Help for the Home Gardener from the Help Desk of the
UC Master Gardener Program of Contra Costa County
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.
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)
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: email@example.com, 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 (http://ucanr.edu/blogs/CCMGBlog/)
Client's Question and Request:
I'm in central county and growing Zinfindel Grapes in my backyard garden. The vines are now several years old and producing fruit this year. The grape leaves have now developed “blotches” and the grapes look “cloudy” and not very healthy. What's the problem and what can I do about it? The pictures below show the leaf damage and what the grape bunches look like.
MGCC Help Desk Response and Advice:
Based on the grape samples and the photos you provided, the problem with the grapes appears to be powdery mildew.
Powdery mildew is a common fungal disease on grapes. It first shows up as faint white powder on the grapes but later can progress to cause brown russeting on the developing grapes. That russeting was somewhat apparent on the grape samples you brought in. Affected fruit cannot ripen normally and may crack as it grows.
Shady conditions and lack of good air circulation favors the development of the disease on grapes. When the vines are pruned iduring dormancy so that shoots are positioned in the next growing season, try to prune so that the plants will allow exposure of the developing fruit to sunlight and good air circulation. Avoid overhead watering of the vines which can spread the fungal spores to new locations.
Next growing season, watch closely for the appearance of powdery mildew. Early signs of a developing problem include young emerging leaves being deformed or showing a puckered condition. Prune out such areas as soon as they appear and it may help to prevent new infections.
If you lose a large percentage of the grapes this season, you may also want to consider the use of fungicides to prevent a recurrence next year. Take a close look at this UC website which gives detailed information about different types of fungicide that can be used and includes directions on how and when they should be applied: http://www.ipm.ucdavis.edu/PMG/PESTNOTES/pn7494.html
We hope this information is helpful. You're welcome to contact us again with any further questions
Help Desk of the Master Gardeners of Contra Costa
Note: The Master Gardeners of Contra Costa's Help Desk is available year-round to answer your 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. We can also be reached via telephone: (925) 646-6586, email: firstname.lastname@example.org, or on the web at http://ccmg.ucanr.edu/Ask_Us/