Black crust complex in rubber tree: influence of different environmental conditions evaluated by UV microscopy.

The black crust has become a concern in the production of rubber trees, as it advances rapidly in the areas of cultivation in Brazil. This work aimed to evaluate the development of Rosenscheldiella heveae and Cladosporium cladosporioides on rubber tree leaflets of clone RRIM600, under the influence of different temperatures and wetness periods. The experiments carried out with each fungus were set up separately. In both experiments, rubber tree leaves were inoculated with aliquots containing 30μL of spore suspension (105 conidia. mL-1). Then, they were kept in a humid chamber at 10, 15, 20, 25, 30, 35 and 40±1°C in the dark with a wetting period of 4, 6, 12, 24, 48 and 72 hours. Samples from the inoculated points on the leaf were fixed in “Karnovsky's” solution, processed and treated with Calcofluor White for observation under a UV microscope. The evaluation was of the quantitative type of fungal development, based on the structures produced by the fungus (mycelium and spores) on the leaf. These analyzes were performed using Basic Intensity Quantification with ImageJ software. The average percentage of leaf area affected by the fungus in each environmental condition tested was statistically analyzed to find the best period in which the fungus developed. Thus, it was possible to verify that the development of both studied fungi is affected by temperature, since their development is greater in the temperature ranges between 20 and 25oC. In the case of the wetting period, it can also contribute to the evolution of the fungus in the plant, since the longer the wetting period, the better the development of the fungus. Thus, these initial observations improve the understanding of the cycle of the crust black fungus complex, since there is little information about this disease in rubber trees.