The Pará rubber tree ("H. brasiliensis") occurs mainly south of the Amazon, as does "H. camporum", but the greatest diversity occurs to the north of the river, in the Rio Negro region, where all the other species occur. In this area where there are variations in soil and topography and the rainforest experiences conditions of all-year-round humidity, the genus "Hevea" has been undergoing a high degree of speciation. The high humidity encourages the growth of fungal leaf diseases, and the species that are deciduous avoid immediate transfer of fungal spores from old leaves onto new growth. The Pará rubber tree has been introduced to and is naturalised in many tropical countries in Asia.