Effect of MgSO4 nutrition on Theobroma cacao L. susceptibility to Phytophthora megakarya infection

A new strategy to reduce the severity of black pod disease (BPD) in T. cacao plants using MgSO4 nutrition was investigated. The dynamics of the tolerance to BPD of 18 susceptible T. cacao plantlets coming from the cross (♀SNK64 × ♂UPA14) was monitored during weekly (8 weeks) supply of MgSO4 into the soil. Prior to MgSO4 application, disease scores of the 18 plantlets (in six sets of three plantlets per set) were varying between 3.5 (susceptible) and 5 (highly susceptible). After MgSO4 application, a substantial decrease in disease scores was observed compared to the control. The percentage of disease tolerance gain of plantlets versus MgSO4 supplied (0-2.96 g) presented a quasi-hyperbolic curve with asymptotic line corresponding to 60% (day 28) and 70% (day 56). Cysteine content was not significantly different between the six triplets before MgSO4 nutrition. On days 28 and 56 of MgSO4 supplementation, cysteine content presented a pattern similar to the tolerance gain of plantlet sets. The monitoring of glutathione content versus MgSO4 supplementation (compared to day 0) showed sigmoid (day 28) and hyperbolic (day 56) curves which were associated with defined mathematical laws determined by MALAB software. Negative and highly significant correlations were observed between disease scores, cysteine and glutathione contents in leaves while positive and highly significant correlations were observed between cysteine and glutathione contents in leaves. These data might mean that MgSO4 nutrition significantly improved the tolerance of T. cacao. The mechanism of tolerance improvement might be associated with the synthesis of sulphur-containing compounds (cysteine and glutathione) which might be directly or indirectly used by T. cacao against P. megakarya.