Newsgroups: rec.gardens
From: cheryln@stortek.com (Cheryl Netter)
Subject: Re: How do you prevent rose leaves getting black dots?
Date: Thu, 16 Mar 1995 17:47:29 GMT

yk@s-crim1.dl.ac.uk (Hi Everyone) writes:
> I would be very grateful if anyone out there could tell me why my
> rose leaves get black dots and how I can prevent this from
> happening.

If its blackspot, then the following applies -

From the rec.gardens.roses FAQ part 1:

[3.1] My rose has blackspot, what do I do?

This fungus causes black spots about 1/16 to 1/2 inches in diameter to form on
the leaves and sometimes stems. The infected leaves turn yellow around the
spots and fall from the plant. In bad cases, blackspot can severely defoliate a
rose bush. The conditions that promote blackspot are wet leaves, splashing
water and warm temperatures.

Ways to combat blackspot. These methods also apply to preventing and treating
powdery mildew.

  1.  Pick a variety of rose resistant to blackspot. Many Old Garden Roses and
     David Austin Roses are quite resistant to blackspot. Some yellow Hybrid
     Teas are especially prone to blackspot. However, yellow Floribundas seem
     to be especially resistant to blackspot.
  2.  Use watering methods that don't get the leaves wet: drip watering, using
     a soaker hose, or just soaking the ground with a light stream from a
     garden hose. If overhead watering is used, do so in the morning so the
     leaves can dry off before evening. If modern fungicides are used, keeping
     the leaves dry is unecessary. Wet conditions actually inhibit the
     development of powdery mildew. Warm dry days, cool dry nights are ideal
     for powdery mildew.
  3.  Remove diseased leaves immediately to prevent further spreading of the
     disease. Infected leaves never get better, they just spread the disease.
     Prune infected canes severely in late winter.
  4.  Prune plant so there is good air flow and the sun can reach more of the
     plant.
  5.  Blackspot is transmitted by water splash. Remove leaves close to the
     ground that are more susceptible to getting water splashed on them. Mulch
     well to minimize water splashing onto leaves.
  6.  Keep the plant well watered. A weak or stressed plant is more susceptible
     to disease.

Treatments for blackspot

  1.  The least toxic spray for blackspot is baking soda. Mix 1 tablespoon
     baking soda and a few drops of Ivory liquid with 1 gallon of water. Spray
     both sides of the leaves once a week. The Ivory liquid helps the baking
     soda stick to the leaves. Reapply after a rain.
  2.  Another treatments is sulfur dust. Dust roses with sulfur dust every 7-10
     days.
  3.  Finally, there are the more toxic chemical sprays such as funginex. They
     are often used once every week or two and are very effective. Follow the
     directions carefully.
  4.  Since a single fungicide may not completely wipe out all the fungi, using
     that fungicide over and over may actually produce a build-up of disease.
     Alternating between two fungicides, such as Triforine (Funginex) and
     Daconil is recommended to keep resistant fungi from building up.

cheryl
cheryln@redwood.stortek.com
USDA zone 4 Golden CO

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