。
If it rains, the match is off.
Literal
By-chance rain [subject-が] fell-[conditional-たら] match [topic-は] cancellation [copula-だ].
中止だ ('it's a cancellation') is the bare noun-predicate form — flat and decisive, asserting the cancellation directly without softening it through になる ('becomes a cancellation'). The terse register fits a quick practical declaration ('match's off') over a more announcement-style phrasing.