Vascular Streak Dieback (VSD) is a disease of cocoa trees, caused by Oncobasidium theobromae. The disease has become one of the major causes of economic losses to cocoa productivity in commercial plantations in Indonesia. A severe infection causes the death of young seedlings and canopy loss in older trees. The maintenance of soil fertility by fertiliser management is one of the recommendations for integrated VSD disease management. The aim of this study was to evaluate the impact of fertiliser application in the plantation environment, and to observe if nutrient management can potentially control VSD disease.
Historical environment data showed that the cocoa plantation of Treblasala Estate, London Sumatra Indonesia experienced a soil pH decline associated with the annual urea application as a source of nitrogen. Soil P status is also relatively high when compared with the suggested critical level for growing cocoa. However, triple superphosphate fertiliser as source of P is applied annually and has resulted in high residual P levels.
Soil and leaf nutrient sampling from selected plantings and genotypes has revealed higher soil pH and soil nutrient levels when compared with historical soil data. Soil phosphorus levels are very high and this might have reduced the uptake of some nutrients, since the levels of these nutrients in cocoa leaves were found to be lower than the critical levels. Magnesium, calcium, copper, zinc and manganese were the nutrients whose levels were found to be below the suggested optimum ranges. These nutrients were also low in the genotype that is susceptible to VSD disease. Calcium and manganese levels of diseased leaves were also lower than healthy leaves, indicating a probable relationship with VSD disease.
To investigate whether high phosphorus supply reduces the micronutrient status, nutrient studies were carried out using young cocoa seedlings in Gatton, Australia under glasshouse conditions, and under the natural environment in Indonesia. The studies revealed that nutrient imbalances may occur in response to high P fertiliser application, although a reduction in leaf micronutrient status was not observed. The results suggest that P application is not beneficial to the soils on the Treblasala Estate.
To examine if the application of micronutrient fertiliser is potentially beneficial in reducing VSD disease severity, responses to a combination of phosphorus, potassium, copper, and zinc fertilisers were tested in tolerant and susceptible genotypes and exposed to natural VSD inoculums in Indonesia. The study showed that none of the fertiliser treatments directly influenced VSD severity. VSD tolerant genotype consistently gave higher leaf nitrogen, magnesium, calcium, copper, zinc, and manganese concentrations when compared with susceptible genotype. The results confirmed that P application is not beneficial in the Treblasala Estate cocoa plantation, and if the soil P is very high, a low rate of application of copper fertiliser may be necessary for better plant growth. However, any copper fertilizer application would need to be at a low rate, because potentially antagonistic effects of copper and zinc were observed in the study.