The fungus causing black spot
produces black, nearlycircular lesions on the upper leaf surface. The lesions
are2 to 12 mm in diameter with fringed margins. Leaf tissuesurrounding the
spots turns yellow and, as the infectionbecomes more severe, the infected
leaves fall. Young
leaves are most susceptible.
Petioles, peduncles, fruit sepals and petals may also be infected.
Raised,irregularly-shaped, reddish-purple to
black blotches develop on the
immature wood of first-yearcanes. Tiny black flecks may form on the leaves of
resistantvarieties in response to the fungus.
COMMENTS on disease
The fungus overwinters on fallen
leaves and diseasedcanes. It tolerates a range of temperatures from 15to 27oC,
but is most able to infect rose leaves attemperatures between 19 and 21oC.
Symptomsdevelop within 3 to 4 days at temperatures between22 and 30oC.
Infection can only occur when leaves arewet. During extended periods of cool,
wet weather any
cultivar may be seriously infected.
Pick up fallen leaves and prune
canes that containblack spot lesions. Allow good air circulation throughthe
leaf canopy, both by spacing plants properly whenplanting and by pruning to
open up the plant. Fungicidesmay be required for susceptible cultivars. Begin
applications as leaves expand in the
spring, spraying atintervals of 10 to 14 days. Fungicides active against
blackspot are benomyl, thiophanate-methyl, triforine, folpet,
ferbam and lime sulphur.