Dendrobium orchids, while resilient, are susceptible to a range of diseases primarily caused by fungi, bacteria, and viruses. Understanding these ailments from a plant's physiological and structural perspective is key to effective treatment and maintaining a healthy specimen.
Fungal pathogens like Fusarium, Pythium, and Rhizoctonia are a significant threat, often manifesting as root rot. From the plant's viewpoint, these fungi attack the velamen (the spongy, white outer layer of the roots), destroying the cells responsible for water and nutrient absorption. The plant subsequently shows signs of thirst and starvation despite adequate watering, with symptoms including yellowing, wrinkled pseudobulbs, and stunted growth. Leaf spots, caused by fungi such as Cercospora, appear as raised or sunken brown or black lesions on the leaf surface, disrupting photosynthesis.
Treatment: Immediately remove the plant from its pot and carefully cut away all soft, brown, or mushy roots with sterilized tools. Treat the remaining healthy roots and rhizome by soaking in a fungicide solution labeled for orchids (e.g., containing thiophanate-methyl). Repot in a fresh, well-draining medium and a clean pot, ensuring excellent air circulation around the roots. For leaf spots, remove affected tissue and apply a fungicide.
Bacterial infections, caused by pathogens like Erwinia and Acidovorax, are aggressive and can kill a plant rapidly. These bacteria enter through wounds or natural openings, secreting enzymes that break down and macerate plant cell walls. This leads to watery, foul-smelling lesions that quickly spread. On leaves, it begins as a small, water-soaked spot that rapidly turns brown or black, often with a yellow halo. The plant's vascular system can become infected, leading to systemic collapse.
Treatment: Immediate isolation and surgery are critical. Using a sterile instrument, cut away all infected tissue, removing a margin of healthy tissue around the affected area. Apply a bactericide or a paste of cinnamon (a natural antiseptic) to the cuts. Drastically reduce watering and increase air movement to keep the plant and its environment dry. Copper-based bactericides can be used as a preventative measure on exposed cuts.
Viruses such as Cymbidium Mosaic Virus (CymMV) and Odontoglossum Ringspot Virus (ORSV) cause incurable systemic infections. From the plant's perspective, the viral particles hijack cellular machinery, disrupting fundamental processes like growth and pigment production. Symptoms are highly variable but can include chlorotic (yellow) streaks or ringspots, color breaking in flowers, distorted growth, and general decline. Stress often exacerbates symptom expression.
Treatment: There is no cure for a viral infection. Prevention is the only strategy. Always sterilize cutting tools between plants (using heat or a disinfectant like bleach). Isolate new plants for several weeks to monitor for symptoms. If a virus is confirmed, the most responsible action to protect other orchids is to dispose of the infected plant.
The plant's best defense is a robust constitution supported by optimal growing conditions. Providing bright, indirect light strengthens tissues, proper watering (allowing the medium to dry slightly between waterings) prevents root suffocation and pathogen-friendly environments, and strong air movement keeps humidity from settling on leaves and crowns, creating an unfavorable environment for fungal and bacterial spores. A regular, balanced fertilizer regimen keeps the plant vigorous and better able to resist infection.
