Diseases and Pests
Potculture (and to some extent gardenculture) often confronts the plants with insects, bacteria or fungi they wouldn't encounter in their habitat. Even if they occur naturally in their surroundings they can proliferate in days because of the favourable environment. A healthy plant can be a dish served on a silver platter. Some diseases are specific for a genus or species, if you don't find the symptoms here you should consult the genera-list and browse for the plant. As a note, we don't really make a distinction between pests (infestation) and disease (infection) here. You should keep in mind that symptoms can be those of an ailment - due to a deficiency or excess in nutrients & environmental factors.
The general idea behind disease-control in your garden, conservatory or greenhouse is trying to create an environment hostile to unwanted critters and ideal for the plants. This can be achieved in a number of subtle ways:
- companion plants can ward off unwanted bugs.
- garden layout can favour natural predators
- a conservatory or greenhouse can be organised to change temperature/humidity/light and air movement. This also goes for gardens.
When this fails and something gets past security, the first thing to do is a risk assessment. A rule of thumb is that pests normally strike when a tipping point is reached on the size of your plant culture. Expert growing for 10 years with a zero history of pest control isn't a sign of expert knowledge in disease control, but of a combination between very green fingers and luck. It's more likely to be an accident waiting to happen. A second rule of thumb is that pests tend to break out when a small part of the collection - somewhere in a corner - is ignored due to lack of time. If you experience chronic lack of time, consider reducing the collection. Every collection has a few species that multiply like mad, and a few that need more experimenting. Be realistic and don't consider terrestrial orchid culture as stamp-collecting.
Consider pests as follows: For growing healthy plants we create an optimal feeding ground, a combination of the soil, nutrition, light, air-movement, humidity and temperature. There are however also sub-optima, a combinations that creates a favourable condition but one of the factors could be improved considerably. What is optimal for your plants could be sub-optimal for insects - the point of view from someone who grows insects.
Allways isolate plants that show worrying symptoms, especially in monocultures of a single species (growers often focus on cultivars of a species) or a single genus. Move them to an isolated area where they don't come in contact with other plants, potting material and substrates. This can be a terrarium or a sheltered place in the garden. Don't keep them in a corner somewhere in the greenhouse. If the affected plant grows in the garden dig it out with a generous amount of soil. Should this be in the middle of a clump of plants you should consider digging out over a few square meters and poting the plants up. The affected plants go to a reserved isolated place, its neighbours to a second spot fit for monitoring. The last scenario is especially advised for Cypripedium.
Terrestrial orchids are not all alike. Those that go dormant in the form of underground tubers or rhizomes are normally less prone to be infected because the organism has a limited time to reproduce. These plants run most risk from soil-bound infestations: underground insect larvae, fungi or bacteria.
Most of these can be eradicated by simple rules:
- clean your pots with water and a brush before storing, give them a rinse before re-using
- when splitting rhizomes or cutting dead roots and remains, clean the knife and utensils with alcohol. Start from clean, sharp equipment and whipe them between different plants.
- when you use a lot of organic material such as leafmould and composted debris you should consider sterilizing the soil. This should definitely be done when the soilmix contains forest soil (often used in garden culture). Bags with commercially available compost don't have a guarantee on sterility and can harbour unwanted visitors such as Botrytis (gray mould), millipedes or nematodes.
- store your soil-ingredients appropiately. Don't keep bags of compost exposed to rain or temperature swings. Once the contents gets soggy or has circular colored rings it can't be used for potculture.
- take out the tubers for inspection after dormancy has started. Try to repot tubers every dormancy, it's not a major problem to skip a year but this is normally not done. Rhizomes that react bad to replanting should at least be inspected by carefully scraping away some of the topsoil. If they can't be repotted frequently you should at least inspect and clean the bottom of the container and the saucers every now and then. You would be surprised what strange microcosmos lies under those pots.
Bringing in plants from the wild is putting the others at risk as they often carry passengers.
Aphids a.k.a. greenfly or plant lice
Blossom midge
Fungus gnats
Mealybugs
Millipedes, Centipedes and Sowbugs
Mites, spider mites, false spider mites
Scale insects
Slugs and snails
Thrips
Whiteflies
gallmidge fly larvae
earwigs
Ants
Cercospora
Anthracnose
Botrytis
erwinia carotovora
erwinia cypripedii
pseudomonas
phytophthora and pythium
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