Good plant health, through good nutrition and environmental control, is the first line of defense against both pests and diseases in the greenhouse. But even in the best circumstances, some pests may bother your plants.
Aphids, mites, and whiteflies are the most common serious insect problems. Use mechanical controls such as vacuuming, squashing, and washing at the first sign of trouble. Yellow sticky traps are effective against whiteflies. If pest populations continue to grow, soap or pepper sprays or appropriate botanical controls such as neem may be useful.
Biological control with predatory insects such as green lacewings and ladybugs, and parasites such as Encarsia formosa are extremely effective. To meet the needs of these insect allies, set up a small “biological island” in a warm, bright spot with pots of parsley-family members (Umbelliferae), such as chervil and dill, and smallflowered ornamentals, such as scented geraniums, lobelias, and salvias. Kept in bloom for the entire greenhouse season, these plants provide nectar and pollen for the beneficial insects.
Tempting as it is to bring outdoor peppers, eggplants, and herbs into the greenhouse at summer’s end, you’ll be running the risk of importing pests with them. It’s far better to start plants or buy transplants expressly for the greenhouse. If you decide to take the risk of bringing plants in, quarantine the plants inside sacks made of tightly woven translucent material for at least 7 to 10 days. Aphids and damage from such pests as mites and thrips will be easier to see after this time. If problems appear, it’s generally best to throw the plant out.
Fungal diseases are usually the greatest disease problems in a greenhouse. Providing adequate ventilation and spacing between plants and monitoring humidity levels are the best ways to prevent fungal problems. Preventive sprays can also help minimize disease incidence. Fermented nettle tea and dilute compost tea, sprayed on leaves at weekly intervals from the seedling stage onward, inhibit many diseases while also providing trace elements. Sanitation is important, too; isolate or dispose of sick plants, and clean up spilled soil and dropped leaves in the aisles and under benches.