Cercospora Leaf Spot (also known as Cercospora hydrangea), is typically brown or purple colored spots at the base of the plant. The spots are small that typically measure 1/8 to 1/4 inches in diameter. Hydrangea is susceptible to several other diseases. Most fungal diseases found on hydrangeas cause leaf spotting on the foliage rather than the stems, so I suspect that the white spots could be the eggs of a sap-sucking scale insect called Pulvinaria hydrangeae, or hydrangea scale. There are two types of fungal diseases can infect hydrangeas - cercospora leaf spot and anthracnose. Since anthracnose can be a difficult disease to treat, when you see the first signs of infection, I recommend taking cuttings from healthy parts of your plant right away. This limpet-like insect also feeds on acers and cherries. 1a. Other Hydrangea Diseases. This is mysterious. Brown spots appear, grow in size and spread over the plant. Other diseases that can affect hydrangea include blister rust (Pucciniastrum hydrangea), bacterial wilt (Pseudomonas solanacearum), viruses, and virescence (caused by a phytoplasma). Here is a brief listing of what they are and how to treat them: Anthracnose – Hot, wet weather conditions help foster development of this disease that normally affects heavily-fertilized hydrangeas. By taking cuttings that you may root and transplant into the garden, if the disease does end up killing up your hydrangea, you will at least have a few replacements ready to go. The blister rust fungus produces masses of orange spores on the underside of the leaves.