Coastal and Tropical South
When algae builds up, there are three steps to take to remedy it. First, physically remove the algae using a broom or rake. I've read that you can put it in the compost, but laying it out so the sun can do it in works, too. Second, fill the now-empty space with floating plants to cover at least two-thirds of the surface. Third, investigate organic ways to control algae, including innovative treatments using barley in liquid or floating bales.
Canna Leaf Rollers
When cannas can't unfurl their leaves, they cannot open their flowers. The culprit is usually a little caterpillar, the canna leaf roller. Mother moth lays eggs in the debris at the base of the canna, then the larvae hatch, eat and spin themselves into the leaves. Cut down the damaged stalks and destroy them; do not compost.
All your hard work can be for naught if squirrels and birds get the tomatoes first. Fake owls, plastic snakes, and metal or glass hanging diversions may work at first, but the crop lasts longer than their effects. Pick tomatoes as soon as its "shoulders" are bright pink and ripen inside. Also pick off any blighted leaves, destroy them, and leave the fruit to ripen. Remove blighted plants once the fruit is harvested.
Once shrub roses have bloomed a flush or two, it's time to fertilize them. Established, vigorous roses like 'Mermaid' or 'Old Blush' may not need much, but stronger stems and leaves mean more flowers next year. Use a formula such as 5-10-10, with less nitrogen and more phosphorus and potassium. If you haven't refreshed the mulch yet this spring, spread an inch of compost/manure under each shrub and top with fresh ground bark or pine straw.
Brown or Bugs
Spring greens up junipers and other conifers, but summer can be tough on them. Watch for green needles turning brown on these usually-durable evergreens. If your weather has been very dry or wet, these plants will be vulnerable to pests. Dry plants and soil make for good spider mite conditions, while flooded roots can get root rot. Use neem for mites and fungicide for rot.