From: dmiller@theory.lcs.mit.edu (Dick and Jill Miller)
Newsgroups: rec.gardens
Subject: Black Spot on Roses
Date: 26 Aug 92 12:00:36 GMT
Organization: Laboratory for Computer Science, MIT

With all the discussion about black spot on roses lately, I thought I'd 
send along this paper from the Cornell Cooperative Extension.

--Jill
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Black Spot on Roses

Black spot (Diplocarpon rosae) is a fungal disease of roses, characterized 
by circular black spots on the leaves.  Similar areas may also be found on 
the petioles, stems, flower receptacles, and sepals.  The spots start out 
small, but then enlarge and attain a size of 1/2 inch or more in diameter.  
On susceptible rose varieties, infected leaves become yellow and fall 
prematurely.  A rosebush severly affected with black spot may lose nearly 
every leaf.

_Diplocarpon rosae_ overwinters in fallen infected leaves or stems on the 
plant.  As with many fungal diseases, warm wet weather and splashing rain 
help to disperse the spores to new, succulent growth.  Leaves are most 
susceptible to fungal penetration while they are still growing and have not 
reached mature size yet.

As the fungus grows, the leaf surfaces above and below the infection site 
blacken.  The time between the beginning of the infection and the 
appearance of a black spot can vary anywhere from 3 days to 16 days.  
Within 10 days of the appearance of a black spot, spores are produced on 
the surface of the infected area, to be splashed again to other leaves.  
During periods of extended wet weather, this disease can be very severe.

What can be done for black spot disease?  Of primary importance is plant 
sanitation.  When foliage is dry, prune and discard or destroy affected 
plant parts.  In the fall, discard or destroy all fallen leaves, hips, and 
branches.  A heavy mulch under the plants can cover missed fallen leaves, 
and thereby prevent their overwintering spores from being splasheed up onto 
new foliage during the next spring.  Be sure to keep mulch away from the 
stem of the plant, to prevent rot.

Other controls include:  planting roses in sites with good drainage, 
spacing plants adequately apart so air moves freely between them, and 
avoiding overhead watering.  Since black spot is spread by splashing rain, 
watering your roses with a sprinkler has the same effect.  Try to water the 
soil underneath the the plant instead.  If the infection is severe, 
fungicides may be necessary.  Apply chlorothalonil or triforine 3 times at 
10- to 14- day intervals.  Other pesticides available include mancozeb, 
copper hydroxide, benomyl, and lime-sulfer; use according to label 
directions.

Finally, certain rose varieties are more susceptible to black spot than 
others.  The American Rose Society publishes information listing rose 
varieties susceptible to (or resistant to) rose diseases.  Look for this 
information in books specifically dealing with roses, in your library.

        * * * * *

8/92: Prepared by Jennifer Tanner, Cornell Cooperative Extension 
Horticulture Assistant, Schenectady County,

Reference: Nichols, L.P., "Rose Black Spot", Department of Plant Pathology, 
The pennsylvania State University, University Park, PA 16802, and Cornell 
Cooperative Extension Publication #74 "1991-92 Guide to Pest Management 
around the Home".

        * * * * *

The information given herein is supplied with the understanding that no 
discrimination is intended and no endorsement by Cornell Cooperative 
Extensison, Schenectady County, is implied.


--
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