Before we start on orchid pathogens, keep in mind that orchids are sensitive plants. If an orchid is well taken care of, it will be strong and thus have high resistance against pathogens. Regular routine of feeding, watering and chemical application on the plant will help keep pathogens away. So take good care of your orchids and in turn they will flower well for you.
Most of the time, one disease can lead to another in a cascading effect if the primary issue/disease is not taken care of. Growing orchid takes a lot of care, observation and dedication.
Bacteria / Fungal Ailments
Brown spots can be caused by mechanical injury to the plant, over-watering and mechanical pest attacks. The ruptured epidermal layer of the orchid leaves provide wet substrate for opportunistic pathogens to colonize.
To remedy, first apply fungicide to the brown spot to prevent fungal growth. If the fungal/bacteria growth continues spreading, cut off the infected portion of the plant. Dip the cut portion in fungicide, dry it before transplanting it.
Black spots are usually due to chemical burns. When mixing your fertilizers, fungicide or pesticide. Ensure that mixing is done well in a container before applying them to the plants. Uneven mixing can result in crystals burns during hot afternoons resulting in small black spots on the leaves.
Black spots may also happen due to too much sunlight. Orchids are picky plants, always remember to provide yours with the right amount of sunlight for the type of orchid that you are growing. If an orchid leave starts turning yellow, this might be due to too much sunlight, provide the plant with some shade and the leaves should turn back to green. Prolong period will end up in burnt spots. Burnt spots can result in further chemical burns as the innards of the plants are exposed on the edge of the burns when treated with chemicals. If fresh burnt spots are kept wet due to rain and watering for prolong periods, secondary bacteria infection can set in.
Root rot occurs due to incompatible basal media or over-watering. This usually happens for epiphytic orchids (e.g. Dendrobiums) as they grow naturally on trees and thus their roots should never be waterlogged.
Root rot also attracts opportunistic microbes and subsequently snails and slugs as the wet substrate provides a good meal for them.
Stem rot usually occurs from bottom up from root to stem to leaves. This happens when root rot is not remedied and occurrence increases with secondary pests such as snails and slugs sucking sap out of the soft fleshy tissues colonized by bacteria/fungi.
Secondary fungal/bacterial infection by a different type of pathogen may also occur after the initial colonization. This is common for badly neglected orchids.
Crown rot usually occurs from waterlogged crown. When watering orchids particularly Vandaceous types, always take note not to allow water retention in the crown. This is also why we frequently advice our customers to water their plants during the morning rather than in the evening as water in crown will dry up in the afternoons.
Vandas with crown rot will stop growing vertically as the apical meristem has been damaged. It is advisable to perform a top cut below the rot to produce keikis from the axillary buds if you wish to salvage the plant.
Movements in your garden due to children, pets or even yourself can caused damage to orchid leaves unknowingly particularly for terrestrial orchids (e.g. Vanda). The exposed areas from the broken leaves will be susceptible to bacteria/fungal infection.
Mold or sooty black fungi may grow along basal axillary joints between the stem and leaves of Vandaceous orchids, commonly due to accumulation of water in these axillary joints. This is usually not serious if the epidermal layer is not damaged. The mold/soot can be easily wiped off with a cloth dabbed with very diluted detergent to clean up the leaves. However, if left for prolong periods, this may result in epidermal necrolysis and develop into similar condition as described for crown rot.
If you have other plants grown within your garden, e.g. this banana tree, you may have more problems with pests like snails and slugs.
Basic remedy for damaged plant parts
- Cut off the infected portion of the leaves, stems, roots or pseudobulb and discard them.
- Wash any remaining dirt off and dip the plant into fungicide (e.g. Thiram).
- Leave the plant out to dry off the fungicide before transplanting the plant.
- Observe. If root rot occurs again, you probably didn’t remove the infected portion of the roots or treat the plant properly.
- Repeat process if required.
The main virus that affects orchid are Cymbidium Mosaic Virus, Cymbidium Necrotic Ringspot Virus, Tobacco Mosaic Virus and Odontoglossum Ringspot Virus. Infection can be very common in Dendrobium and Oncidium orchids in the tropics. This virus is transmitted from cell to cell through their plasmadesmata and impedes the orchid from growing properly. The virus forms a distinct circular pattern on the orchid leaves.
When the virus spreads dominantly across the leaves, they will start to yellow and die off together with the virus.
This infection is usually a considerably slow process as compared to bacterial/fungal infection, especially if the plant is well taken care of.
It is not possible to completely rid the orchid plant of viruses; however the infection can be gradually contained in young plants. Notice in the first picture that this virus is more prevalent on the older leaves. This virus replicates itself in the cells of the orchid leaves and gradually gets transmitted to newer leaves.
To contain the virus in a young plant, simply strip off all infected leaves by pulling them downwards or making a clean cut at the base of the leaves. Dip the plant in fungicide to prevent any fungal growth and allow the plant.
New leaves produced will be more resistant to the virus. Continue removing any infected leaves overtime as the plant grows and the prevalence of the virus will gradually decrease. This can be a long process but if done correctly, the viral infection will be kept low gradually.
The virus will lay dormant in strong plants and may surface again as the plant senesce. Plant resistance, similar to human, grows weaker over time as it accumulates damages, diseases and infection throughout its growth. However, do not fret when your orchid gets old. You can always produce keikis or produce seedpods from them to have new orchid plants for growing again.
As the name suggests, genetic ailments can never be corrected. Not all genetic mutations are bad, those that results in the plant having slower growth, lousy flowers and disease prone are bad. Sometimes in genetic mutations can produce beautiful effects like Dendrobium Liberty Mutation, splashes on the petals and petal fusion that results in rare prized collectible orchids.
Common Insect Pests
Insect pests that affect orchids include spider mites, thrips, slugs, snails and midgets. These are large pest, that can be easily observed and removed by applying insecticides.
Spider mites, also commonly known as red spiders, hide on the underside of orchid leaves and suck sap out of its host. Severe pest infection can result in dwarf plants, reduced in flowering and small and deformed flowers due to their feeding on the plant’s sap. This pest spreads fast and can be persistent.
Thrips and midgets are common pest for Dendrobium orchids. These insects use the buds of orchids, particularly Dendrobium to house their eggs. When their young hatch, they feed on the sap of the plant from within the buds, resulting in the buds falling off prematurely or having poorly formed flowers. Thrips/midgets can be killed and warded off using pesticides such as Imidacloprid.
Slugs and snails feed on wet substrates of the plant. Aside from being secondary pest after primary plant infection by microbes, snails/slugs are also common after a heavy rainfall. The Oncidium and its alliance are particularly vulnerable to snails/slugs after rain due to their soft petals. After heavy rain, often the soft lips of the Oncidium flowers will go missing where snail/slugs are rampant. Snails/slugs can be remedied by throwing snail/slug pellets (molluscicide) around the vicinity of your orchid. These molluscs will dissolve into liquid with a uric smell upon consuming these pellets.
Caterpillars can also appear on poorly maintained orchids. Caterpillar attacks the flowers and soft chewy parts of the plant, e.g. the crown or spiny tip of a Vanda teres. This can end up with parts of the leaves dying and drying up as the sap were sucked out by the caterpillar or flowers damaged. Caterpillar eggs are usually laid on the underside of the leaves as it provides shelter for them, and also making them not easily visible. Where infestation is low, caterpillars can be removed manually using a pick or pointy petiole of a leaf to transfer instead of killing them with pesticide. Personally I don’t think caterpillars are that bad for private garden as they will moult and change into a beautiful butterfly later on. =P
Precautions for Treatment
- After using an orchid shear to cut off infected portion, remember to sterilize it by flaming the blades with a lighter, dipping in fungicide solution etc before cutting the next piece to reduce transmission.
- When cutting off infected portions, always cut an extra bit more of the healthy plant so as to ensure that the remaining portion will never be incontact with the infected portion.
- Be careful not to damage other plant parts with the sharp edge of the scissors, open wounds are susceptible to infection by microbes. Expose areas should always be minimize and treated with fungicide where required.
- Never use old media for transplanting as it’s likely to be contaminated.