Response of rubber tree clones to black crust

Abstract

The occurrence of black crust, a disease previously considered secondary, has become a cause for concern due to early leaf fall in rubber trees (Hevea brasiliensis). This study aimed to identify sources of resistance to black crust in 21 clones of rubber trees in a clonal garden. To achieve this, the disease severity was evaluated through eight assessments conducted over a 2-year period. In each evaluation, 15 leaflets from three different parts of the plant (bottom, middle third and crown) were randomly collected, resulting in a total of 45 leaflets per plant. These leaflets were then taken to the laboratory, and the software Leaf Doctor was used to measure the percentage of leaf area exhibiting symptoms of black crust. The results were analysed using the Scott-Knott test at a 5% probability level. The clones with the highest infected leaf area observed during the evaluations were IAC300, IAC418, IAC503, PB311 and RRIM600. Clone IAC511 did not show any symptoms of the disease in any of the evaluations. Clones IAC301, IAC411, IAC501, IAC507 and IRCA111 exhibited low percentages of affected leaf areas. Additionally, this study revealed the progression of the disease over the evaluation period. The infection initially starts on the youngest leaves (crown) in March and subsequently intensifies in the middle third of the plant, corresponding to the youngest leaf previously infected in the crown. Furthermore, environmental data collected in the experimental area over the 2-year evaluation period indicated that the disease manifests with greater severity when the average temperatures fall below 25°C after periods of heavy rainfall, followed by dry periods with low relative humidity. The data presented in this study significantly contribute to the management of this disease in the field by identifying clones where the disease manifests with lower severity.