Blackspot on roses (Diplocarpon rosae)

GPI · Last edited by GPI on Mon Aug 18, 2008 4:16 pm; edited 2 times in total

Blackspot on roses (Diplocarpon rosae)
by GPI

Plants affected
Roses.

Season(s)
Spring to early autumn.

Signs and Symptoms.

Initially appears on leaves as purple or brown/black circular spots with yellow-fringed halos, which may join up over time. The spots will alway have irregular edges.

These spots may also be visible on the plants leaf stalks and the stems.

Eventually the infected leaves will turn yellow and fall before time from the rose.

. Blackspot as it appears on leaves of the rose, photo / picture / image.

Cause.
The fungus Diplocarpon rosae which is encouraged by much the same conditions that encourage potato blight, which are warm, moist locations with stagnant air. Fungal spores are released under warm, wet conditions and spread by rain-splash and breezes.

Trouble?
Yes, most Irish roses especially those grown in areas of high rainfall and clean air are destined at some stage of their growing life to play host to blackspot. Although black spot will rarely kill a rose outright, it will however leave you with a sickly, twiggy rose, which flowers poorly due to a lack of its life-giving leaves.

Organic or cultural control.

Try to grow only varieties labelled as resistant to Apple scab wherever possible. See list below...

Amber Queen (golden yellow),
Bonita (orange-apricot)
Iceberg (white),
Trumpeter (red),
Electron (Deep pink),
Helmut Schmidt (Golden yellow),
Just Joey (Creamy peach),
Keepsake (Dark pink),
Las Vegas (Dark peach with yellow highlights),
Peter Frankenfeld (Dark pink),
Polarstern (White),
Precious Platinum (Medium red),
Silver Jubilee (Salmon pink),
Voodoo (Orange),
Love (Crimson red with white backs),
New Year (Orange),
Tournament of Roses (Rose pink),
Bonica (Rose Pink),
Escapade (Mauve-pink),
Europeana (Dark red),
Impatient (Orange-red),
Liverpool Echo (Orange),
Matangi (Red),
Mermaid (clear yellow)
Orangeade (Orange-red),
Play Girl (Bright pink),
Playboy (Reddish orange),
Redgold (Golden yellow edged in dark pink),
Regensberg (Pink and white),
Royal William (velvety-red)
Sarabande (Orange-red),
Sexy Rexy (Rose-pink),
Showbiz (Scarlet red),
Viva (Red).

Note: Even varieties of rose touted as very resistant to blackspot have been know to fall foul of it, as the blackspot fungus has several strains, some of which your rose may not be resistant to.

Dig in lots of organic matter when planting your roses and keep plants well fed with a fertiliser high in potash spring and summer of each year. Occasionally digging in a banana peel around each bushes is a good way to apply a trickle of slow release potash to each plant .A well fed plant can resist blackspot much better than a hungry specimen which will let anything overtake it.

Regularly rake around the plant, collecting fallen leaves, then burning them. This is especially important in autumn as it will help prevent the disease overwintering. Also, pick or snip off any live leaves that exhibit black spots, as well as looking unsightly they aid the spread of the disease.

When pruning your roses each year be sure to cut out any stems which show signs of infection. Again like the leaves burn the prunings.

Note: Check with your local authority as to the legality of burning garden waste outdoors. If there is an issue, infected leaves and shoots can instead be buried deeply.

To prevent disease transfer between plants, you must wipe down your pruners with alcohol or surgical spirit before and after pruning each rose.

Apply a layer of mulch to the base of your rose plants each spring to help prevent previously fallen blackspot spores being splashed on to roses by rain. If that mulch is well rotted farm-yard manure rather than bark mulch, then you will be feeding the plants as well.

There are two organic home remedies which are possible. I have had varying success with both, depending on time of application, rose variety etc. They use either milk or baking soda.
Method 1, Milk.
Mix equal parts milk and water, then apply this each week with an atomiser or a sprayer to the upper and lower section of the roses leaves. This milky solution causes an invisible and friendly fungus to form, which will help prevent the formation of the dreaded black spot.
Method 2, Baking soda.
Mix one tablespoon of baking soda or baking powder into one litre of water and add a drop or two of washing up liquid for stickiness. Again, apply this each week with an atomiser or a sprayer to the upper and lower sides of the roses leaves. The baking soda (Sodium bicarbonate) causes the rose leaf surface to become exceedingly alkaline which again prevents the blackspot from thriving. Both method 1 and 2 are effective only if used before (early April) or at the first sign of symptoms.

Another “organic” blackspot spray is known as Burgundy mixture, this is not a totally organic spray, but is a deemed acceptable by most organic growers.
Burgundy mixture is created by mixing......
50 grammes of copper sulphate (bluestone)
60 grammes of sodium carbonate (washing soda) or sodium bicarbonate (baking soda)
All in five litres of water.

Blackspot disease is most prevalent in areas of low air pollution occurring less frequently in polluted city areas. Its not very practical, but if you are a rose fanatic, you may be better moving closer to an area with atmospheric pollution problems. Smile

Other Control.

Spray with a fungicide containing the systemic active ingredient myclobutanil such as Bayer Garden Systhane Fungus Fighter, Multirose or RoseClear 3

Spray with a fungicide known as Captan

Spray with a fungicide containing the systemic active ingredient penconazole such as Scotts Fungus Clear

Spray with a fungicide containing the systemic active ingredient flutriafol such as Rose Clear Gun

You can also spray with a fungicide containing the protectant active ingredient mancozeb such as Bio Dithane 945.

Note: Spraying with two of these fungicides mentioned above should begin in early April and continue every two weeks up until October. Selecting two different products/active ingredients and alternating between one or the other on each spraying will reduce the chances of resistance to just one building up.

Note: Please avoid using dual action products containing insecticides as well as fungicide such as Multirose or RoseClear 3, unless a pest problem is actually present as well.

Nature notes.
Please note that Irishgardeners.com does not advocate the removal of fungal, animal or insect life, instead, this forum encourages wildlife preservation. However, there are occasions where a wildlife becomes a problem within the garden for certain people. Be aware that wildlife is a link in the chain of life, benefiting something or someone down the chain. Please at least think about this before you remove wildlife from your site.

(DISCLAIMER: The control methods are suggested here as a matter of general information. Under Irish and EU law it is illegal to use any preparation as a pesticide/fugicide/herbicide that is not approved for such use. The author and the website accepts no responsibility for how a user may mix, use, store, or any effects the mixture or its elements may have on people, plants or the environment. The information here is for reference only and does not imply a recommendation for use. If you disregard this warning and make any of the preparations, you do so entirely at your own risk.

Associated content...
Rose pruning

Secrets Of Organic Rose Plant Gardening in Ireland

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Dr. Sunny Thomson · Posted: Fri Jul 04, 2008 1:35 pm Post subject:

MMMMMMega post, thanks for that gpi.