripe pods for most of the clones was high, exceeding 75%, and was correlated with severe and light incidence, both of which varied more. The data support a model that predicts severe infestation inflicting bean losses occurs above a critical threshold of total CPB incidence. The cumulative CPB incidence in infested pods was significantly lower in a local selection, Aryadi 2, which also had fewer larval entry holes and a low exit/entry ratio indicating a degree of resistance to CPB. The incidence of lightly infested pods was significantly lower in the susceptible controls. Pod hardness was moderately high in Aryadi 2. However, VSD3 (a local selection), Scavina12, KKM22 and BR25, had low levels of severe infestation and/or high incidences of light infestation, but had relatively soft pods. This suggests that factors in addition to pod hardness might be involved in resistance, a possibility also supported by the positive correlation of CPB incidence in ripe pods with that in immature pods and the larger seasonal fluctuations of CPB incidence observed in resistant clones, compared to susceptible clones. In a mixed genotype stand, the lower CPB incidence observed in some clones might be explained partly by pest non-preference.

On-farm selection for quality and resistance to pest/diseases of cocoa in Sulawesi: (II) Quality and performance of selections against Phytophthora pod rot and vascular-streak dieback.

The cocoa industry in Sulawesi, the main region of cocoa production in Indonesia, is threatened by destructive diseases, including vascular-streak dieback (VSD) caused by the basidiomycete Oncobasidium theobromae and stem canker and Phytophthora pod rot (PPR) or black pod, caused by Phytophthora palmivora. Using the considerable genetic diversity of cocoa on farms, host resistance was identified and tested with the participation of farmers. Forty-nine local and international cocoa selections with promising resistance characteristics (as well as susceptible controls) were side-grafted onto mature cocoa in a replicated trial with single-tree plots. Developing grafts were assessed in the dry season for severity of VSD infection, scored from 0 (no infection) to 4 (graft death). All of the 49 clones in the trial became infected with VSD in at least some replicates. Average severity varied from 0.2 to 1.6. Potential VSD-resistance was found in eight clones, including DRC 15, KA2 106 and a local Sulawesi selection, VSD2Ldg. Some of the most susceptible clones were local Sulawesi selections from areas with a history of little or no VSD. Thirty-four pod-bearing clones were evaluated over a 2-year period for yield, quality and resistance to natural infections of PPR. Cumulative PPR incidence for all clones was 22% but varied from 8.6 to 43% among clones. Clones with less than 15% PPR incidence were designated as resistant, including DRC 16 and local Sulawesi selections, Aryadi 1, Aryadi 3 and VSD1Ldg. Scavina 12 was moderately resistant in the trial with a PPR incidence of 23%. Cumulative incidences of the mirid, Helopeltis spp., determined in the same evaluation period, indicated that DRC16 was the most susceptible clone with an incidence of 52% in ripe pods and 23% in immature pods. In comparison, KKM4 showed evidence of resistance to Helopeltis spp., with incidences of 34 and 0.8% in ripe and immature pods, respectively. The impact of diseases and pests (including cocoa pod borer) on bean losses and bean quality varied between clones but generally the bean size (or bean count) was affected more than the fat content or shell content.

EFFECT OF ORGANIC AND INORGANIC AMENDMENTS ON PRODUCTIVITY OF COCOA ON A MARGINAL SOIL IN SULAWESI, INDONESIA

Reduced soil fertility and damage from pests and diseases have contributed to a decline in productivity of cocoa (Theobroma cacao L.) smallholdings in Sulawesi, Indonesia over the last decade. In a trial on a marginal, acidic soil in South Sulawesi, young PBC123 cocoa trees were supplied with compost, mineral fertiliser (NPK fertiliser and urea) or dolomite, alone and in combination. After 20 months, the trees supplied with compost were taller, flowered more profusely and had a five-fold higher dry bean yield than other treatments. Treatments had no impact on incidence of cocoa pod borer, Phytophthora pod rot and vascular streak dieback. All of the trees supplied with compost survived, while the control, mineral-fertiliser- and dolomite-treated trees had a mortality rate of 22–45% and symptoms of interveinal necrosis. Leaf concentrations of N, P and K were within the normal range in all treatments. In the control and mineral fertiliser treatments, leaf concentrations of Ca (0.28–0.30%) and Mg (0.11–0.15%) were deficient, but were higher in trees supplied with compost (0.78–1.21% and 0.26–0.29%, respectively). The Mg/K ratio in soil-exchangeable cations and leaves was increased three-fold by the combined compost/dolomite treatment. Supplying mineral fertiliser alone resulted in 3.3 cmol kg−1 exchangeable Al, compared to 2.2 cmol kg−1 in control soils. Since 10 kg tree−1 year−1 compost was supplied, a rate that is not practical on most cocoa smallholder farms, further investigation of cost-effective applications of organic matter in conjunction with appropriate formulations of inorganic fertilisers is recommended.