Sea weed extract is the magic elixer of plant stress protection. There are active hormones in the extract that boost plants' immune systems. It's good to treat plants with this 2-3 times a summer (some have suggested that much more often than this may be detrimental).
At transplant time, flowering, or when plants are under particular stress (prolonged heat spell, or when you notice they are yellowing and you're going to need to side dress) are good times to spray the plants with this hormone mix, which they'll absorb through the leaves. It is not a fertilizer ( because it's applied at a great dilution), but will help the plants make better use of the available fertility. By boosting the immune system the plants will ward off diseases and insect pests better - one of the ways to be plant positive, rather than pest negative.
It's mostly my tomatoes that I spray with sea weed extract. I buy Sea-Com from Johnny's seeds, but there may be other brands available locally. Look for brands that say they have active cytokinens (though there may be good brands that don't say this).
update: As we're facing frost tonight I'm remembering that sea weed extract may confer a couple of degrees of frost protection, too, perhaps by strengthening the cell walls.