Answer: The white residue sounds like powdery mildew, which is a fungal disease. Aphids are a soft-bodied insect, but they are not a scale. Powdery mildew overwinters on living plants, and can be difficult to eradicate. There are some things you can do to minimize its effects. Powdery mildew is unique among plant diseases in that it doesn't require a wet leaf surface to spread. It can thus thrive during hot, dry weather. The first line of defense is to grow resistant varieties. Next, remember that while the disease is unsightly, it doesn't cause any real harm to many of its victims (though it will damage some plants.)
Here are some general rules for helping control fungal diseases. Start by making sure that your plants are getting enough direct sunlight. (Eight to ten hours a day is generally the minimum for plants that flower or bear fruit.)
You'll also want to make sure that there's enough room between plants for air to circulate freely. Overcrowding not only makes plants more susceptible to diseases, if leaves touch other plants, those diseases can easily be spread.
The general advice to inhibit the spread of fungal diseases is to avoid wetting leaf surfaces. In the case of powdery mildew, you can actually inhibit infection with periodic strong sprays of water (not so strong as to damage the
plant.) After the new growth of affected plants reaches about six inches long and UNTIL THE TEMPERATURE REACHES 90 degrees F, you may apply dusting sulfur to foliage every 14 days. If necessary apply fungicide. Fungicide works much better as a preventative measure, before the powdery mildew becomes a problem.
For aphid control, I always start with the simplest method first, and if that isn't successful, move on from there. Healthy vigorous plants will withstand insect attacks best and sometimes a little damage is acceptable. If your aphid problem is not too severe, a strong blast of water from the hose should work. Spray underneath leaves, in between, etc. Do this daily.
If that doesn't work, try a soapy water spray. Use 1 teaspoon to 2 tablespoons of liquid detergent soap per gallon of water. Use regular, not concentrated soap. Don't use soaps with lemon, as the citric acid can burn plants. Start with the lower amount and work up as needed. Spray as often as needed. As with any spray you might wish to test it on a few leaves first before you treat all your plants. Next on my list would be an insecticidal soap spray.
Ladybugs and their larvae are voracious eaters of aphids. They often "arrive" a week or two after the aphids, so not spraying with chemicals is a good idea if you'd like to attract them to your garden to consume aphids for you.
As for growing gardenias, they are a marginal plant here in the low desert. They prefer a cooler, moister environment with acidic soil. Our soil is highly alkaline. You can try amending the planting soil with peat moss or pine needles, using a product such as Miracid, and planting where they will receive protection from hot afternoon sun. Because they are marginal, it is likely they will be stressed without continual care. Research shows that insects and disease seek out stressed plants for attack, which seems accurate in your case as the gardenias have both aphids and probably powdery mildew. You might be better off replacing them with more appropriate desert-adapted plants. Good luck!
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