MGCC Help Desk Response: Thank you for contacting the Help Desk of the UC Master Gardener Program of Contra Costa County. Based on the photo you provided, it is likely that your rose are infected by Powdery mildew (Podosphaera pannosa). This fairly common fungi produces mycelia and spores on leaves and shoots, and sometimes on flowers. It looks like a gray (older infections) or white (newer infections) powder on both sides of the leaves, differentiating from Downy mildew, which tends to appear only on the underside of the leaves.
Powdery mildews spread with windblown spores. They do not need water to germinate and die when wet for an extended period. They like moderate daytime temperatures (60-80 degrees F), cold nights and shady and low light conditions. UC Agriculture and Natural Resources (UCANR) recommends a number of measures to control infestations but once a plant has extensive infections it is usually too late to control and prevent damage to the plant. It is hard to determine from the photo how badly infected your rose is, but the following recommended measures may be helpful in managing it.
- Prune during the dormant season to increase air circulation and light penetration. Trim or remove adjacent plants to avoid crowding.
- Remove infected plant parts to reduce spore production. This should be done when the plant is wet and the spores can't generate.
- Irrigate using an overhead sprinkler in the morning when the spores would otherwise be released.
- Some infections may require fungicides, these usually only prevent new infections and may need to be applied repeatedly when conditions favor disease development. Use less toxic pesticide products wherever possible and carefully follow instructions on proper use, storage and disposal to reduce adverse impacts to other species and the environment. Mild to moderate infections can be treated with horticultural oil or plant-based oils, such as neem oil. Do not apply in bright sunshine or when outside temperatures are 90 degrees or more as it may burn or otherwise damage the plant and some beneficial insects. Also, read instructions carefully on preventing harm to bees.
- Disease prevention can also be effective with the application of sulfur products, especially ready-to-use soap like surfactants. These are not effective once the disease has appeared.
If your plant is substantially infected, consider removing it and growing a more resistant cultivar and species. Some examples are Meidiland shrub roses, Rosa rugosa, glossy-leafed hybrid teas, grandifloras, that are trademarked as Care-free, Knock Out, and Home Run. Plant in a sunny location and provide ample adjacent open space for air circulation.
For additional information please see this link to UCANR Integrated Pest Management Program Pest Notes: http://ipm.ucanr.edu/PMG/PESTNOTES/pn7463.html
We hope this information is useful. Feel free to contact us again if you have any further questions.
Help Desk of the UC Master Gardener Program of Contra Costa County (NHP)
Note: UC Master Gardeners Program of Contra Costa's Help Desk is available almost year-round to answer your gardening questions. Except for a few holidays (e.g., last 2 weeks December), we're open every week, Monday through Thursday for walk-ins from 9:00 am to Noon at 2380 Bisso Lane, Concord, CA 94520. We can also be reached via telephone: (925) 608-6683, 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 Biog.
Advice for the Home Gardener from the Help Desk of the
UC Master Gardener Program of Contra Costa County
MGCC Help Desk Response: Thank you for contacting the UC Master Gardener Program Help Desk with your question about the change in the appearance of your rose blossoms.
The most common reason for a change in the appearance of rose blossoms is root stock take over, and we suspect this may be the case with your rose. It is not unusual for the rootstock rose to send up suckers and even take over the preferred rose.
Many roses are grafted. This means that a bud from a preferred species is grafted to the rootstock of a different species usually known for its vigor and longevity. Any shoots that grow from stems above the graft union will produce flowers from the preferred species. But any shoot that grows from stems or below the graft union will produce flowers from the rootstock. The two will usually differ in flowers and foliage form and color. You can trace the canes from the rootstock to their point of origination and remove them; otherwise, they will eventually replace the preferred species.
Here are some helpful links to UC information on rose care:
http://ipm.ucanr.edu/PMG/PESTNOTES/pn7465.html
http://ipm.ucanr.edu/PMG/GARDEN/PLANTS/rose.html
http://ipm.ucanr.edu/PMG/PESTNOTES/pn7463.html
If rootstock takeover doesn't appear to be the cause of the change in your rose blossoms, please give us a call and let us know and we can discuss further.
Help Desk of the UC Master Gardener Program of Contra Costa County (SLH)
Note: UC Master Gardeners Program of Contra Costa's Help Desk is available almost year-round to answer your gardening questions. Except for a few holidays (e.g., last 2 weeks December), we're open every week, Monday through Thursday for walk-ins from 9:00 am to Noon at 2380 Bisso Lane, Concord, CA 94520. We can also be reached via telephone: (925) 608-6683, 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.
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: 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 (http://ucanr.edu/blogs/CCMGBlog/)
Advice from the Help Desk of the
UC Master Gardeners Program of Contra Costa County
Client: I'm having some early summer problems in my garden. I'm reluctant to use pesticides in my garden since I have several small children. Would you please provide some recommendations to either cure and/or minimize the problems:
- Tomato plants are doing reasonably well but there are some funky looking lower leaves that I'm concerned about.
- My roses' leaves are mottled with brown-red markings.
- The flowering pear tree's leaves are mottled and dropping; an arborist recommended injecting a chemical into the tree to cure the problem
- I suspect that my garden soil is too alkaline (i.e. previously tested pH@7.1). How can I test it and what can I do to reduce pH? Will just adding compost reduce pH?
UCMGP's Help Desk Response: Thank you for the photographs, which were very helpful. Our responses follow:
Flowering Pear Tree: The photograph from the flowering pear confirms that it is infected with Entomosporium, another fungus that is spread by splashing rain. http://ipm.ucanr.edu/PMG/GARDEN/PLANTS/DISEASES/entomoslfspt.html. We do not think that this disease warrants fungicide treatment. I do not know what chemical the pest company was
Soil pH: Home test kits for soil are fairly reliable as long as the instructions are followed carefully. A pH of 7 - 7.5 is borderline satisfactory, but may go up over time because our municipal water tends to have high pH. We recommend that you wait until the plants have been harvested and then add garden sulfur to the soil in the fall according to label directions. Lowering the pH with sulfur takes time (i.e., months), so you must be patient. Adding compost will not lower the soil pH.
I hope that this information is helpful. Please do not hesitate to contact us again. Good luck on a successful summer garden.
Editor's Notes: It is likely that the tomato, rose, and flowering pear tree problems may have been caused by the late rains creating conditions conducive to fungus: cool, humid conditions. Warmer, drier weather should minimize these types of fungus infestations provided there is adequate air circulation and not overhead watering early evening.
Help Desk of the UC Master Gardener Program of Contra Costa County (JL)
Note: The UC Master Gardeners Program 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: 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 (http://ucanr.edu/blogs/CCMGBlog/).