HortFACT - Plant Health of Greenhouse Capsicums
Thrips: Small, long, thin, brown/black insects, with similar shaped larvae which are white/yellow. Thrips feed on flowers and on the lower side of the leaves, causing silver or grey-white spots with black faecal dots. Another symptom is small ‘warts’ on the lower leaf side, caused by egg deposition. High numbers of thrips will prevent flowers setting and can affect plant growth and reduce production.
Two thrips pests are found on New Zealand capsicums: onion thrips and western flower thrips. Both species can transmit tomato spotted wilt virus which can mark fruit and kill plants. Western flower thrips is resistant to most pesticides and for effective control it is important to know which thrips you have. Both thrips can be controlled by a predatory mite, Amblyseius cucumeris.
Two-spotted mite: These are tiny (0.5 mm) web spinning mites, coloured yellow brown with two dark marks on the sides. They feed on the lower side of the leaves, and cause very yellow feeding marks. In severe infestations whole leaves turn yellow and webbing will cover the plant. Successful biological control using the predatory mite Phytoseiulus persimilis depends on finding infestations of the pest early and releasing the predator immediately.
Broad mite and strawberry (or cyclamen) mite are so tiny that they are difficult to see, but they cause distinct virus-like symptoms (distortion of leaves) and can prevent new growth. If pesticides are used, the plant must be thoroughly wetted. Amblyseius cucumeris may control small infestations of these mites.
Below is an example of broad mite damage on capsicum fruit. The marking on the surface of the fruit was caused by mites feeding when the fruit was young.
Aphids: Various aphids are fond of capsicum plants, especially the young leaves in the top of the plants. Symptoms of aphids include sticky honey dew, white empty skins on the leaves, and feeding injury in the form of curling leaf margins, yellow spots, leaf distortion. Severe infection may cause leaf drop and as a result low production and sun scald on fruit. Aphids stain the fruit with their excrement and may transmit viruses.
Caterpillars: Green loopers are the commonest caterpillar damaging capsicum plants. The first signs of caterpillars are holes in the leaves. Large caterpillars may damage fruit. Green looper and tomato fruit worm are controlled effectively using a biological spray containing Bacillus thuringiensis toxins. This is a selective and environmental-friendly spray. The most damaging and difficult to control caterpillar is the poroporo fruit borer, a native species that fortunately only occasionally gets in to crops. The white or pink caterpillars can be found in the fruit.
Whitefly: This is not a severe problem in capsicum. Adults and larvae feed on plant sap and excrete a sticky honey dew staining leaves and fruit. They can be controlled by a parasitic wasp, Encarsia formosa.
Three common root diseases are associated with soil that remains too wet for long periods. This is usually the result of poor drainage. So improving drainage is the primary solution. Some of the root diseases can develop in hydroponics. To kill inoculum of all three disease, the soil or medium must be sterilised before the next crop is planted.
Pythium and Rhizoctonia: are two fungi causing damping-off of seedlings. They arise when plants are weak or under stress (too cold, too wet), also in older crops. Generally Pythium is more active at lower temperature, and Rhizoctonia at higher temperatures. Pythium thrives under wet conditions, also in hydroponics. Symptoms of Pythium are brown roots: while the inside root is rot, the root skin is still intact and can be easily removed. Rhizoctonia typically has brown lesions at soil level.
Phytophthora: infects the main root and the lower stem part. Later the above-ground stem part turns black and the plant wilts and decays. Phytophthora causes soft rot (watery rot). It spreads through soil and water. Infected plants must be removed carefully and good sanitation practices are required.
Botrytis (grey mould): attacks many crops. It thrives under humid conditions, and needs very high humidity or free water for germination. Infection may occur on leaves, flowers, fruit and stems, mainly in stem wounds. Symptoms are brown lesions, expanding rapidly, and later symptoms are a grey mass of fine fungal tissue on dead plant material. The pathogen is resistant to many fungicides and for their effective use, resistance management strategies must be practised. Botrytis infection can be minimised by keeping plants dry and maintaining low humidity (also at night) by heating and venting at the same time (especially early morning!).
Downy mildew: shows as a white fungal tissue on the lower side of leaves, and later also on the upper side. Early symptoms are light green spots on the upper side of the leaves (similar to spider mite symptoms). These spots grow and later cover the whole leaf until it falls off.
Sclerotinia: causes shoots to turn brown, often without others signs of fungal infection. When there is enough water, a dense white fungal tissue will develop. It can also affect fruit. Infected material must be removed. When the infection is serious, the soil or medium must be sterilised.
Capsicum plants can be infected by several viruses, including tomato (or tobacco) mosaic virus (mechanically transmitted) and tomato spotted wilt virus (transmitted by thrips). Viruses cannot be seen but are detected by plant symptoms which include mottling, distortion or discolouring (flecking, ring patters, mosaic) of leaves and fruit, and stunted or bushy plant shape. It may reduce the production and fruit quality. A virus disease in the crop can not be cured. It is therefore important to prevent virus infection, or when it is detected, to reduce spreading.
It is important to identify the cause of a problem so that informed actions can be taken, because some virus-like symptoms are not caused by a virus but by mites. Also, viruses are transmitted in different ways (mechanical, by aphids or by thrips). So identification and hygiene are the key.
Mechanically transmitted viruses can be spread by knives, hands, clothing, boxes, etc. Viruses can stay alive for over a year in living and dead plant material e.g. in seed, old plant or roots parts in the system, or in soil or water. When there was virus in the old crop, the greenhouse needs to be thoroughly cleaned and sterilised before planting the new crop. Remove all plant material, even small parts, from the greenhouse and dispose or completely isolate them. Sterilise the medium, greenhouse, nutrition recycling system, boxes, sorting machine, all tools, shoes, etc.; thoroughly clean the whole work area including the cafeteria; hot-wash all clothes. Removal of all weeds from inside the greenhouse and the surrounds will reduce the risk from insect borne viruses and from insect infestation.
The new crop must be planted with all possible precautions. Always start from high-quality seed, as inferior seed is a major source of virus. Propagate the plants elsewhere in a greenhouse with insect screening, and have them arrive when your property is perfectly clean. After planting, be alert for the virus symptoms and have these identified. Any infected plant plus the neighbouring plants should be taken out, put in a plastic bag on the spot, and disposed carefully.
If a virus has established, some practices will help to prevent spreading and minimise the damage. Control insects if these are a vector. Wash hands in soapy water and dip knives in trisodium phosphate, Virkon, or milk containing at least 3.5% proteins. The milk must be renewed daily. For all work in the crop, always go in one direction. Have large shoe disinfection trays at the entrance of your property and at every greenhouse entrance. Make enough time for the big clean-up after ending the crop.
Several bacterial diseases may affect capsicums. They can cause soft rot, wilting, spots, deformation, leaf drop, and thus devastate the production and quality. Transmission may be by seed, insects or mechanically. It is very difficult to control bacterial diseases in a crop. Prevention is the only viable approach and seedlings and young plants should be treated with full precautions to prevent bacterial diseases. See virus diseases above. Sometimes copper sprays can be helpful.
Nematodes are tiny worms normally living in the soil. Some of them may feed on plant roots. One species can cause small knots or galls on the roots of capsicum and other plants. Above ground symptoms may include stunting, yellowing, wilting, lack of vigour. Nematodes can be killed by good fumigation or steaming between crops, but this must be done after the roots are decayed.
See also: Growing Greenhouse Capsicums for a comprehensive guide on growing capsicums.
(The author gratefully acknowledges the help of Nicholas Martin and Phil Broadhurst of Crop & Food Research, Auckland.)