Q. My once-healthy dwarf oakleaf hydrangea has black spots on some of its leaves, especially the interior ones. It appears to be a fungal disease. Should I treat it as soon as possible, or can the plant live with it? I prefer to garden organically and would appreciate the lowest impact solution. I was planning to spray Sulfur Guard, if appropriate.

A. There are a number of fungal leaf spot diseases that commonly afflict hydrangeas. Fungal diseases can easily be separated from those caused by other pathogens by the appearance of dark dots -- the fruiting bodies of the fungus -- inside the leaf spots.

A common mnemonic device used by plant pathologists to help students remember this characteristic of fungal diseases is "spots with dots, fungus ya gots!" It helps to look at the spots with a magnifying glass or under a microscope to see the fruiting bodies, although they can often be seen with the naked eye.

The above-average rain in June, followed by the warm weather we are experiencing now is quite conducive to hydrangea problems that we do not normally see until later in the summer and early fall.

The most common leaf-spot diseases are Phyllosticta, Colletotrichum and Cercospora. On oakleaf hydrangea, Cercospora leaf spot appears as brown to purple, angular spots. Severely infested leaves turn yellow and fall prematurely. It is typical for Cercospora leaf spot to start at the base of the shrub and spread up through the canopy. Although these leaf spot diseases rarely kill hydrangeas, they certainly spoil their appearance.

Control options include cleaning up and disposing of the fallen leaves. If they are left in place, they will serve as a reservoir for the fungus, and your oakleaf hydrangea is likely to be troubled by the disease whenever weather conditions favor its development. Avoid overhead irrigation, which wets the leaves, and use drip irrigation, soaker hoses or a watering wand instead. It is helpful if your oakleaf hydrangea is planted in an area where there is good air circulation. Make sure it is not crowded by other plants. This permits the leaves to dry quickly after rain, which helps prevent leaf spot diseases.

Finally, you can make fungicide applications to protect the new growth. Fungicides work best preventively; they will not "cure" existing leaf spots. Organic gardeners can use a copper-based fungicide such as Soap Shield. A sulfur-based fungicide may also help. (I did not find Sulfur Guard labeled to control leaf spot on hydrangeas, but other sulfur-based fungicides were.) Conventional gardeners can use Daconil (chlorothalonil) and Dithane (mancozeb) to control fungal leaf spot on hydrangeas.

Q. How can I make my pink hydrangea turn blue?

A. The blue color of hydrangeas (Hydrangea macrophylla cultivars) is actually dependent on the concentration of aluminum ions in the soil. The aluminum in the soil works in concert with the blue pigment in the flower. Aluminum is most available for the plant to absorb when the pH of the soil is acidic. A pH range between 5.0 and 5.5 allows the plant to absorb enough aluminum for the flowers to be blue. At higher pH ranges, not enough aluminum is available to the plant, and the flower is pink.

Start with a soil test so that you know what the pH is now. That will determine how much acidifying material will be required to lower it into the 5.0 to 5.5 range. Soil test kits are available from your local extension office for a nominal fee. The cost of the kit covers the test as well, so there are no additional fees unless you want additional tests beyond the basic macronutrient and pH test. The kit includes instructions for taking a proper soil sample and paperwork for you to fill out describing the crop(s) you wish to grow in the area being tested. Mail it to Penn State's Agricultural Analytical Laboratory with the paperwork. Results come back in a week to 10 days.

Changing the pH of your soil can be a slow process. When applied to the soil surface, sulfur moves down through the soil profile at about one-quarter inch per year. That slow rate of movement explains why it takes so long to produce the desired results.

For hydrangeas, aluminum sulfate will do a better job of producing blue flowers because it not only acidifies but also adds additional aluminum to the soil. Aluminum sulfate is a granular fertilizer that is available at garden centers and farm supply stores. It is most effective when tilled into the soil prior to planting. When that is not an option, surface applications will eventually help.