Why Is there sugar on my tomato leaves?

By Connie Mehmel
WSU Chelan/Douglas County Master Gardener
June 1, 2022

Sugary substance on tomato leaves.
Psyllid sugar and potato/tomato psyllid nymphs are visible on the leaves of a tomato plant. – Provided photo/Connie Mehmel

Connie Mehmel
Connie Mehmel – WSU Extension Chelan/Douglas County Master Gardener – Provided photo

In the summer of 2020, many home gardeners in Chelan County were surprised to find what looked like granulated sugar on the leaves of their tomato plants. This “sugar” was the excrement of an insect called the potato/tomato psyllid (Bactericera cockerelli), an aphid-like insect with piercing-sucking mouthparts that it uses to suck fluids from the leaves and stems of potatoes and tomatoes.

This insect can feed on other related plants such as peppers, eggplants, tomatillos and wild nightshade, but only potatoes and tomatoes appear to be injured. Infested plants often developed a condition called “psyllid yellows”, with discolored leaves, deformed growth and poor quality fruits. The insect also vectors a serious potato disease called “zebra chip.”

Although a number of infestations were reported to the local Master Gardeners in 2020, home gardeners did not report infestations in 2021. Will they show up again this year?

Potato/tomato psyllid was long thought to occur mostly in Mexico, the U.S. Southwest and Rocky Mountains, but in 2011 an outbreak in the Pacific Northwest showed that it can occur here as well. The insects are killed by freezing temperatures, but if they find adequate shelter they can survive.

Adult psyllids are about one-tenth of an inch long and resemble miniature cicadas. In late spring, females lay their eggs on the undersides of leaves. Eggs hatch in four to 15 days, depending on temperature. Newly-hatched nymphs are pale brown or tan, becoming increasingly green as they develop. They become adults in two to three weeks, and can produce three to four generations in a season.

So back to the question, will psyllids cause damage to tomatoes and potatoes this year? You can place yellow sticky traps in your garden if you want to check for these insects. Psyllids begin to show up in late May. If you have an infestation, you will see psyllid sugar and damage by mid-July. But what can you do about it?

Most pest control products have not been evaluated for potato/tomato psyllid control. If you see psyllids in your garden, be wary about using broad-spectrum insecticides. While these may kill the psyllids, they also eliminate important natural predators and may increase populations of other damaging insects such as spider mites and leaf miners. Predatory bugs are important natural enemies of potato/tomato psyllid, especially damsel bugs and minute pirate bugs. Depending on the severity of the damage, you may be able to harvest your tomatoes and remove the infested plants.

To reduce the chances of repeated infestation, clean up debris from any tomatoes, potatoes or other plants in the Solanaceae family at the end of the growing season. Eliminate any nightshade growing in the area, since psyllids can use nightshade as a host.

For more information, email the the Master Gardeners Plant and Insect Clinic at chelanmastergardeners@gmail.com or call the WSU Extension Office at (509) 667-6540 and leave a message.

or visit:

Plant & Insect Clinic | Chelan & Douglas Counties | Washington State University (wsu.edu)

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