It is a generally accepted standard that editors should attempt to follow, though it is best treated with common sense, and occasional exceptions may apply.
Finer compass points take a hyphen after the first word, regardless, and never use a space: Use sentence case, not title case, capitalization in all section headings.
While the ICNCP has recently preferred the term "Group" (used by itself and capitalized) to refer to the cultivar group concept, please use the lower-case phrase "cultivar group" (aside from "Group" within an actual scientific name), as it is both less ambiguous and less typographically confusing to the average reader.
Orders, families and other taxonomic ranks above genus level have an initial capital letter (and are not italicized): wikiprojects for some groups of organisms are in the process of converting to sentence case where title case was previously used.
When an independent clause ends with a dash or semicolon, the first letter of the following word should not be capitalized, even if it begins a new independent clause that could be a grammatically separate sentence: The same usually applies after colons, although sometimes the word following a colon is capitalized, if that word effectively begins a new grammatical sentence, and especially if the colon serves to introduce more than one sentence. In a list, if each item of the list is a complete sentence, then it should be capitalized like any other sentence.
If the list items are sentence fragments, then capitalization should be consistent – sentence case should be applied to either all or none of the items.