BLACK SPOT of Roses

Disease Information

BLACK  SPOT

Symptoms and damage

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.

Management

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.



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