Pests and Diseases
Hilary Hobbs
Pests and diseases affect all plant species, not just orchids, but the effect on these valuable ornamentals can be deadly. Pests damage cells, suck sap, and generally reduce vigour leaving unsightly plants that can take years to recover – if at all. Prevention is by far the best control, and just a few simple steps can go a long way. These include good (optimum) culture, greenhouse and tool hygiene, avoid overcrowding, remove non essential plants such as ferns, regular inspections, water in the mornings to keep plant crowns dry overnight. Despite best efforts it is almost impossible to avoid pests for an active grower. Pests and diseases can come into the greenhouse from purchased plants; contact with plants at shows etc. Quarantine, isolate, and if necessary be prepared to destroy plants badly infected.
There are 3 methods of dealing with pests – ignore, control, and prevent. The first isn’t a viable solution. You really need to know your enemy to understand how best to treat/prevent them. Diseases are harder to identify at first alongside normal wear and tear, but once seen won’t be quickly forgotten.
The pests
Mealybug is probably the most common pest, and attacks most genera, but especially phals. They suck sap from the leaves, and leave white cotton like marks everywhere. An adult can lay 100-200 eggs which hatch at 14 days into crawlers who are highly mobile and will rapidly spread to adjacent plants. They have a tendency to hide in sheaths, under leaves, in compost and elsewhere. Males eventually pupate and turn into flies to go off elsewhere to mate and die.
Treatment is to kill the adults by either a systemic spray such as Provado or Bifenthrin, or treat with cotton buds and meths. Neither treatment will kill the eggs, so must be repeated 2-3 times at weekly intervals. Neem oil (no longer available) will help to suffocate the bugs, and a bio control – Cryptolaemus montrouzieri (common name Mealybug Ladybird is a ladybird species endemic to Queensland and New South Wales, Australia) – should work, but Hilary hasn’t tried them.
Scale incorporates both hard and soft versions. The scale is a membrane that acts as armour from other pests, and unfortunately acts as a shield from spray. The females live under the shell where the eggs are laid, and look very unsightly. Once hatched, the nymphs are mobile, and the males pupate and become flies. Systemic sprays will have an effect, but manual treatment is more effective.
Boisduval Scale looks very similar to mealybugs, but is more aggressive. They originated in South America, and have a liking for native genuses such as Cattleyas, Odonts, Pleuros etc. Their lifecycle is similar to the other bugs, taking 33 days to become adults. Treat as for bugs at fortnightly intervals for 6 weeks. A weak solution of Physan will work on the crawlers.
Aphids are quite easy to see as they tend not to hide and give birth to live offspring. Consequently they are easy to treat with insecticides. As aphids are winged insects, they are very mobile, and can spread viruses.
Thrips are tiny insects that are hard to spot. Like the others they are sap suckers, and can quickly turn leaves brown. They are often widespread in greenhouses, and rarer in the living room. They spread diseases, viruses and fungus, so are quite dangerous. They pupate in the compost before flying off elsewhere. Treat by spraying insecticides, or use Neem oil. A better prevention is to use sticky fly tape around the greenhouse.
False Spider Mites are too small to see without a magnifying glass. They are reddish brown mites that damage cells leaving unsightly marks. A large infestation will leave a silvery mesh on the leaves. They like it hot and dry, so a humid greenhouse helps. Insecticides work, and regular leaf cleaning is useful. They prefer softer leaved plants. Biological controls are available – Phytoseiulus persimilis, a predatory mite.
Slugs and snails are large pests that cause rapid damage. Use slug pellets for protection, and use a torch at night to spot them, them dispose/despatch as required.
Vine Weevils are often unseen as they tend to eat the roots. A serious problem will see plants simply keel over. They tend to be nocturnal. A root drench of insecticide is the best treatment.
NB. When using insecticides, check that they are for ornamental plants, carefully read the instructions and use the correct strength. Use outside if possible, and protect yourself.
Fungus
Black & Brown rot, phytophthora & phythium affect many genuses, especially Cattleyas, Paphs and Phrags. Results are leaf spots that spread out. Trimming leaves back to green matter with a sterile knife and dusting the wound with cinnamon or sulphur should suffice. In serious cases remove the whole leaf or cane in the case of Cattleyas.
Bacteria
These are difficult problems to prevent. Psuedomonas is soil borne and affects Cattleyas and the Cyp group. Using a weak solution of Physan can be effective. Erwinia affects the Cyp group, and spreads rapidly. Affected leaves must be removed immediately and cinnamon/sulphur applied.
Be vigilant with rots and bacterial infections. Fungicides can be effective, but not on bacteria. Isolate plants when being treated. Good air movement and dry crowns at night should help avoid problems. Identification can only really be made through lab analysis – which usually takes too long to be of great help, and is expensive.
Viruses
These are simple organisms and are classed as Intracellular parasites. They contain RNA and a protein coat/cover. Good husbandry can keep them at bay, but once infected, they are likely to remain in the host for life weakening plants.
Cymbidium Mosaic Virus (CymV) is common in several genuses, and can often be seen as a brownish diamond pattern within the leaf rather than external. It can also be seen as chlorotic spots on leaves and flowers.
Odontoglossum Ring Spot Virus (ORSV) is uncommon, and can be identified by necrotic rings on the leaves. This virus is stable, and can survive outside of the plant, so is more mobile.
Bean Yellow Mosaic Virus is spread by aphids when sucking infected cells then moving on. It affects Masdevallias where new growth is deformed and mottled.
No treatments for viruses are available. Affected leaves can be cut off and burned. If possible divide and isolate the plants to see if the division is clear – then burn the infected one.
Summary
Prevention is far better than treatment or death. Carefully inspect new plants and quarantine them. Greenhouse hygiene is essential. Avoid overcrowding. Water in the mornings, and avoid drips onto other plants. Good air movement is essential. Try not to reuse pots, canes or ties. Sterilize tools between plants and wash hands. Deal with pests when seen.
An excellent and well received lecture. Thank you Hilary.
NB. Further information is available in general orchid growing books, or use the names in a search engine on the internet. Biological controls are available to purchase online –www.ladybirdplantcare.co.uk is just one site.