You may be surprised to find out that perennials can get sick by their own versions of the same organisms that attack people — fungi, bacteria, viruses, and microplasma. Plant diseases are primarily water-borne. Don’t overwater, and let the soil dry out between waterings to slow down their spread. Following is a list of the diseases that are most likely to attack your perennials:
Aster yellows: Plants infected with this disease become bizarrely deformed and distorted — the flowers may start to grow strange protrusions and the leaves curl and twist.
Gray molds: Fuzzy brown or gray mold can form on leaves and flowers, and stems may become soft and rotten. Cool temperatures and humidity encourage their growth. Where this disease is a problem, water in the morning, so that plants dry quickly. Remove damaged leaves and flowers and destroy badly affected plants. To prevent mold, space flowers in the garden widely enough that they don’t touch and clean up dead plant debris promptly.
Leafspots: Both viral and bacterial infections can cause brown or black irregular blotches or circular spots.
Nematodes: Nematodes are actually microscopic worms that can damage plant roots or foliage.
Powdery mildew: Plants infected with powdery mildew look as though they’ve been dusted with talcum powder. This disease requires heat and a brief period of high humidity to form; the attack often occurs after the flowers have finished blooming. Some perennials are highly susceptible to powdery mildew, so plant resistant varieties.
Rust: Rust fungi causes yellow, orange, or brown bumps to appear on stems or leaves. Keep plant foliage dry and pick out infected leaves.
Viruses: Insects transmit viruses. Infected leaves may be mottled in irregular or sometimes circular patterns or may be yellowed overall. Destroy infected plants and practice good sanitation (wash hands and tools thoroughly).
Wilts: When the whole plant wilts and dies, sometimes overnight, fungal or bacterial root rots may be responsible. Nematodes or gophers can cause the same symptoms. If root rots are the cause, continually wet soil encourages their growth. Improve the drainage and don’t replant the same flower in affected soil.
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Source:http://www.dummies.com/how-to/content/diagnosing-perennial-plant-diseases.html
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