The infection cycle starts when spores are spread by rain or
overhead irrigation from leaves or canes infected from the
previous season. The conidia must be wet for several hours
to infect plant tissues. Symptoms begin to appear in three to
16 days after infection. Mature conidia can be produced 10
to 18 days after infection and initiate a new cycle. Conidia
are colorless and two-celled (Fig. 2b). A temperature of 64°F is optimal for black spot development, but conidia
germination still occurs from 59 to 81°F. This wide tem-
perature range allows the disease to continue to develop as
long as the moisture is adequate during the season.