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Diagnosing Plant Problems: Is It Insects or Disease? Tips for Effective Identification & Treatment

by | Jul 7, 2023 | Gardening | 0 comments

Plants and shrubs are vulnerable to attack insects and diseases (primarily fungi). It can be hard to diagnose the source of the problem when the results are similar: damage to leaves, bark, or other parts of the plant.

Guessing wrong can make the problem worse. If we use the wrong treatment products, not only are we failing to help the plant, but by the time we make a correct diagnosis, it may be too late to save a plant already too stressed to recover.

How to Correctly Diagnose the Problem

Bug or Disease?

(GARDENER’S TIP: “Picture This” App ) The Picture This app can diagnose most problems and tell you if they’re insect or disease-related. It will inform you which insects or diseases are involved and how to treat the plant.

The app can also identify plants from a photo you’ve taken. This is extremely helpful if you want to know if a new plant is a weed or a good plant. And it can expand your botanical knowledge when you’re out on walks or visiting the park, and you’d like more information about the plants you encounter.

While apps like Picture This are fabulous tools for the gardener, they’re not foolproof. Therefore, I always recommend a second check.

  • Examine the plant’s leaves for evidence of insects.
  • Starting by brushing your plants with your hand or a stick to disturb the leaves. If insects fly off them, then insects are the likely culprit.
  • Next, examine the leaves, especially their underside, for insect larvae or resting insects. Many harmful insects like thrips and spider mites are almost invisible to the naked eye. The good news is your cell phone can serve as a magnifying glass with the help of your camera or a magnifying app.

Your 5-Step Plant Treatment Plan

  1. Determine if you can save the plant.
  2. Correctly identify the problem (type of disease or insect)
  3. Eradicate the disease or insects
  4. Implement preventative maintenance & management

Determine If You Can Save the Plant

The good news is that most infected plants can be brought back to full health. The key is early treatment. If over 50% of the plant is infected or infested, then removing the plant might be the best option. It can be challenging to save plants this far gone, but there are other things to consider:

  1. Plants with an insect infestation are easier to save than plants with a comparable infectious disease. Killing insects and larvae is more immediate, while disease will require more protracted treatment. The disease may be systemic at the roots or inside the plant.
  2. It’s harder to stop a disease infection on over 50% of a plant without further damage occurring.
  3. Disease-infected plants are more likely to infect neighboring plants. Removing the infected plant might save other plants.
  4. Some plants can be cut hard to remove the infection or infestation. Typically, perennials will regrow if cut hard (trimmed close to the ground), as will some annuals — as long as you keep some stems and leaves on the plant.

Correctly Identify the Problem (Type of Disease or Insect)

As mentioned, I recommend the Picture This app for preliminary diagnosis of the problem, followed by a manual check for pests. I’ve outlined the process earlier in this blog.

Eradicate the Disease or Insects

Thrips, spider mites, and aphids are common in Southern California and can be eliminated with the same insecticide or natural organic product.

Brown spots, black spots, and white powdery mildew are common infections and are easy to identify. Most anti-fungal disease products will fight all three of these as well as some forms of blight.

Natural and organic products are best if you catch the problem early. They’re also best for ongoing management and disease and insect control. I recommend Bondie products, sold at Anawalt and other fine garden centers.

Chemical products might be necessary for severe outbreaks that demand immediate results to prevent the spread of the infection. The severity of the infection will determine what type of products you should apply. Chemical products like Sevin (insecticide) and Daconil (fungicide) should be used only in extreme cases.

Insect eradication with natural products takes more time. Organic treatments don’t necessarily kill insects but make the plant inhospitable to them. They encourage insects to take up residence elsewhere and interrupt their breeding cycle, preventing additional damage to the plant.

Infection eradication with natural organic products won’t cure the existing infection on leaves (infected leaves should be removed), but it will stop the spread to other parts of the plant. Extreme infections may demand chemical treatments, which will kill the existing infection and stop the spread.

Implement Preventative Maintenance & Management

Preventative maintenance and management begins in the spring and lasts all summer.

2Spraying your plants with a good natural organic insecticide and anti-fungal product will help prevent outbreaks during the growing season. It can also help manage small outbreaks and make it easier to eradicate them.

Most natural products recommend a 7 to 14-day cycle of treatment. These products are best for the environment, and they won’t harm creatures that eat the “bad” insects. They’re also safer for people and pets.

I recommend Bonide natural products for insect infestations or disease infections. These are sold at all Anawalt Lumber, Hardware, and Nursery locations in ready-to-spray bottles and concentrates.