These grapevines tend to be hearty plants with good natural resistance to pests and diseases. However, some can be susceptible to downy mildew and powdery mildew, two different fungal conditions. Downy mildew symptoms include yellowing of the leaves and are on the underside. It will form fungus inside the plant's leaves, and if not treated quickly, it can kill the vine. Powdery mildew creates circular spots all over the foliage of your plant. You will also notice yellowing of the leaves if this disease has gone untreated for a few weeks.