Crown and spear rot

Phytophthora megasperma is a species of water mould in the family Pythiaceae. It is well known as a plant pathogen with many hosts. It often causes a plant disease called root rot.
This is a poorly defined species which is generally called a species complex. Its name applies to water moulds of many forms, functions, and host preferences, many of which are actually different species which have not yet been separated and described. Some species previously treated as part of the complex include Phytophthora rosacearum, P. sansomeana, P. sojae, P. medicaginis, and P. trifolii. A form of P. megasperma is suspected to be a separate species when it is recognized to have host specificity, when it is found attacking just one host plant; alfalfa in the case of P. medicaginis, for example.