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Diplocarpon rosae - Arghhh!!!
August 21, 2013

Welcome to the Flying Butterfly!

Rain, rain go away 'cause now my Rudbeckia have
Diplocarpon rosae, or black spot disease.

If you are seeing spots to, read on....

Black spot disease thrives in moist conditions,
in my case a very rainy gardening season. It's most prevalent
in many rose types (giving it the more common name Rose Blackspot)
but will also show up on plants that have fleshy leaves and stems.

This is a fungus that usually shows up late summer. Symptoms are:

  • Black spots on both sides of leaves
  • Plant tissue around spots turns yellow, causing leaves
    to fall off prematurely
  • If left untreated, fungus can get into stems
  • Can inhibit blooms

As summer advances so will the disease. If left untreated the plants energy will go to fighting the rapidly spreading fungus and weaken plant.
If the disease becomes more difficult to control aphids can move in.

Although it's highly likely plants will survive, before disease accelerates to the above level some basic things to start with:

  • Remove infected leaves, pick up fallen leaves, especially in autumn
  • Destroy leaves or throw out. Don't throw leaves in compost pile.
  • Prune plants to:

      1) Increase sunlight to all leaves of plant
      2) Increase air circulation. If leaves are clustered densely, sunlight can't reach all leaves and humidity sets in.

  • If mulch is present, it's a good idea to replace old mulch with new.
  • Don't get leaves wet when watering. Just water roots.

Once established, Methods of eradicating

1) Baking Soda (Sodium bicarbonate)
Baking soda has fungicidal properties
which change pH on leaf to the point where fungus can't survive.

Baking soda has also been shown to
help with powdery mildew (warm, humid weather), not the same as downy mildew (cool, humid weather).

Depending on the job size, recipe(s):
    1 teaspoon.. /.. 1 Tablespoon baking soda
    1 quart....... /.. 1 gallon warm water
    1 teaspoon.. /.. 1 Tablespoon (optional) of liquid dish soap
    ...................... to help cling to leaves longer.
Use baking soda one time weekly until temperatures get above 85 degrees.

2) Neem Oil
This is oil pressed from evergreen trees. This is a natural alternative and can be effective. The draw back is if oil doesn't work it's recommended to wait 30 days before applying other methods of elimination.

3) Sulfur
This has been used for centuries to control plant pathogens and pests on contact. Sulfur can be purchases alone or mixed with other insecticides. Use when temperatures are between 65 degrees and 85 degrees.

4) Recommendations
  • Avoid planting vegetation close together
  • Plant vegetation in well drained, sunny areas
  • Consider rotating plants since fungus can continue to grow

Because spores are spread by splashing water and wind, as fall arrives winds pick up. If leaves aren't picked up Diplocarpon rosae is not eliminated. Fungus can over-winter and appear again in spring.

If not used before, apply mulch in late winter
over fallen leaves. This smothers any fungal spores on them.

What to look for after winter:

In spring when temperatures reach 60 degrees the blackspot disease will begin to spread. When temperatures are at 70 degrees, fungus grows fast but slows drastically with hot temperatures of 85 degrees and higher.

Prevention recipe after winter:
Spray skim milk one month or so before your areas last frost date. Spray stems and both sides of leaves. Do this in the morning. Re-apply every
5-7 days for 6 weeks. The skim mild encourages and invisible 'milk fungus' that is harmless to plants but keeps black spot disease away. Don't use milk with fat in it. This clogs pores in leaves.

With all recipes - spray on dry days in between rainy one's.
Also, avoid watering plants on cloudy days.

Until next time, Happy Gardening!

Elizabeth @ Easy Butterfly Garden

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