Wednesday, July 30, 2014
YOLO COUNTY NEWS
99 CENTS

Garden doctor: Heat causes maples’ sunburned leaves

These nectarines are infected with powdery mildew, a fungus that results in dwarf-sized fruit that is distorted and covered with this white, powdery growth, as well as rough, corky-like spots on the skin of the fruit. Apricots, apples and peaches also are vulnerable. Courtesy photo

Question: The ends of the leaves on my Japanese maples are brown, dry and fall apart with a mere touch. What’s wrong?
Answer: With the extreme heat we’ve had, seven days of over 100 degrees, many plants exhibit forms of sunburn also known as sunscald, usually appearing as brown blotches in the middle of leaves. Also, the problem you describe can be the result of the boron in our water, leaving the leaves with brown tips. However, in Japanese maples during prolonged hot temperatures, the leaves will exhibit dry, brittle ends on the delicate leaves. This is due to insufficient water.
We all know extra water is needed as temperatures climb but Japanese maples’ roots are shallow, running along the surface. The roots are sometimes unable, in extreme heat, to get sufficient water to the very tips of the branches. Thus, the leaves appear to have a tip burn.
Be certain your irrigation system is working properly, check for leaks and be sure sufficient amounts of water are being distributed beneath the tree canopy. Increase the watering time or amount if needed. Watch weather forecasts and try not to fertilize before extremely hot days. Fertilizing will produce new tender growth that is easily sun-damaged.
Plants that are prone to problems with extreme sun exposure can be protected by planting near or around a large tree that will offer filtered shade during the day. An overhead trellis also can provide adequate protection. If the plant is especially important to your landscape, make a temporary sunscreen trellis by stretching shade cloth over tall tree stakes.
The leaves may look sad and unappealing but be patient. Fall will come, leaves will drop, and spring will bring new, lush growth. The tree should not be permanently damaged from these hot days.
Question: What happened to our nectarines? Some are fine, but many, as in the picture, have this white growth on them.
Answer: What you have is a powdery mildew. Most of us think of powdery mildew as something that affects only roses, but there are many types of powdery mildew. This is one that affects the smooth skin of nectarines.

Powdery mildew is a fungus. The most common one for stone fruit, as well as apple, is Podosphaera species. All powdery mildews like our Mediterranean climate so are more prevalent in our gardens than other plant diseases. The spores are carried by wind to the fruit. The thin layers of mycelium grow on the surface of the fruit and will be white to gray in color. The affected spots on the fruit eventually can grow together to form a large patch (this is what you see in the picture).

Powdery mildew usually attacks new growth and the outcome is dwarf-sized fruit that is distorted and covered with this white, powdery growth. Looking at some of the samples you have, we see scars as well as rough, corky-like spots on the skin of the fruit (“russetted”). This is where the infection occurred. You also may see this on apricots, apples and peaches.

We have had a wet June with showers and rain. The last week of June brought 48 hours of 100 percent humidity, which is perfect for the spores to spread. One cannot control the weather, but there are a couple of things that can be done to help prevent the infection. Thinning the fruit as well as good tree pruning will help in the prevention of the spores and mycelium from spreading. The picture shows two fruit side by side and the mycelium spreading between the fruit.

The spores and mycelium are sensitive to extreme heat and direct sunlight, so this heat wave we have just had will help to kill the fungi. Still, you may want to seriously consider spraying with a fungicide in the fall to prevent any residual spores from infecting the tree next year.

The UC Davis integrated pest management website is always a good source of information: www.ipm.ucdavis.edu. See the “Identify” and “Manage Pests” tab at the top.

— Send questions, addressed to the “Garden Doctor,” by email to mgyolo@ucdavis.edu, voice mail to 530-666-8737 or regular mail to UCCE Master Gardeners, 70 Cottonwood St., Woodland, CA 95695. Be sure to include your contact information because any questions not answered in the Garden Doctor column will be answered with a phone call or email to you.

You can request the Yolo Gardener newsletter delivered by email and learn more about the Master Gardener program in Yolo County at http://ceyolo.ucdavis.edu/Gardening_and_Master_Gardening.

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