Answer: What you describe sounds like powdery mildew, a fungal disease. Insecticidal soap is for use against soft-bodied insects such as aphids; it has no effect on fungal diseases. I think the yellowing and dropping of leaves is a result of the use of Insecticidal soap - some plants, such as Akebia, are not tolerant of the fatty acids in the soap. I'd stop using it on the Akebia.
Fungicides available to the home gardener for powdery mildew control are myclobutanil (available as various products and formulations, including Bio Systhane Fungus Fighter (liquid concentrate), Bio Fungus Fighter (ready-to-use spray) and Roseclear 3 (which also contains an insecticide for pest control)), penconazole (Fungus Clear), flutriafol ( Roseclear Gun!: also contains an insecticide), and sulphur as a dust (Vitax Green or Yellow Sulphur).
Check the labels carefully before choosing, as pesticides may only legally be used on the range of plants specified on the label. Occasionally, the label will also list certain cultivars that may be damaged by the application of a specific fungicide.
An alternative to chemical treatments would be to prune off the infected plant parts and dispose of them.
The following measures will reduce susceptibility to the disease:
Keep plants well watered, so they are not dry at the roots.
Mulch to preserve soil moisture.
Improve air-flow around plants to reduce humidity.
Avoid high nitrogen fertilisers, as these encourage soft sappy growth that is more easily colonised by fungi.
Ensure plants are in their ideal position. For example, a sun-lover will struggle in shade and be at greater risk of infection.
Hope your Akebia recovers quickly!
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