Bacterial, viral and fungal diseases on Phalaenopsis and other orchids
Even in modern production plants and when strict hygiene measures are observed, infection of the plants with viruses, bacteria and especially fungal diseases cannot be completely ruled out. Especially infections of Fusarium oxysporum may lead to existential losses in the production of potted products and young plants. The same applies to bacterial infections with Erwinia and Acidovorax (Pseudomonas). In the following, the economically most important pathogens are listed, including their biology, damage pattern and control options. Recommended chemical control methods must be checked against the applicable approval regulations of your country. All plant protection recommendations are made with the exclusion of any liability.
Erwinia cypripedii [Hori] Bergey et al
Acidovorax avenae subsp. cattleya [Pav.] Will. (=Pseudomonas cattleya [Pav.] Savul.)
Bacteria are unicellular rod-like organisms that move actively in the water with the help of flagella. Reproduction is done by division and only takes a few minutes in a humid and warm environment.
Bacteria are not able to infest a healthy plant surface, they always need open parts such as wounds or gaps.
Most bacteria reach their optimum in high temperatures between 25°C and 30°C and high humidity, so repeated explosion-like infestation usually has to be expected during the summer. But even in the darker and cooler seasons they can cause severe damage to orchids, even if the pest develops considerably slower.
There may be several bacterioses on orchids at the same time. Usually the degree of infestation of the host plant is therefore not directly connected with the respective bacterium, but rather with the conditions of cultivating and climatic conditions and the condition of the plant. Thus, only microbiologic examinations can reveal the type of bacterium that causes a certain disease.
- Water is the most common source of spreading bacterioses on plant populations. The plants must be watered carefully, keeping the leaves as dry as possible, the plants should not be placed too close to each other.
- Wounds are open doors for bacterioses and must be avoided.
- All plants with visible symptoms must be removed from the greenhouse immediately and destroyed. Cutting off individual infected leaves hardly improves the situation and only slows down the course of the disease. In the case of an infection in young thinned out plants, the whole box should be removed.
- Stress situations such as marked temperature or light changes or increased nitrogen fertilisation have to be avoided at all costs.
- Cultures should be cultivated as regularly as possible.
- Hygiene must have top priority for all cultivation: washing your hands, changing or disinfecting work materials.
- Cultivation vessels, shelves, greenhouse desks and tools that were in contact with infected plants or water must be disinfected.
- In order to decrease spreading of the intruder in the stand, regular and repeated prophylactic spraying with copper-containing products can be done. Copper hydroxide or copperoxichloride crystals have the effect of mechanical barriers on the plant surface. However, treatment with copper over a longer period might result in phytotoxity on leaves and air roots.
- Common benzoic acid can also be used for disinfection in gardening when carefully poured over the plants, however, this product may not reach the roots. Most other disinfectants are phytotoxic and are not suitable.
- Excellent results are achieved using chloride dioxide. As an oxidating agent, it takes effect on the organic parts of the water. Chloride dioxide can be mixed with the water for watering.
The causative organism of leaf and stem mould and marked blossom rot on Phalaenopsis and Cattleya is the weed mould Botrytis cinerea Pers. This common intruder has a very broad spectrum of host plants; in fact it grows nearly on all parts of the plant. A characteristic feature is the grey sporal area on the infested spots (grey mould) which can be seen in high humidity. When slightly touched or due to air movement, the spores detach from their carriers and spread in the greenhouse. In favourable conditions Botrytis can also form dark sclerotical diseases.
- Botrytis is a secondary parasite; well nourished and optimally cultivated plants will hardly be infested.
- Insufficient light, unbalanced nitrogen fertilisation or too high a salt content in the substrate increase the susceptibility of the plants.
- During blossom formation it has to be ensured that the conditions are not below dew point to avoid the resulting water sprinkling from damaging the plant. Air humidity must be decreased especially in winter, stagnating air has to be avoided. In some cases the use of fans may be recommended.
- In non blossoming stands repeated prophylactic treatment may be done with fungicides. As Botrytis becomes resistant to fungicides very quickly, the agent groups have to be changed constantly.
- Direct chemical control of Botrytis blossom rot does not make any sense.
Anthracnose on orchids is caused by the weed mould Colletotrichum gloeosporioides Penzig (= Glomerella cingulata [Ston.] Spauld et Schrenk). It appears in glasshouses when humidity is high or when the leaves have been wet for a long time. Circular spore deposits are found on leaves and the spores are spread in the stand with constant high humidity or wet leaves. Water sprinkling is the most common form of spreading. Infection via the roots is not possible.
- If there is a risk of infection, air humidity should be decreased. Watering from above should always be avoided, at least drying off the leaves should be carried out immediately.
- When there is only slight infestation at the beginning, infected leaves can be cut off, markedly infested plants have to be destroyed.
- Prophylactic, repeated treatment with organic fungicides is possible.
- When symptoms are detected, spraying with fungicides that are effective against leave spotting diseases should be carried out.
The main pest on Phalaenopsis and Paphiopedilum is Fusarium oxysporum Schlecht. The fungus forms its fungal spores on the usually whitish to pinkish spore hymenia (sporodochiae). They are either long and sickle-shaped and serve to survive (macro conidia), or they are small, globose to oval (micro conidia). The conidia spreads the infection in the stand and infests further host plants. Most parasitic Fusaria are vascular parasites, they obstruct the conducting tissue and cause wilt diseases. They are typical secondary parasites, the reasons for infestation can be too high a salt concentration, low ground temperatures, gnawing by organisms in the ground and inappropriate substrates that are too wet or contain too much peat. Fusaria and other fungi multiply particularly quickly in sterile plant substrates with a high percentage of peat, because in those materials there are no other micro-organisms as natural antagonists.
- Control should be generally prophylactic by improving the respective cultivation methods.
- Severely infested plants should be destroyed or at least planted elsewhere and all dead roots have to be removed carefully.
- Treatments with fungicides against Fusarium oxysporum on roots are only sufficiently effective in the early stage of the disease. Watering with fungicides are not effective enough and can negatively influence the growth of the plants. In the case of Fusarium infestation at the base of the leaves, spraying can be useful to stop it spreading in the stand and get the infestation under control.
Pythium & Phytophtora
The causative organs of black rot on roots, rhizomes, stems and leaves of orchids belong to the species of the Oomycetes group. Main species found on Orchids are Phytophthora cactorum Schroet., Pythium ultimum Trow., Pythium debaryanum Hesse and Pythium splendens Braun. Characteristic features of these fungi are round to lemon-shaped sporangia. This is where the flagellate zoospores are formed that makes the causative organ spread. Sufficient humidity is the decisive factor for multiplication and spreading of the fungi in the plant stand. Constant substrate humidity helps to grow fruiting structures and zoospores. Spraying or irrigation spreads the rot from plant to plant. Most Phytophthora species prefer high temperatures and sufficient humidity, many other fungi, however, grow best in cool and humid conditions. The fungi intrude their host plants with the help of specific enzymes and excrete toxic substances in them. Within a very short time infested plant parts may decompose and there will be extreme soft rot. Whereas Pythium only infests underground plant parts as a parasite, Phytophthora may occur on all plant parts.
- Plants showing symptoms must be removed from the stand immediately.
- Endangered stands should not be sprayed with water as this might help the zoospores to spread quickly, in companies with recycled water the pest might spread via the roots.
- Cultivation vessels, shelves, greenhouse tables and tools that were in contact with infected plants or water must be disinfected.
- To prevent infection, cultivation conditions must be improved. In addition to avoiding high temperatures (early shading required) water treatment must be done carefully. Humidity may only be increased moderately.
- Stagnant moisture is not to be expected in orchid substrates, but the substrate used for Phytophthora prophylaxis needs to meet certain requirements. The substrates used should not warm up too much, which would lead to the considerable weakening of plants in extreme conditions and contribute to spreading the disease more quickly.
- Pythium and Phytophthora belong to a special group, so direct treatment must be done with special fungicides.
The causative organism of root and crown rot on Phalaenopsis, Miltonia, Oncidium and other orchid varieties is the fungus Rhizoctonia solani Kühn. Rhizoctonia does not form any spores, but only dense mycelium settling on organic material. When the plants are more infested, there will be curling, usually at the roots. The fungus is widely spread and often gets into orchid cultivations in substrates. Wet, cold and badly ventilated substrates help the organisms to spread. In substrate, Rhizoctonia may form persistent sclerotial diseases which may be infectious for a very long time.
- Different species of Rhizoctonia solani are found on orchids. Not all species cause damage, some are also Mycorrhiza fungi.
- Those species react very differently to the fungicides used. A change of the active principle group is therefore recommended to control Rhizoctonia in orchids. Using products to strengthen the plant such as Trichoderma harzianum or Bacillus subtilis is a good solution for young plants. These organisms are antagonists, they form deposits at the roots of the orchids and make them resistant to harmful fungi.
- When infested, the young plants should be kept drier and sprayed not more than is absolutely necessary.
The causative organism of Southern Blight, Sclerotium rolfsii Sacc. (= Athelia rolfsii), is a fungus that lives on the ground and does not form spores, similar to Rhizoctonia. High temperatures help it to grow quickly, optimal temperatures are between 27°C and 30°C. The fungus needs a lot of oxygen, infestation starts in the upper substrate layers at the root neck or stem base. Roots in the substrate are not infested. Sclerotium can hardly grow in cool, humid substrates.
- Fungicide treatment is only possible in the initial phase of the disease. As soon as mycelia and small round sclerotia occur, direct treatment is no longer possible.
- Sclerotia in substrates, pots and transportation boxes can survive for a long time and are easily spread in greenhouses in the water.
- Infested plants should be immediately removed from the stand, because of the high risk of infestation, and destroyed.
- In case of widespread Sclerotium infestation in greenhouses, the greenhouses must be thoroughly disinfected and hygiene measures have to be carried out.
The young shoots of Cymbidium show small longish bright spots on the leaves, initially as longish stripes in the middle of the leaf, later on as marked spots on the leaf. With the disease progressing, older plants show black stripes on the upper and undersides of the leaf. Assimilation is considerably decreased, the leaves drop off. Blossom symptoms are frequent on Cymbidium.
Cattleya shows sunk in, dark brown to black discolouring on fully grown leaves. These discolourations may be found over the entire leaf surface or as round spots. New shoots are usually very bright. Small round brown spots can be seen on the open blossoms that join later to form brown stripes. Cattleya, Cymbidium and Phalaenopsis are the most susceptible orchid varieties to ORV and CyMV. However, these two viruses also occur on Odontoglossum and Laelia, but they are not frequently seen on Paphiopedilum.
Viruses can easily be transmitted mechanically, but there is no risk of sucking insects transmitting the virus. The risk of spreading the virus in water should not be underestimated. Both viruses are often found in mixed infections on orchids, attributing symptoms to one of the two viruses is hardly possible.
- If a virus is suspected, examinations should be carried out in the laboratory.
- Viroses cannot be controlled chemically, thorough positive selection must be carried out in the greenhouse.
- Plants showing symptoms must be completely removed immediately, cutting off individual leaves is insufficient
- Sap transmission from plant to plant by touching, sprinkling water or work on the plants must be avoided.
- After each work process and most definitely after changing to a different species, hands must be washed thoroughly. Work on plants is the most common way of transmitting viroses in horticulture.
- For disinfection of cutting tools, tables, shelves and vessels, disinfectants can be used to destroy CyMV and ORV. The surfaces of cutting tools may also be cleaned with high percentage alcohol.
- The vectors of viruses, especially thrips and Individual viruses can also be transmitted by pests. The vectors, especially of thrips and aphids should be controlled consequently
Weeds & funghi in the substrate
Marchantia polymorpha L. grows mainly in nutrient-rich pot substrates, it is an indicator for high nutrient supply. Frequently, it comes together with the plants. Once established, the moss spreads when drops of water hit the so-called breeding cups. Marchantia polymorpha L. is very sensitive to higher pH values and dry substrate surfaces. Effective agents can be used that selectively effect the moss.
Creeping yellow wood-sorrel (Oxalis corniculata L.) is a soil-covering weed with green to dark red leaves and yellow flowers. The plant forms seed capsules from which the seeds are catapulted over a long distance. Therefore, the plant can spread quickly in the stand. As it also forms sprigs above and below the ground, it has good conditions on pot substrates. Creeping yellow wood-sorrel prefers pH values in the slightly sour range. Chemical treatment with herbicides is not possible in orchids.
Only hygienic measures help against spreading this fungus. All pots with fungus phases must be disposed of. Direct treatment of the fungus is hardly possible. Early use of fungicides against the spores is the only way to improve the situation.