Yellow Leaves on Phlox - Ask Extension
I am seeing the leaves on several garden phlox plants turning yellow. I've experienced powdery mildew before but his is something different. There are...
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Yellow Leaves on Phlox #798545
Asked June 29, 2022, 12:20 PM EDT
I am seeing the leaves on several garden phlox plants turning yellow. I've experienced powdery mildew before but his is something different. There are several other phlox plants nearby but they don't seem affected. I am attaching several photos. Any thoughts? Thanks, Ed
Carroll County Maryland
Expert Response
Hello Ed,
These are interesting symptoms, and we have a few theories which more information might support or disprove. We agree this is not likely powdery mildew.
The fungus causing Verticillium Wilt, which Phlox (and many other perennials) is susceptible to, can create leaf discoloration like this without necessarily progressing to actual wilting. It would be difficult to determine this is the culprit, though, since the fungal growth remains hidden inside plant tissue and does not create obvious external discoloration in the roots the way root-rot infections can. It might stain the tissues in the center of the affected stem, if you were to trim it off and split it apart.
We notice what looks like some wire caging/fencing around the base of the Phlox clump. Has this been in place for some time, and is the metal rusting or degrading at all, especially below the soil line? This hypothesis may be more of a stretch, but nutrient toxicity such as from certain metal coatings (like zinc) in this case might be leaching into the soil in this spot and creating issues with nutrient absorption by the Phlox roots. Even though no nearby plants look affected (the Butterfly Weed looks yellower than usual, but maybe that's the photo exposure), perhaps Phlox is more sensitive to these levels than other species. A laboratory soil test of the soil within the plant's root zone might reveal abnormal nutrient levels.
How long as the decorative statue been in that location? Was it recently cleaned to remove algae or other residues, where runoff might have seeped into the Phlox root zone? If so, what cleaner was used?
Was any herbicide used near the Phlox? Symptoms are usually a bit different or more drastic with herbicide exposure, and the weeds below this plant look healthy, but it's worth asking.
Miri
These are interesting symptoms, and we have a few theories which more information might support or disprove. We agree this is not likely powdery mildew.
The fungus causing Verticillium Wilt, which Phlox (and many other perennials) is susceptible to, can create leaf discoloration like this without necessarily progressing to actual wilting. It would be difficult to determine this is the culprit, though, since the fungal growth remains hidden inside plant tissue and does not create obvious external discoloration in the roots the way root-rot infections can. It might stain the tissues in the center of the affected stem, if you were to trim it off and split it apart.
We notice what looks like some wire caging/fencing around the base of the Phlox clump. Has this been in place for some time, and is the metal rusting or degrading at all, especially below the soil line? This hypothesis may be more of a stretch, but nutrient toxicity such as from certain metal coatings (like zinc) in this case might be leaching into the soil in this spot and creating issues with nutrient absorption by the Phlox roots. Even though no nearby plants look affected (the Butterfly Weed looks yellower than usual, but maybe that's the photo exposure), perhaps Phlox is more sensitive to these levels than other species. A laboratory soil test of the soil within the plant's root zone might reveal abnormal nutrient levels.
How long as the decorative statue been in that location? Was it recently cleaned to remove algae or other residues, where runoff might have seeped into the Phlox root zone? If so, what cleaner was used?
Was any herbicide used near the Phlox? Symptoms are usually a bit different or more drastic with herbicide exposure, and the weeds below this plant look healthy, but it's worth asking.
Miri
Miri, Thanks for the suggestions, but the wire cage has only been there for several weeks because the height of the phlox was causing the stems to bend. The statue goes in usually later in the spring once the plants get some height and I like to see the moss or algae on the statuary. No herbicides have been used. I read your article in the paper about Southern Blight. Might this be an issue? I have eight phlox plants around the garden and so far only the phlox have been infected, and now the foliage is dead or nearly dead. Thanks, Ed Gottleib
Hi Ed,
Southern Blight generally has fairly easy-to-spot white fungal growth on bases of dead/dying stems and mustard seed-like spore capsules on the soil surface, so should be fairly straightforward to confirm if it's the culprit if the disease has progressed to the point of plant collapse. Otherwise, it's hard to say what the cause was once the symptoms have advanced as far as leaf loss/death and plant failure. Since it's hard to tell at that point if the roots were affected as well, you could just remove all the above-ground growth and see it if resprouts next year. Or, you could dig up one plant to check the roots; those dead from root rot tend to be mushy and their stringy inner core will easily separate from the outer sheath of the root. (Roots dead from other causes tend to just snap cleanly in two, and take more effort to break.) Live roots won't necessarily be white or pale-colored, but any that are darker brown or black are highly suspect as dead due to soil over-saturation or infection by root-rotting pathogens.
We have found that high mite infestations on Phlox have caused leaf yellowing and eventual browning and desiccation, even if the plant doesn't have any obvious webbing around the leaf bases. While this didn't look like mite damage in the initial images, which is why we didn't mention it, but it's worth investigating. You may need a magnifying glass to see the mites, though, since their tiny size and pale or rusty color will be hard to see on yellow or tan leaves. If mites were the culprit and no other issues were co-occurring, we expect the plants will recover and sprout normally next year.
Our only other suspect for the time being is a nutrient abnormality, perhaps from any compost or mulch that was applied fresh this year that had an odd pH or makeup, or perhaps from an accidental over-fertilization that is interfering with normal nutrient absorption. Usually, however, over-fertilization results in "burnt" foliage from the accompanying moisture stress, and a nutrient deficiency in the soil would be unlikely to affect only the Phlox, particularly in several different planting sites. If the Phlox around the garden are all the same cultivar, perhaps that implies they are exhibiting their shared vulnerability to the same pathogen. Feel free to send us additional, closer images of the foliage if any is not completely brown yet, though we might not be able to narrow-down potential culprits much more than this.
Miri
Southern Blight generally has fairly easy-to-spot white fungal growth on bases of dead/dying stems and mustard seed-like spore capsules on the soil surface, so should be fairly straightforward to confirm if it's the culprit if the disease has progressed to the point of plant collapse. Otherwise, it's hard to say what the cause was once the symptoms have advanced as far as leaf loss/death and plant failure. Since it's hard to tell at that point if the roots were affected as well, you could just remove all the above-ground growth and see it if resprouts next year. Or, you could dig up one plant to check the roots; those dead from root rot tend to be mushy and their stringy inner core will easily separate from the outer sheath of the root. (Roots dead from other causes tend to just snap cleanly in two, and take more effort to break.) Live roots won't necessarily be white or pale-colored, but any that are darker brown or black are highly suspect as dead due to soil over-saturation or infection by root-rotting pathogens.
We have found that high mite infestations on Phlox have caused leaf yellowing and eventual browning and desiccation, even if the plant doesn't have any obvious webbing around the leaf bases. While this didn't look like mite damage in the initial images, which is why we didn't mention it, but it's worth investigating. You may need a magnifying glass to see the mites, though, since their tiny size and pale or rusty color will be hard to see on yellow or tan leaves. If mites were the culprit and no other issues were co-occurring, we expect the plants will recover and sprout normally next year.
Our only other suspect for the time being is a nutrient abnormality, perhaps from any compost or mulch that was applied fresh this year that had an odd pH or makeup, or perhaps from an accidental over-fertilization that is interfering with normal nutrient absorption. Usually, however, over-fertilization results in "burnt" foliage from the accompanying moisture stress, and a nutrient deficiency in the soil would be unlikely to affect only the Phlox, particularly in several different planting sites. If the Phlox around the garden are all the same cultivar, perhaps that implies they are exhibiting their shared vulnerability to the same pathogen. Feel free to send us additional, closer images of the foliage if any is not completely brown yet, though we might not be able to narrow-down potential culprits much more than this.
Miri
