Correct way to type a genus name with or without its species part is always italics. For eg Genus spp or Genus. We don't use italics for "spp" because it is a short form used to refer to a group of organisms and is not a latinised part of the scientific name.
Genera and species names should be in italics when referring to the organisms, but words (or abbreviations) like "species" should not. Thus, if saying Rhizoctonia spp., the genus is in italics while the abbreviation for species is not. Similarly, for a single, unknown species, one would not have the abbreviation in italics, like Fusarium sp. If you are specifying a known species, that also would be in italics, like Colletotrichum coccodes.