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.
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.