Wilting & Discolored Phlox ‘Jeana’

Question: Hi there, I planted two phlox ‘Jeana’ this past spring and unfortunately dealt with some spider mites shortly thereafter. I treated with neem oil and the plants seem to be cleared of that problem, however, the leaves have now been discolored for a few weeks. I think the discoloration started due to damage from the mites, but now they appear to be showing a nutrient deficiency. I fed them with a light organic liquid fertilizer on 7/12 and they improved a little. The stems were a bit floppy so I stuck some plant supports into the soil to prop up the stems on Sunday, but as of yesterday two stems on one plant are wilting heavily. Could I have damaged the roots when inserting the stakes, or is this yet another disease/nutrient issue? These plants receive about 7 hours of sun throughout the day and I water every 2-3 days, or every day if it’s quite hot out. Thank you! Danielle

responder: The Question Asker Replied July 18, 2023, 9:25 AM EDT
response: I forgot to mention- I haven’t seen any insects other than the original spider mites and some aphids, bees, etc. I read online about phlox bugs so I’ve been checking everyday but I haven’t found any yet. Thanks again, Danielle

responder: Answered by the Home and Garden Information Center (HGIC), University of Maryland Extension. For timely gardening information and tips, subscribe to our Maryland Grows blog and quarterly newsletter. Follow us on Facebook, Instagram, and YouTube. Replied July 18, 2023, 2:16 PM EDT
response: Hello Danielle,  It's difficult to determine exactly what the cause is, though we suspect something abiotic (that is, not an insect or disease), though more than one factor could be overlapping. You mention watering every couple or so days; is the soil checked for moisture level before watering happens? This sounds like a frequent schedule, but with a plant still establishing, might be needed. The soil should feel like it's becoming somewhat dry to the touch a few inches deep before it should need another soaking.  We ask since chlorosis, the type of foliage nutrient deficiency symptoms shown here (greener veins as the leaf yellows), can arise from declining roots due to overly-wet conditions causing rot or root stress. It's not often the case that the soil itself is markedly deficient in certain nutrients, though we can't rule it out without the results of a laboratory soil test. (Usually not needed for perennial gardens, but harmless to do if you're curious.) Fertilizers might add to the stress on roots dealing with rot or surplus water, but time will tell as there is nothing that can be done to cure roots afflicted with rot. Additionally, some nutrient deficiencies won't resolve in older foliage even if fertilization is beneficial, because those newly-absorbed nutrients aren't able to move into old leaves, only new growth.  We do think we see some spider mite damage, but that might be older injury that has since been treated by your neem applications. Phlox Bugs can create heavy damage when abundant but we don't see evidence of the insects or their feeding in the photos. Root physical damage from inserting stakes is always possible but probably not drastic enough to cause notable symptoms.  Floppy growth can result from insufficient light (not likely here given the 7 hours of sun), too much moisture, or too rich of a soil (over-fertilization or lots of added organic matter like compost). Taller perennials tend to need support from stakes or similarly-tall perennials planted alongside them, as they would in nature receive support from nearby plant stems acting like natural stakes to lean on.  For now, just monitor the plants for new or worsening symptoms, in which case feel free to send us more photos. Decrease the watering frequency if the soil isn't drying out as rapidly as every couple of days. Phlox can contract the virus-like disease Aster Yellows, but we don't clearly see indications this is present here, though also can't rule it out yet either since symptoms can overlap with other ailments. Prune out wilted stems that don't recover within about a day after being watered (or if the soil is already damp when they wilt) since this suggests they are infected with a root, stem, or crown rot or a stem-boring insect that cannot be treated. Southern Blight is one common fungal disease that takes advantage of heat, high humidity, and over-mulching that reduces air circulation at the base of the plant (if the mulch touches or covers the base of the main stems); stem wilt is a classic symptom, though not exclusive to that pathogen.  Miri