Mildew may appear as a chalky or smooth coating in black, gray, white or pink on many plants, including the acanthus. Acanthus plants are perennial plants that grow quickly and in profusion if they are not contained. They have bright spikes of flowers and aesthetically pleasing foliage when they are not in bloom. While it is unlikely that powdery mildew will kill your acanthus, it certainly will cause a fair amount of cosmetic damage if allowed to grow unchecked. Fortunately, there are several easy cures for mildew on an acanthus plant.
If you notice mildew spots on your acanthus on the leaves or flowers, the most effective way to get rid of the problem is via sterile pruning. Use rubbing alcohol to clean the blades of your clippers after each cut as you remove all affected foliage. Place the foliage in a sealed plastic bag rather than allowing it to fall to the ground, and remove all dead foliage from underneath the plant as well. According to the Acanthus National Collection, when cut back acanthus will quickly re-sprout.
Baking Soda Fungicide Treatment
If you notice larger spots of mildew on the leaves of your acanthus and do not want to remove as much foliage or as much of the plant as you fear may be required to eliminate the infection, you can spray the plant with a solution of baking soda and water to kill off the mildew and prevent new growth. Use a solution of an ounce each of baking soda and horticultural oil per gallon water, and spray both the tops and the bottoms of the leaves. Spray at least once a week or after a rain for four weeks, at which time you should see new, healthy foliage and the mildew should have stopped spreading.
Behavioral Prevention and Cure
In many cases you can cure mildew on an acanthus plant simply by changing the way that you care for your plant. Water using a drip hose to prevent leaves, stems and flowers from getting wet. Only water in early morning. Make sure that other plants have not overgrown the acanthus to the point where it is not getting any air circulation, and prune the other plants back to allow for some sunlight on the plant. By keeping the leaves dry and the air around the plant moving, you will make the environment untenable for mildew and it will die.