Dear Deb,

Thank you for reaching out to the UVM Extension Master Gardener Program with your question.

Nutritional deficiency can cause yellowing of the leaves. As it sounds like the affected plants are near healthy phlox plants I would think nutritional deficiency is not the issue. Feeding by aphids can result in yellow, curled or stunted leaves, but I would have expected you to see the aphids on your plants. Spider mites are a common problem for phlox. Their feeding on the leaves usually causes a stippling on the leaves but over time can progress to yellowing of the leaves. Spider mites are tiny so you need to use a hand lens to look for them on the underside of the leaves. Webbing on the underside of the leaves is another clue that you are dealing with a spider mite problem. My biggest concern is that the findings in your phlox plants may be from Aster Yellows, which is a plant disease caused by a phytoplasma (bacteria like organism) that is transmitted by the leafhopper insect. Infection with Aster Yellows manifests as yellowing of the leaves and stems, stunted growth and malformed flowers. When the insect feeds on an infected plant it takes in the bacteria and then can transmit the bacteria to the next plant when the leafhopper feeds. Many different type of plants can be infected with Aster Yellows. Once infected a plant will not recover and needs to be removed from your landscape and destroyed to prevent further spread.

You should inspect the undersides of the leaves carefully using a hand lens to look for evidence of spider mites. Recommending removal and destruction of plants is serious so I want to be sure that that is the necessary course of action. Additional photos may be helpful. I am including our plant pathologist in my response.

The resources below provide more information.

https://extension.umn.edu/plant-diseases/aster-yellows

https://extension.umn.edu/flowers/tall-garden-phlox

Dear Deborah,

Thank you for providing additional photos. I cannot see the underside of the leaves in any of the photos. I was not able to zoom in on the photos for greater detail. Although you saw the one beetle and some slugs on your plants I do not think that those are playing a major role in your phlox problem. In the photo showing the top of a plant that is wilted with browning of the leaves, there appears to be webbing and tiny white structures. I could not zoom in for finer detail but I wondered if the white “spots” were aphids. The webbing suggests spider mites. The phlox in the second photo appears to be very close to a cement wall. In the following photo both of the plants have the same yellowing pattern to the leaves and it is notable that all of leaves over the entire length of the plant are involved. That pattern suggests a more systemic process. You didn’t mention the plant location. Phlox like full sun and you can see yellowing of leaves if the plants are not getting adequate sunlight. The fact that the one photo shows the plant in front of a cement wall raises the issue of adequate sunlight. Phlox like moist, well-drained soil. Too much water could contribute to yellowing of leaves. As mentioned previously, nutritional deficiency can also result in yellowing of the leaves. You did not mention if you have fertilized the plants. The best way to look for nutritional deficiency is by doing a soil test.

I am still concerned that you could be dealing with Aster Yellows as the pathogen lives within the vascular system of the plant and spreads through the entire plant from roots to flowers. The Aster Yellows pathogen can affect plant growth, development, and ability to process nutrients. As the pathogen can be spread from plant to plant by the leafhopper it would be important to make this diagnosis and remove the infected plants to prevent further spread.

I have included our plant pathologist as collaborator on my response. She may provide additional information and suggestions such as submitting plant material to the lab for further investigation.

See resources below that provide additional information to the resources included in my prior answer:

https://apps.extension.umn.edu/garden/diagnose/plant/annualperennial/phlox/leavesdiscolored.html

https://www.missouribotanicalgarden.org/gardens-gardening/your-garden/help-for-the-home-gardener/advice-tips-resources/insects-pests-and-problems/insects/plant-bugs/phlox-bugs

https://www.uvm.edu/extension/agricultural-and-environmental-testing-lab

Dear Deborah,

Thank you for the additional information and photos. I am sorry to hear that the findings in your phlox seem to be spreading. It sounds like your phlox are getting enough sunlight and appropriate irrigation. Our plant pathologist reviewed your photos and her opinion is that spider mites are playing a major role. The fact that there is webbing present suggests a significant spider mite infestation. She recommended treatment with insecticidal soap. You most likely will need to use repeated applications of the insecticidal soap 5 days apart. It would be important to monitor the new leaves that appear after the spider mite infestation has been treated. If the new growth does not appear healthy then an alternate diagnosis will need to be considered. Her other concern is that the yellowing of the leaves could also be due to iron or magnesium deficiency. A soil test may be helpful to assess nutrient levels and soil pH. Even if nutrient levels are adequate, if the soil pH is high it can interfere with a plant’s ability to use nutrients such as iron. You could supplement iron by spraying the leaves of the plant with chelated iron which should be available in a garden store. Our plant pathologist did not think that Aster Yellows was the cause of the findings in your phlox.

Hope this helps.