Effects of cassava allelochemicals on rubber tree pathogens, soil microorganisms, and soil fertility in a rubber tree cassava intercropping system
The rubber tree (Hevea brasiliensis) cassava (Manihot esculenta) intercropping system is one of the most popular intercropping patterns for small holder rubber planters. However, this practice is not recommended in some rubber producing countries, since it is believed that the rubber tree and cassava share some identical pathogens and that the intercropping system will deplete soil fertility. The effects of cassava allelochemicals on rubber tree pathogens have never been reported. Using controlled plate tests, pot tests, and field experiments, this study aimed to assess the possible adverse effects that cassavas may impart to rubber plantations. The results showed: (1) although the leaf extract, decomposed leaf extract and root exudates of cassava inhibited or had no effect on the mycelial growth of the rubber tree pathogen Corynespora cassiicola, 10 mg mL 1 and 100 mg mL 1 cassava leaf water extracts and testing concentrations of cassava root extraction significantly promoted the mycelial growth of the rubber tree pathogen Rigidoporus lignosus, which increased the risk of rubber tree white root disease; (2) the overall soil fertility was increased significantly in the rubber tree cassava intercropping system. However, the soil phosphorus content was reduced significantly in the field experiment; (3) the average girth of rubber trees was significantly reduced, and the girth uniformity was decreased in the intercropping system. Based on the experiment results, we suggested that an optimised fertiliser recommendation, particularly on the phosphorus amount, should be developed and implemented for the rubber tree cassava intercropping system. In addition, since the intercropping of cassava will increase the risk of R. lignosus induced rubber tree white root disease, which is very hard to prevent and eradicate once rubber trees are infected, the intercropping of cassava in rubber plantation rows should not be recommended in the R. lignosus prevailing areas.
Journal of Rubber Research