My Orchid Turned Yellow With Black Spots

Viruses

• Viral illnesses are among the most serious pathological conditions that affect orchids. Only a few of orchid viruses, such as the Cymbidium mosaic virus (CymMV) and Odontoglossum ringspot virus (ORSV), are common problems for cultivated plants, according to The North of England Orchid Society. An infection of CymMV can cause yellowing and other discoloration of orchid leaves and flowers. Dark, decaying spots may also develop as the leaves and stems decay. ORSV also causes superficial symptoms, including light streaks and circular discolorations on foliage.

Bacteria

• Orchids can contract several bacterial diseases that mar the beauty of their flowers and leaves. Bacterial brown spot is a common threat to orchids and can spread throughout infected plants quickly. The pathogen destroys tissue on all parts of an orchid. The initial symptom of an infection is the emergence of moist patches on the surface of leaves, stems or buds. The wet spots spread quickly and turn to a darker color, such as brown or black. Damaged areas are bordered by yellowing tissue.

Fungi

• Infestations of several fungal genera, including Cercospora and Phyllosticta, can cause yellow and black discoloration of orchid foliage. Infections of these fungi are described collectively as leaf spot diseases, because they produce many of the same symptoms and require similar management techniques to eradicate. Leaf spot diseases produce spots, specks or patches of dark or yellow tissue. Small black spots on lighter colored tissue are large colonies of fungal cells. All leaves, stems and flowers infected with a fungal leaf spot disease should be pruned and plant debris must be removed regularly to prevent the infection from returning.

Other Damage

• Orchids are also vulnerable to damage from cold temperatures, harsh growing media and other environmental conditions. Excessive exposure to sunlight, particularly for shade-loving orchid species, produces black patches that are surrounded by rigid, yellow or brown tissue. Sun damage is untreatable, but it can be prevented by acclimating plants to sunlight slowly, according to the St. Augustine Orchid Society. Orchids planted in soil with high salinity or excessive moisture can also develop discolorations on leaves and buds.