There are lots of different powdery mildews and they tend to be host specific, meaning that honeysuckle powdery mildew will only infect honeysuckle etc. On top of that the plant has to be in a poor condition (usually suffering from drought) so that it is susceptible.
I wouldn't use liquid seaweed every week - in my opinion that would be too much and result in soft growth and possibly an imbalance of nutrients in the soil - once a month would be enough - but if your apple tree is against the fence it needs more liquid than one can a week, even with rain. I water my 2 year old espalier pear with a bucket of water 3 times a week at the moment.
If planting shrubs in dry areas I sometimes get a large plastic tonic bottle (other mixers are available ), remove the screw cap, cut the bottom off and sink it into the soil neck downwards so that the neck is right by the roots of the shrub - you can then water direct into the bottle and the water is directed straight down to the roots. I do this for my courgettes too.