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DIY Plant Doctoring

DIY Plant Doctoring

Common Summer Pests – Do It Yourself Plant Doctoring

by Judy Quan 

Did you ever wonder how the Help Desk magic of the Master Gardeners happens? Here are some clues about how the Help Desk answers unfold, using six common summer pests as examples.

The first steps to becoming a DIY Plant Doctor is to be observant in your garden.

Walk your garden frequently.  Notice the leaves, the soil, the moisture level.  Are your plants growing? Are they dropping leaves? The leaves don’t look so great; why? 

The next step in answering that question might be to examine your plants more carefully.  

Do you see yellowing leaves with stippling; if so, what plants are affected? Are there any signs of insects on top or bottom of leaves? Is the whole plant affected? Is webbing present? Or as you stroll by your plants, pause and look closely at the leaves including the underside. 


You might see the actual insect or

You might see a folded leaf, a home for a caterpillar.

Have you seen leaves that have gone from green to a silvery color?  Is there a stippling pattern?  Is there black speckling on the leaves or fruit? 

Have you noticed leaves or fruit that are covered in black soot?  What plants are affected? When did it start?

Do you see a white powdery growth on either the top or bottom of the leaves or on the flowers or fruit?  Do your leaves have yellow or brown blotches on the upper or lower sides? Which of your plants have white powdery spots? Are they in the shade or the sun?

Now that we have noticed that something is amiss in the garden, we need to start our investigation. We need to collect information about the garden, the plant and how much is affected.

Identify the affected plant or plants.

We need to identify the possible causes for the symptoms we are seeing and often that starts with the plant. Amazingly, insects and diseases are very attuned to specific plants. If we can identify the plant, then we have narrowed our list of possible diseases and pests. If we don’t know the name of the plant, then we have to do more detective work looking at the plant damage.

Sources/tools to help narrow the choices for diagnosis

Use the UC IPM website, where you may choose a plant category to find the most likely source of your pest/disease problem

Using the UC Plant problem diagnostic tool

The tool can use some explanation. Check out this blog that provides an overview of how to use the plant pest diagnostic tool.

What next after pest identification? If your diagnosis ends with the conclusion that the problem has been caused by a pest of some sort, the next step needs some more thought and observation. There are many aspects to effective management of pests, including cultural care of the plants to make them stronger and more able to tolerate pests. The degree of control of a pest need to be balanced with environmental needs and the health of the plant.

Is the pest still present? By the time we notice the damage, often the pests are gone, and there is little we can do until the next season. Look for insect presence on the top and underside of leaves. You can check for thrips as you would for mites, by shaking injured parts of the plants over a white piece of paper.  A small 10x hand lens might come in handy to see the tiny pests such as insects and arachnids (e.g., mites and spiders). Timing often determines the effectiveness of the strategy.

Question? Is there a need for management/control? To have a healthy garden with pollinators, perhaps we need to tolerate some damage to our plants.  What impact will our management methods have on the beneficial insects or the garden overall? Is the damage more cosmetic than damaging to the health of the plant?

Science-based information for management/control The University of California encourages home gardeners to manage pests by using Integrated Pest Management (IPM), a science-based strategy to manage the damage pests cause in the garden. IPM is helping the home gardener solve pest problems while minimizing the impact on people and the environment.

Sometimes the best control is prevention. Work to know the cultural care for the plants in your garden. Strong healthy plants resist pests and disease better. Correct watering, especially in times of extended drought, is very important. Encourage pests’ natural enemies such as beneficial insects and birds. For new plants, choose disease resistant plants suited to your area.

Try physical controls first.  Using water to spray pests off the plants. Squish or trap the pests. Use physical barriers.

When using chemical controls, use the right one at the right time, in the right amount. Always follow the directions on the label. 

Science-based information for management/control The statewide UC IPM Program has produced publications (Pest Notes) about specific pests. Pest Notes (http://ipm.ucanr.edu/PMG/PESTNOTES / ) contain information on pest identification, biology and habitat, and life cycle, with description of pest  damage and impact. Pest Notes include multiple management suggestions including monitoring, cultural control, biological control and chemical control. There are also Quick Tips (http://ipm.ucanr.edu/QT/index.html) that offer quick advice related to pests and environment-friendly gardening practices; often they are the abbreviated versions of the Pest Notes.

Here are the Pest Notes and Quick Tips associated with the common summer problems associated with the photos that appeared in the article earlier.


Spider mites




Spider mites have many natural enemies which help to reduce the pest numbers when the environment is not disrupted by the use of insecticides. Spider mites prefer hot, dusty conditions, and plants under water stress.  Watering appropriately to reduce plant stress has the added benefit of reducing the damage caused by spider mites. Spraying down dusty pathways or washing trees and vines with water to remove dust can help prevent mite infestations. Spider mites can become a problem after applying insecticides.



http://ipm.ucanr.edu/QT/whitefliescard.html, http://ipm.ucanr.edu/PMG/PESTNOTES/pn7401.html 

Whiteflies are closely related to aphids, another sap-sucking insect, and they too can cause sooty mold. Whiteflies can proliferate rapidly in warm weather, especially when their natural enemies are ineffective due to use of insecticides, the presence of dusty conditions, or interference by ants. Early stages of whitefly population can be managed by removing infested leaves and/or hosing down with water sprays. Management is difficult once populations are high.


Leafrollers and other caterpillars

http://ipm.ucanr.edu/PMG/PESTNOTES/pn7473.html, http://ipm.ucanr.edu/PMG/GARDEN/FRUIT/PESTS/leafroller.html 

 It is important to correctly identify the pest that is causing the damage first before deciding on management techniques. The management of leafrollers differs from that of the other caterpillar-like larvae. Keep in mind that healthy trees can withstand the loss of some leaves, making insecticide application unnecessary for small numbers of leafrollers.



Although thrips damage is unsightly, the use of insecticides is usually not effective; by the time damage is noticed on ripening fruit or distorted buds or shoots, the thrips that caused the damage are often gone. Damage will remain until the damaged plant parts (leaves, buds, fruit) drop off or are pruned off, or the fruit is harvested. Insecticides will not fix the damage.



Sooty mold 

Sooty mold is a black fungal mold that grows on sticky honeydew, the waste of many plant-sucking insects.  The insects excrete honeydew, and  the honeydew lands on plant leaves, fruit, twigs, concrete, sidewalks and whatever else is below the insect.  The key to controlling sooty mold is to manage the insects that produce the honeydew. These insects include aphids, leafhoppers, mealybugs, psyllids (including eucalyptus redgum lerp psyllid), soft scales, and whiteflies. Sooty mold is not damaging to the plant, but it can block light to the plant, reducing photosynthesis.


Powdery mildew

Powdery mildew affects many plants, and is common in warm, dry conditions. Moisture during the spring inhibits growth. Try washing spores off infected plants with overhead sprinkling but try to do this in the morning so plants have time to dry off during the day. Prune out affected plant parts and dispose of them so as not to spread the spores. Fungicides may be needed for susceptible varieties of apple, grape, strawberries, rose, and crepe myrtle.

Need more help with understanding the Pest Notes or finding more information? 

Plant-doctoring in your own garden can be an interesting and addictive activity or it can be one where you think you might need a “lifeline” to solve your plant pest problem.

The Master Gardeners can be that lifeline.

Email the Master Gardeners of Alameda County at acmg@ucanr.edu. Or contact us through our website http://acmg.ucanr.edu/Contact_Us/