Pest Scouting Pointers For Greenhouse Vegetables

Pat Rorabaugh


When should you begin scouting your greenhouse crops? How do you know if it is a disease or insect you are up against?

American Vegetable Grower caught up with Patricia Rorabaugh, an assistant professor of plant sciences at the Controlled Environment Agriculture Center at the University of Arizona, to answer these questions and more.

Q1 When should growers begin scouting in the greenhouse?

Patricia Rorabaugh: As soon as the plants are introduced. Growers should also make sure that the plants they are putting in are clean of pests.

Q2 In a typical vegetable greenhouse producing crops such as tomatoes or peppers, what conditions are conducive to bacteria and viruses to becoming a problem?

Rorabaugh: It depends on the fungus: bacterium or virus. Humid conditions do tend to be conducive for many fungi, and some fungi and bacteria need free water to spread. Some fungi and bacteria like warm conditions, others prefer cooler conditions.

Viruses usually need a vector (insect, people, wind, etc.) so environmental conditions may not be an issue. Also, diseases will not become a problem if you use resistant varieties. Remember the disease triangle: For a disease to be a problem you must have a susceptible plant, the proper conditions, and the disease organism (and sometimes a vector). If you can alter any of those, you can stop the disease.

Q3 From the symptoms, how can growers tell the difference between a disease or a nutrient deficiency?

Rorabaugh: It depends on the disease and/or nutrient deficiency. I know that sounds obvious but take a look at yellow spots, for example. If there is interveinal chlorosis (yellowing in between the veins) on lower leaves, this could be a magnesium deficiency (but tomato leaves will show this as they are dying). If on upper leaves, this could be iron deficiency. If anywhere on the plant, it could be powdery mildew (which could also be white patches, depending on the species of powdery mildew). And if the yellow spots are tiny (stippling) associated with webbing, this is most likely spider mites.

Q4 What types of tools are available to growers to help them make these determinations?

Rorabaugh: A hand lens is needed, for sure. Books and web references can be used to compare what you see to what the nutrient deficiency, disease, or insect/mite problem looks like.

Q5 If a disease is determined to be present, what are growers’ options?

Rorabaugh: I can’t speak to fungicides as I don’t use pesticides. We typically only have problems with two types of fungi: powdery mildew and botrytis. With powdery mildew, which almost always appears on the cucumbers, we use a combination of water (3 gallons), vegetable oil (87 milliliters) and baking soda (9 rounded teaspoons). This is a contact spray and should be sprayed directly onto the powdery mildew spots on a regular basis (once or twice a week). We also get powdery mildew on the peppers which, unlike the cucumber powdery mildew, goes into the underside of the leaf via the stomates and is then untouched by the contact spray. To get to the underside of leaves, a trans- laminar spray must be used that goes into the leaves and kills the powdery mildew from the inside.

Q6 From your experience, what are some of the mishaps growers need to avoid so they don’t lose a crop to disease or mistake a nutrient deficiency for a disease?

Rorabaugh: Before they embark on greenhouse hydroponic produce production, they need to become as educated as possible about what a healthy plant looks like, what disease problems look like, what nutrient deficiencies look like, and what insect/mite problems look like. Then they need to be scouting weekly, if not every time they go into the greenhouse.

Finally, they have to know the problem and how fast it can move in the crop. For example, if the problem is spider mites, they must act immediately (like that moment), maybe even spray down the infested plants to kill as many mites as possible, then order spider mite predators. If the problem is russet mites, which move much more slowly than spider mites, they can say, “Okay, we’ve got russet mites, make sure we spray the stems tomorrow with water, oil, and baking soda.” Remember, there are no beneficials for russet mites so you have to suffocate them!

If it’s a nutritional problem, they need to check their fertigator to make sure all is working properly — if not, fix it! If a disease is suspected, they should take a sample of the plant to a specialist to see if it can be identified and steps taken to make sure it doesn’t spread to other plants in the greenhouse.