Photinia is very susceptible to the fungus leafspot, called Entomosporium, that can defoliate photinia plants. Plant photinia in full sun and provide good air circulation. Remove and destroy fallen leaves, prune off diseased twigs. Avoid overhead irrigation. Fungicide sprays (copper-based products) are also generally necessary for this disease, beginning from bud break and applied every ten to fourteen days until all foliage has matured. Mature foliage is no longer susceptible to infection.
As for the Oleander, water when the soil is dry (about an inch weekly). During droughts, water a couple of inches each week. Keep in mind, however, that while the soil may not need much water, the leaves and petals can dry out quickly. Mist the leaves with water (early in the morning) to avoid this, especially after a hot, sunny day.