Growing gardenias (Gardenia jasminoides, USDA plant hardiness zones 8 to 11) can feel like a significant achievement for both novice and professional growers alike, which is why any sign of something harming your garden babies can feel like a huge deal. If you start to notice white fungus on your gardenias, it's not a plant-killing disease. You're most likely dealing with a case of powdery mildew.
Powdery mildew is a white fungus that you can find on gardenias and other plants. This fungus will not kill your plants, though.
What Is Powdery Mildew?
Erysiphe polygoni is a fungus that can cause powdery mildew on gardenias. The fungus is mostly located on the upper leaf surface, but it can be found on the lower leaf surface too. You'll be able to spot signs of infection easily because a whitish- to grayish-colored powdery fungal growth will show up in patches or spots. Sometimes, these patches or spots can overtake the entire leaf or other plant parts.
Gardenias can become susceptible to powdery mildew in the summer, and infection peaks in late summer. Although the spots or patches of powdery mildew may appear white to grayish, the fungal growth can darken. When the fungal growth darkens, you can also see the formation of small, rounded reproductive structures called cleistothecia.
Cleistothecia start as pinhead-sized, rounded structures. These structures are initially whitish, and then they darken to yellow-brown. Eventually, the color turns to brown or black. Powdery mildew forms when plant foliage is dry, when there's low lighting and when temperatures are moderate. Powdery mildew forms when there is high humidity too.
Is Powdery Mildew Dangerous?
If your gardenias get powdery mildew, it's not fatal to your plants. Unfortunately, though, it can hurt the overall aesthetics of your gardenias. When powdery mildew isn't treated, it will leech nutrients off your plants. The major effects will be noticed on the gardenias' young leaves and shoots. You'll also see deformed leaves and buds, leaf yellowing and leaf drop.
Preventing Powdery Mildew
The best way to ensure your gardenias don't get powdery mildew is through prevention. When planting gardenias, make sure there is enough room between your plants so that there's adequate airflow around them. When you plant your gardenias, make sure that they are planted in an area with enough light. Also, check to see that your soil can drain adequately.
When growing gardenias, they need to be properly maintained. Powdery mildew spores can spread, so as soon as you notice powdery mildew on your gardenias, treat the fungus and always remove dead gardenia parts. Another option is using preventive treatments, like a fungicide, so powdery mildew doesn't form at all.
Treating Powdery Mildew
If powdery mildew does form on your gardenias, you can use an organic fungicide to treat the fungus, and you can use milk as well. All of the science isn't yet known, but milk is becoming a popular treatment due to its compounds. Milk's compounds act like an antiseptic and fungicide. Also, milk can strengthen the plant's immunity.
Mix 1 part of skim milk to 2 to 3 parts of water. Use skim milk instead of regular milk because skim milk doesn't have any fat content. This means when you spray the milk mixture, there's less chance you'll notice an odor. Spray the milk mixture on the powdery mildew and around the plant as well. Repeat weekly to keep your leaves free of fungus.