| Disease (causal agent) | Symptoms | Survival of pathogen and effect of environment | Comments on control |
|---|---|---|---|
| Black rot * (Pythium ultimum) |
Rapid, black rot of pseudobulb and rotting of roots. Bulb, usually firm at first, gradually desiccates, forming a hard mummy. | Soilborne pathogen. Spores spread in water. Favored by poor drainage and excess water. | Provide better drainage. Avoid excess irrigation. Drench plants with oomycete (water mold) specific fungicide. Steam or chemically treat growing medium. |
| Flower spotting (Botrytis cinerea) |
Small black, brown, or colorless spots often surrounded by water-soaked areas. | In plant debris. Spores airborne. Favored by cool (45° to 60°F), moist conditions and condensed moisture on flowers. | Avoid wetting flowers. Keep humidity as low as possible. Eliminate old flowers and plant debris both inside and outside growing area. |
| Sclerotium or collar rot (Sclerotium rolfsii) |
Rapid rotting and collapse of the leaf bases and stem. White fungus growth and small resting structures (sclerotia) that resemble mustard seeds usually present on plant and planting medium. | Sclerotia survive in soil for many years. No spores form. Favored by warm, moist soil. Fungus has a wide host range. | Destroy infected plants. Heat-treat soil, fumigate with chloropicrin or another chloropicrin combination, or mix granular PCNB with planting medium before planting. |
| Virus or viruslike disease | Symptoms | Host range and natural spread | Comments on control |
| Bar mottle (Cattleya severe flower break virus) |
Yellow bar-shaped streaks and blotches on leaves. | Green peach aphid. Cymbidium, Cattleya, and its hybrids. | Same as for mosaic. Also, control insects. |
| Diamond mottle (Odontoglossum ringspot virus) (Tobacco mosaic virus, orchid strain) |
Elongated chlorotic areas are sometimes diamond-shaped. Older leaves sometimes develop brown-to-black flecks and streaks. | Can be transmitted by juice inoculations. Cymbidium, Cattleya, Odontoglossum, Phalaenopsis. | Same as for mosaic. |
| Mosaic (Cymbidium mosaic virus) |
Symptoms vary in pattern and severity. Small, elongate pale areas in young leaves may later develop into dead, dark spots or streaks. Mottling of young leaves, sometimes becomes inconspicuous in old leaves. No flower variegation. | Transmitted by pruning tools. Cymbidium, Cattleya, Epidendrum, Zygopetalum, Angraceum, Laelia, Oncidium, Spathoglottis. | All virus diseases are propagated with plant. Once infected, plant remains so for life. Destroy infected plants. Disinfect tools between cuts or heat-sterilize in a flame. |
| Ringspot virus (Cymbidium ringspot virus) |
Necrotic ringspot patterns on young and old leaves. Plants severely stunted. Can be lethal. | Can be transmitted by juice inoculations. Cymbidium, Cattleya, Spathoglottis, Trichosoma. | Same as for mosaic. |
| * For additional information, see section on Key Diseases. |