Rubber tree resistance to black crust: two years evaluation in clonal garden

The occurrence of black crust, whose causal agents are Phyllachora huberi and Rosenscheldiella heveae, disease previously considered secondary, became a cause for concern due to the early fall of leaves on the rubber tree (Hevea brasiliensis). There are no agricultural pesticides registered for this disease in Brazil. This study aimed to identify sources of resistance to black crust in 21 rubber tree clones in a clonal garden and assess the ideal climatic conditions for the development of the disease. For this, the severity of the disease was evaluated based on eight assessments carried out over two years. In each evaluation, fifteen leaflets from three parts of the plant (bottom, middle third and crown), totaling 45 leaflets per plant, were randomly collected from three different plants of each clone. The collected leaflets were taken to the laboratory, and the percentage of leaf area with symptoms of black crust was measured with the aid of software. The results were submitted to the Scott Knott Test at a 5% probability level. The clones with the highest infected leaf area during the evaluations were IAC300, IAC418, IAC503, PB311 and RRIM600. Clone IAC511 did not show symptoms of the disease in any of the evaluations. Clones IAC301, IAC411, IAC501, IAC507 and IRCA111 also showed some disease resistance, as they had low percentages of disease-affected leaf areas. The climate data obtained during the tests, at a climatological station located close to the collection areas, shows that the disease manifests with greater severity when the average temperatures are below 25°C after periods of high rainfall. This study identified highly resistant rubber tree clones (IAC 511) and partially resistant to black crust (IAC301, 411, 501, 507 e IRCA111), providing informações for future tests focused on the genetic control of this disease, as well as helping producers to choose resistant clones in areas with a history of the disease.