One of the most difficult problems in orchid growing is orchid diseases. Here we will talk about only infectious plant diseases. Pathological plant conditions such as a sun burn, fertilizer burn, and heat/cold damages are not transmitted by pathogenic microbes and therefore not contagious and not infectious. Infectious plant diseases are caused by viruses, bacteria and fungi, and therefore are called viral diseases, bacterial diseases and fungal diseases.
There are plenty of orchid diseases, most of them are indistinguishable from other common plant diseases and caused by the same pathogens, but most viral diseases are caused by specific orchid viruses and can infect only orchids, and there are some specific orchid pathogenic fungi and bacteria such as Pseudomonas cattleyae, Gloeosporium cattleyae, Botryodiplodia oncidii, Cercospora odontoglossi, Diplodia laelio-cattleyae, and very dangerous pathogen Fusarium oxysporum f. sp. cattleyae.
Most orchid diseases are caused first of all by improper care, healthy orchids, grown in proper growing conditions are particularly resistant to pathogenic fungi, but bacterial and viral diseases are highly contagious even for healthy and properly grown plants.
Also, there are some differences between different orchid species in their resistance to pathogens – some types of orchids are susceptible to plant diseases, for example Zygopetallum is susceptible to Phyllosticta infection.
Most common orchid diseases are fungal diseases because fungi produce numerous spores which are easy transported from an infected plant to another. It is important to distinguish these conditions to make correct diagnosis and to choose the best treating technique to save the plant and collection, because untimely noticed disease could kill an entire collection.
Viral diseases such as Flower break, Mosaic and White cell necrosis are caused by specific orchid viruses such as Odontoglossum ringspot virus (ORSV) and Cymbidium mosaic virus (CymMV). They are transmitted with plant juice by not properly cleaned instruments while orchids are being trimmed or cutted . They are manifested by regular yellow, black and white spots on leaves and especially by flower abnormalities (absent lips and tepals, irregular shape of flowers, color is unusual with patterns of more dark and light shades). Viral diseases are untreatable and therefore infected plant should be incinerated as viral infections are highly contagious. The best caring for orchids is to eliminate and incinerate infected plants to save the entire collection from viral diseases. It is best to not use the same cutting and trimming instruments when working with orchids and thoroughly clean and disinfect instruments between applying them to plants.
Bacterial diseases are caused by Erwinia carotovora (Bacterial soft rot) and Pseudomonas cattleyae (Brown spot). Bacterial diseases are very dangerous, they are characterized by appearance of soft, brown and black, rapidly spreading spots. Infected parts become soft, juicy (bacterial enzymes dissolve plant tissues). The plant could die within days, simply turning to a very unpleasant mash. One of characteristics of these diseases is a fishy smell from this mash. These infections are also factually incurable and very dangerous, that’s why it’s better to discard an infected plant and incinerate it, because one single infected plant can kill an entire collection of orchids.
Fungal orchid diseases are more diverse in symptoms. Black spots, yellow orchids parts (leaves, stems and pseudobulbs), potted orchids – theses are all symptoms of fungal diseases. Most common fungal diseases are Canker, or anthracnose, caused by Colletotrichum crassipes, Glomerella cingulata, Colletotrichum gloeosporioides and Gloeosporium cattleyae. Black rot is caused by Phytophthora and Pythium fungi, Gray mold is caused by Botrytis, leaf rots and spots disease is caused by Alternaria and other fungy, and very dangerous pseudobulb and root rot is caused by Fusarium oxysporum, and Rhizoctonia causes root rot. Fungal infections are characterized by decreased turgor, yellow, black and brown dry spots, parts of plant become brown and black. Fusarium wilt is characterized by decreased turgor and purple spots, affected tissues quickly become purpule and brown after cutting. Fungal diseases are generally caused by improper care – overwatering, too cold or too warm temperatures (for intermediate and cool climate orchids), insufficient lighting and so on. When growing orchids, it is a good idea to follow growing instructions. Also, fungal infections could be caused by insufficient air moving and by water dropping on leaves and new growths, which can not dry quickly because of poor air moving. Generally, fungal infections are curable, if the plant is not very affected by fungus. First you should remove affected plant parts – leaves, pseudobulbs and rhizome parts, which show signs of the infection. If full rhizome is affected, but pseudobulbs seem green and healthy – the plant most likely is incurable and dead. After removing all infected parts, you should apply fungicide and place the plant to quarantine to ensure that the infection is not spread to another plants. Also, you should consider your growing technique – if fungal infection is caused by improper care, none of remedies will save the plant from another infection, you should know that only proper care will prevent plants from pests and diseases.
See also: Orchid pests