It's usually impossible to spot the offending fungi (though some are just visible), viruses and bacteria, but their symptoms are invariably obvious. In all cases, take prompt action, such as spraying, and cutting away and destroying the affected growth, and the problem should be relatively easy to minimize and control.
Fungal and bacterial infections are spread mainly by spores, facilitated by moisture on the leaves or in the soil. Keeping infections out is impossible, especially when spores are spread by wind, but you can minimize their effect for example, by promptly removing leaves and stems attacked by powdery mildew before it puffs out more spores. If you're growing roses, choose disease-resistant varieties. Where conditions are such that a rose regularly gets black spot, either grow something else (in severe cases) or spray it with an appropriate fungicide every week or two from early spring onward as a prevention. When spraying, always wear gloves or wash your hands thoroughly afterward. Sterilize pruners after using them on diseased plants.
Viruses can be a major setback, reducing vigor and growth, and invariably producing discoloration. They are often spread by sap-sucking insects, one very good reason to create natural habitats for birds, ladybugs and other natural aphid predators. You can also grow plants like marigolds that attract parasitic wasps, which feed on aphids. If you decide to spray the aphids, it may kill their predators, giving the next generation of these pests a free run. Wait and see if the predators tackle the problem before spraying. Group 9 Rembrandt tulips are particularly prone to a virus that produces \"broken\" flowers with wonderfully attractive white and feathered markings, but it can also be transmitted to other tulips.
Small bright orange or dark brown blistering on the leaf or stems, with affected areas withering and possibly dying, caused by fungi in damp conditions. Rarely a major problem; remove affected areas and spray with an appropriate fungicide.
Rose Black Spot
Evident in warm, moist conditions, producing black spots on leaves, then yellowing and leaf fall. Some roses are more susceptible than others; tackle by repeat-spraying and/or pruning from spring, and destroy the fallen leaves.
White powdery spores and yellowing usually appears on upper leaves initially, caused by fungi encouraged by dry conditions. Treat by watering regularly, pruning affected areas and promptly destroying infected parts, and spraying.
Rhododendron Bud Blast
Flower buds covered by tiny black fungi fail to open and turn brown, possibly hanging on for years, though not all buds may be affected. Spread by leafhoppers in midsummer. The only action is to remove affected buds.
Usually black (sometimes green) mold caused by fungal growth on the honeydew excreted by sap-sucking insects, which may actually be active on the higher leaves of an adjacent plant. Control by removing the insects, where possible.
Gray Mold (Botrytis)
A common fungus producing fluffy grayish white mold on decaying and dead tissue. Remove and destroy affected parts. Prevent by improving air circulation, avoiding overcrowding, and removing fallen debris.