Certified GKH Gardening Expert
I notice the heavy, un-amended, clay soil. This will be the reason for infection, as clay soils remain too moist for too long to support good growth.
There are only a handful of actions that you can take from here. I would introduce worms to the soil, as well as topping with a layer of compost mixed in with potting soil. This is important to prevent smothering roots. This will help to work up the soil, as well as introduce nutrients and air the soil.
Here are some articles that will help: