Since annuals die after only a single season, diseases are less of a concern than with perennials. Often, it's best to just pull out and dispose of individual plants that become diseased. Not growing the same species in the same place in consecutive years can help. If diseases persist, try using resistant varieties.
If foliar diseases such as powdery mildew are a problem, provide better air circulation by spacing plants farther apart. Keep foliage dry as much as possible by watering in the morning. If root rots and diseases are a problem, avoid over-watering and improve drainage.
Damping off is probably the most serious disease of annuals, causing seeds to rot and small seedlings to die. It spreads quickly and can be carried on soil, tools, and containers. Use sterile soil and containers to prevent its spread when starting seeds.
In a diverse and healthy garden, beneficial insects prey on and parasitize pests, helping to keep their populations in check. Keep in mind that when you use insecticides, you also kill the good guys that prey on the pests. If you use insecticides, follow directions precisely.
Where aphids and other pests that suck plant fluids (below) are a problem, avoid excessive nitrogen fertilizer. This makes plants more attractive to these insects. Also look for resistant varieties. A few of the common sucking pests you may encounter include:
Tiny, soft-bodied insects (often pear-shaped) that pierce tender plant parts and suck out fluids. Sooty mold (a black fungus) often grows on the sweet sticky honeydew associated with aphid colonies. You can wash aphids off plants with a hard stream of water.
Adults are small and yellowish with dull white wings. Immature whiteflies are oval, flattened, and yellowish scale-like insects. Heavily infested plants send up a cloud of adults when disturbed. Do not purchase infested plants. Pull out and dispose of plants that become infested.
These tiny 8-legged arthropods suck liquid from plants, causing yellowish stippling on leaves. Webbing, which looks like strands of spider's web, is sometimes visible. Rogue infested plants. Wash off with hard stream of water.
Slugs and Snails
Slugs and snails can also be a problem, especially in wet years. Removing mulch and other garden debris can reduce the moist hiding places they need during the day.