It may seem that the closed environment of a greenhouse would be protective, keeping pests out and plants disease-free. The same closed environment can also trap pests and disease inside, harboring them while providing conditions that enable them to thrive. A greenhouse that is used year-round allows insects and diseases to live without a natural winter kill. Use a system to regularly check plants in your greenhouse, and try to catch and treat problems early. Always check new plants before introducing them into your greenhouse. Prevention and organic controls are preferred methods of dealing with greenhouse pests and diseases.
Powdery mildew is common in greenhouses, and it affects a variety of greenhouse plants. Mildews do not kill a plant, but they spoil the appearance and make the plant unmarketable. Several strains of powdery mildew thrive in a greenhouse, including erysiphe, leveillula, microsphaera and spaerotheca. Unlike other fungal infections, mildew does not need wet leaves to grow. The high humidity in a greenhouse provides perfect conditions for it to grow on the host leaves. Proper plant spacing and good sanitation practices will help prevent mildew. It survives on old plant matter, so keep dropped leaves and plant refuse cleaned up.
Black Root Rot
Black root rot, caused by Thielaviopsis basicola, thrives in greenhouse conditions. It is a widespread air- and soil-borne fungus that can live in soil or dust for years as spores without harming plants. Healthy plants can be infected and show no signs, but as soon as the plant becomes stressed, such as through improper temperatures or poor fertilization, the fungus kicks in. It causes cankers on the roots and yellowing, stunting or wilting of foliage, and can eventually kill plants. Correcting the stress factor usually helps the plants recover. Disinfect benches and do not reuse trays. Sanitize growing medium, especially if it contains sand or topsoil that may contain spores.
Aphids in the greenhouse are a serious problem. They damage plants by sucking the sap, seeking tender new growth and causing leaves to curl or die. They excrete a liquid known as honeydew, which attracts ants. Both ants and aphids transmit diseases between plants and from outside the greenhouse. Aphids reproduce asexually by parthenogenesis and they multiply rapidly. Aphids are resistant to many insecticides. For organic aphid control, insecticidal soaps and neem-based sprays are effective. Biological controls include tiny parasitoid wasps, which lay their eggs in aphids and kill them, and ladybugs, which feast on aphids.
Greenhouse thrips (Heliothrips haemorrhoidalis) is a common insect pest. It feeds on more than 100 plant hosts, eating mainly inner leaves and sometimes fruits. Foliage will appear silvery and then turn yellow and drop. Affected fruits will turn brown with cracks and sunken spots. Unsightly black excrement on foliage is a sign of a thrips infestation, although all the various developmental stages of the insects are visible anytime throughout the year in an infested greenhouse. Thrips reproduces by parthenogenesis, and once established, a population is difficult to eradicate. A parasitic wasp, Megaphragma mymaripenne, lays its eggs in thrips eggs and can kill 25 to 50 percent of thrips eggs. Beneficial thrips, Franklinothrips aeolothripidae, feed on greenhouse thrips and may also be used as biological controls. Pyrethrin is an acceptable organic spray for greenhouse thrips infestations.