I have an 'Amethyst Falls', and I have never had to spray it for bugs or fungus. Wisteria in general are fairly pest free. I'm surprised you are having trouble. The yellowing I see on your tree might actually be too little water, or getting too dry between watering. Reason is it seems the oldest leaves are yellowing. This could be an artifact of looking at a photo, rather than live in person. If it is the newest leaves yellowing it could be over watering.
In general wisteria are water hogs when in full summertime growth. Many set their wisteria in shallow trays of water during the warmest months.
I'm up north of you, within the next 2 weeks we will have a hard freeze, or at least should have a hard freeze. No sign of a freeze coming. If I were you I would not worry about it. Autumn is here, even though your first frost will be a month later than my first frost, trees are hardening off and going dormant.
By the way, these native wisteria species are hardy enough that I leave mine out, under the bench, on the ground but otherwise fully exposed for the winter, and so far -17 F (-28 C) was no problem. Caution if your pot is not freeze tolerant you will need to protect the tree to save the pot.
You should probably repot in spring - if only to get the tree into your own potting mix. This way you will be more familiar with how often to water. What mix you use is up to you, if at all possible use the same mix for the majority of your trees. That way you will be familiar with how quickly or slowly the mix dries out.