Fungal disease?

Question: Hello Mira or whoever else gets this question.  I am one of your MOCO Ask Extension colleagues, and my Master Gardener colleagues could use your help in determining what is affecting some of our plants.

I help manage six cutting garden beds in the Demonstration Garden in Derwood.  Earlier this season, we observed that small portions of the conoclinium coelestinum were turning black and dying, but the rest of the plants were, and remained, healthy.  We cut down the affected stalks to the ground, but that was all.  Occasionally over the summer the condition recurred, but was confined to the conoclinium.

Today, however, I observed that the condition has spread to other beds in the garden.  A large stand of phlox Jena was completely destroyed, and a clump of phlox David next to it also was affected (fortunately, the rest of the David was unaffected and continues to flower nicely).  I also found a good amount of the condition in leucanthemum 'Becky'.  I should note that these affected beds are not next to each other and in fact are a good 30 to 50 feet apart.  Other plants in beds in between the affected areas  are fine and disease free.

I asked other MGs what they thought might be happening, but no one knew for sure.  We all agreed it looked like a disease (no sign of insect infestation), perhaps fungal given that the bottoms of the affected stalks turn black and moist, like they're rotting.  The attached photos will give you an idea.  (I have more photos, but the system will only allow me to attach these three.)  If it's a fungus, perhaps it's been exacerbated by the alternating dry and wet weather we've had.  I've been working in these beds for many years, and I have never encountered this before.

What do you think is causing this?  Is there a treatment?  As mentioned, we've confined ourselves to cutting down the affected stalks to the ground--since these are perennials we've avoided pulling them out by their roots.

Your help will be greatly appreciated!

Bob Loesche

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 26, 2023, 3:46 PM EDT
response: Hi Bob,  Our plant pathologist had a look and agrees that cutting back the foliage is helpful and also recommended digging one or a little section of one - maybe the part of the plant where the rest looks healthy still ( try to take a spade and cut that wedge of the plant and roots out) so you can examine it for Southern Blight nodules on the roots.  It could also be a wilt disease like Verticillium which given the weather patterns like you mentioned could have stressed the plants and caused the infection and in that case they would have to be removed. Did you notice any reddish center or streaking in the cut stems? Maybe you could cut some more to look.  Garden phlox unfortunately get so many ailments so it is difficult to say for sure when they are this far gone.  Examine some of the roots and see if you notice anything and feel free to send more photos.  Emily

responder: The Question Asker Replied July 26, 2023, 4:45 PM EDT
response: Emily, thanks so much for your prompt response.  I will look more closely at the plants as you suggest.  Unfortunately my colleagues and II won't be back in Derwood until next Tuesday, but I'll keep you posted as to what we find. Thanks again,BobOn Wed, Jul 26, 2023 at 3:46 PM Ask Extension <<personal data hidden>> wrote:

responder: The Question Asker Replied August 03, 2023, 9:03 AM EDT
response: Hi Emily.  Yesterday, I had another Master Gardener take some photos of the roots and leaves of our diseased plants, attached.  Do these in any way help you diagnose the problem?    Fortunately we didn't have any major spread of the disease over the last week, although there were a few spots where there was new die-back. Thanks,Bob

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 August 04, 2023, 5:08 PM EDT
response: Hi Bob,  The white fungal threads are probably mycorrhizal in nature or are decaying the mulch itself; they don't appear to be tied to disease and we do not see Southern Blight spores in the images. Some other form of crown rot is still possible, and we also suspect root rot may be involved, given that several roots pictured have the cortex showing (the stringy inner core) as they have lost their outer layer, a typical symptom associated with root rot diseases.  The leaf spot might be Cercospora (the symptoms of several pathogens can look alike) but this would not cause rampant plant dieback, only cosmetic damage to foliage that does not need treatment. Removing the infected shed foliage in autumn and discarding, not composting it, may reduce re-infection odds next year, though it's not a foolproof approach.  Miri

responder: The Question Asker Replied August 04, 2023, 8:11 PM EDT
response: Miri, thanks so much for this info.   If the problem is indeed some form of root rot, can that be caused by overwatering?  All the beds in the Demo Garden are watered by a drip line system, but it's not a precise system and I wouldn't be surprised if there are spots where too much water is applied.  Having said that, we didn't immediately observe that the areas with plant die-back were unduly wet.  We weren't looking for that, but if there were really wet spots chances are we would have noticed.  In any event, I thought I'd ask in case overwatering is something we should look out for.Beyond checking watering, is there any other response we can take besides promptly discarding the affected plants?BobOn Fri, Aug 4, 2023 at 5:08 PM Ask Extension <<personal data hidden>> wrote:

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 August 07, 2023, 11:53 AM EDT
response: Hi Bob,  It could be from overwatering for sure. It could also be that something else affected the roots and then caused a root rot. It is difficult to pinpoint but in our opinion doesn't warrant a fungal treatment especially in the demonstration garden. Fungicides are difficult to use since reapplication is needed to keep the fungal growth at bay - usually every 7-14 days, so they aren't curative and long term use is often more harmful to the beneficial soil microbes and fungi than to helping the plants.  Yes dig up the plant that seem to have been affected, and maybe try another variety of phlox or a plant that seems to tolerate the wetter soils more? Veronicastrum, Monarda, Aesclepias incarnata, Eupatorium (https://plants.ces.ncsu.edu/plants/eutrochium-maculatum/) now Eutrochium  are some ideas that may fair better.  Emily

responder: The Question Asker Replied August 07, 2023, 5:34 PM EDT
response: Thanks for this response Emily.  I agree that applying fungicide is not a good idea in the Demo Garden.  We'll double check the level of watering in the troubled spots and remove affected plants as needed.  Hopefully we can get the watering under control so that we don't have to look for plants that work in wet soil BobOn Mon, Aug 7, 2023 at 11:53 AM Ask Extension <<personal data hidden>> wrote: