Greenhouse Growing – The Good, The Bad and The Bugly

Greenhouse Growing

Diseases, pests and good bugs in the greenhouse

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I love to spend time in my greenhouse. It is like my personal oasis. I turn off my phone and go in and get lost in the comfort of my plants. Unfortunately there are some diseases and bugs that may desire this warm, friendly atmosphere as well. If you are diligent in your efforts to monitor for this and keep your greenhouse environment clean, you may never have an issue at all. A healthy environment is critical for your greenhouse production, be it for pleasure or profit.

The Good – Do these to prevent diseases

Keeping it clean – The best way to fight disease in the greenhouse is to keep everything clean. If you are reusing your soil it should be sterilized. If you are purchasing new soil or soilless mix you should be certain that they are pathogen free.

Always keep your tools, bench surfaces and floors clean. Clear all debris from previous crops out of the greenhouse. Many of the diseases will go dormant in these dead leaves, etc just waiting for the proper conditions to reappear.

Bugs such as thrips, whiteflies, and aphids may find a home in weeds that have been let grow in the greenhouse.

Circulating Fans – This is an often overlooked greenhouse accessory. They provide many beneficial features to your greenhouse. They should be running 24 /7 in the spring, winter and fall. These fans provide a slow, steady air flow to the greenhouse. They will help to avoid any hot or cold spots in corner, etc. They help to keep condensation down, therefore curbing back on disease. They mimic a gentle, blowing breeze which will increase the strength of your plant stems, giving a stronger, healthier plant.

Greenhouse Circulating Fan

Greenhouse Circulating Fan

Bumble Bees or Honey Bees – These are both excellent pollinators. Bumble bees are least aggressive and require smaller colonies. Also, if you use honey bees you have the added task of taking care of the honey that is produced.

Yellow Sticky Traps – These are used as early detection for adult whiteflies, thrips, adult leafminer flies, fungus gnats and aphids. Early detection is critical for elimination of these pest. Hang these in your greenhouse around your active crops.

The Bad – Diseases of the greenhouse

Botrytis – Botrytis is a fungus that can affect any plant in a greenhouse. It presents as a gray, fuzzy  mold. It usually appears as a Spring or Fall issue, as it grows best in temperatures of 55 – 75 degrees F. Excess humidity can cause Botrytis to appear. To avoid this, keep the greenhouse clean and remove any plants which appear to be affected. Make sure that your plants are not overcrowded. You need good air circulation. This is where your circulating fans come into play. Water only in the morning. Avoid placing plants where water is dripping from the roof.

Tobacco Mosaic Virus – This may affect petunias, tomatoes and tobacco plants among others. It may affect up to 350 species of plants. Some plants are carriers but show no sign of the disease. This virus can stay active for long periods of time in plant debris and even on surfaces such as greenhouse floor or benches. You can spray the affected plants with a 20% nonfat dry milk mix. This will coat the virus and inactivate it. This presents as a mosaic pattern of light and dark green on the leaves. There is no chemical cure for this virus. Discard infected plants.

Bacterial Blight – This is incurable, so the best way to handle this is with prevention. The most obvious sign of this is wilting of the leaves. The root may appear rotted, but not always. If you over fertilize you may see similar symptoms. Make sure to differentiate.  Be sure to sanitize all tools and surfaces. Affected plants must be destroyed and removed. The soil should not be reused.

The Bugly – Good bugs and bad bugs

Biological control – To me this is a fancy way of saying good bugs. There are some bugs that will naturally prey on other bugs. This is a whole lot better than using chemicals. The most important step, once you have found you have an issue, is proper identification. You must pair up the proper predator with your pest.

  • Parasitic wasps will control aphids. Aphidius wasp.
  • Ladybugs are good for spider mites and aphids.
  • If you have a mite issue there are predatory mites.
  • Parasitic wasps will control whitefly. Encarsia formosa.
  • Parasitic nematodes are for controlling larvae or grubs of some beetles and weevils

The early detection is very important. Once a pest has been established it is harder for the beneficials to completely eradicate the problem. This is because the beneficial bugs have a hard time reproducing as fast as the bugs already in residence.

Well, just as there are good bugs for the greenhouse, there are bad bugs as well. They can cause a whole lot of damage in a greenhouse. The best way to keep this under control is to monitor your plants for signs of bugs on a regular basis. Also, the yellow sticky tape mentioned earlier will help you to monitor your bug situation.

Caterpillars – The easiest way to keep these out of the greenhouse is to keep butterflies and moths out of the greenhouse. They will chew holes in leaves. You should be able to spot these with regular monitoring. They can be manually disposed of.

Thrips – Thrips are small slender insects. They have wings, but do not fly well. They may affect plants such as onions, beans, carrots and squash. They feed by cutting the leaf and drinking the sap. They spread tomato spotted wilt virus. The leaves will have stippling and discolored flecking. Insecticidal soap can be used to rid your plants of these pests.

Aphids – There are many different types of Aphids. They are relatively small and may or may not have wings. They prey on the plants by sucking the juices out. Their population will increase exponentially. You can manually remove the infected leaves to help curb these bugs. Ladybugs and lacewings are natural predators.

Whiteflies – These pests are appropriately named for the color of their wings. They are small and moth like in appearance. They lay their eggs on the bottoms of plant leaves and suck the sap out of the plant leaves. Immature stages are more difficult to see and locate. The Whiteflies can transmit several viral  diseases.  Yellow sticky cards will help to monitor this situation, but they are not really effective as a control measure. Ladybugs and lacewings are natural predators.


The best way to keep a healthy greenhouse is to keep a clean greenhouse. If you are unfortunate enough to run up against one of these issues make sure to properly identify first so you can take the appropriate measures.