Managing Several Common Garden Rose Diseases

Help for the Home Gardener from the Help Desk of the
UC Master Gardener Program of Contra Costa County

Client's Request (via phone):  Help! My roses are showing some sort of diseases. My neighbor says that there are both Powdery Mildew and Black Spot on the roses. Would you please tell me what to do to make them healthy again?

MGCC Help Desk Response (via email):  Thank you for calling the UC Master Gardener Program Help Desk with questions about your roses with (probably) Black Spot (Diplocarpon rosae) and/or Powdery Mildew [Podosphaera (previously Sphaerotheca) pannosa var. rosae].

Without the roses in-hand or recent photos, we can't be sure that the problem is appropriately identified. However, both of these diseases are common rose problem this time of the year and can be confirmed by pictures in the linked references below.

The link following contains information about rose diseases, including the Black Spot and Powdery Mildew that you are most likely experiencing on your roses:  http://ipm.ucanr.edu/PMG/PESTNOTES/pn7463.html

Both of these are fungal diseases. Both may be the result of the uneven weather we have been having with rain and warmth and cool weather all at almost the same time.

As we previously discussed, the initial treatments for the two are somewhat contradictory. Black Spot results from water sitting on the leaves. To avoid Black Spot, the leaves should be kept as dry as possible with no more than seven hours of moisture on the leaves. Powdery Mildew can be treated by washing off the leaves. The compromise would be to wash the leaves off during a warm period (say by 10am) to ensure that the water evaporates quickly. Also make sure that any fallen leaves are cleaned up from below the plants and disposed of into your garbage not into your compost. See the Pest Note link following : “Powdery Mildew on Ornamentals” for more details on management. http://ipm.ucanr.edu/PMG/PESTNOTES/pn7463.html.

Thank you for contacting Master Gardeners with your questions. Please do not hesitate to call again.

Help Desk of the UC Master Gardener Program of Contra Costa County (ECS)

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Note: The  UC Master Gardeners Program of Contra Costa's Help Desk is available year-round to answer our gardening questions.  Except for a few holidays, we're open every week, Monday through Thursday for walk-ins from 9:00 am to Noon at 75 Santa Barbara Road, 2d Floor, Pleasant Hill, CA  94523, although we will be moving this spring. We will notify you if/when that occurs. We can also be reached via telephone:  (925)646-6586, email: ccmg@ucanr.edu, or on the web at http://ccmg.ucanr.edu/Ask_Us/  MGCC Blogs can be found at http://ccmg.ucanr.edu/HortCoCo/ You can also subscribe to the Blog  (//ucanr.edu/blogs/CCMGBlog/)