Plant Disease

Question: Hi,

I'm trying to identify the cause of the following conditions (see photos), affecting boxwood and azaleas.   I have additional photos.  Treatment of fungicide has not helped.  Condition has worsened over the last few months.  Branches within the bush become brittle and break off.  

Thank you
Kyle

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 June 08, 2023, 2:55 PM EDT
response: Hello Kyle,  The shrubs in the photos aren't boxwood but instead a similar-looking evergreen called Japanese Holly. (These and a few other holly types do not have the leaf spines familiar to most hollies.) We mention this distinction as it is important to distinguish which pest or disease issues are more likely. With Japanese Holly, the most common ailment in our experience, as they are not pest-prone, is a fungal root-rot type of infection called Thielaviopsis. Conditions that increase the vulnerability of shrubs susceptible to Thielaviopsis include stress from growing conditions like soil acidity (pH) that is outside the plant's preferred range and cold soils that don't drain well. Even though we of course cannot avoid cold soil temperatures in spring and autumn, this overlapping with compacted or overly-wet root zone conditions or soils not acidic enough for holly vigor can allow an infection to set in opportunistically.  Fungicides can be broad-spectrum but rarely treat root pathogens (especially if not applied directly to soil if the product is designed to be used that way), nor can fungicides cure existing infection. Instead, they only help to protect healthy growth from becoming infected, but they don't always work effectively, even when applied as directed and with the proper ingredient selected among the various fungicide products. Although the linked publication about Thielaviopsis does mention that some fungicide drenches (the term used for soil-applied treatments) can help suppress infection, they are not practical to use as the conditions creating the vulnerability in the plants in the first place must be addressed to prevent recurring issues with the disease, and yearly application(s) of the fungicide would be costly and not guaranteed to work.  Both hollies and azaleas thrive in acidic soil pH levels (roughly 5.0 would suit them both), so perhaps a laboratory soil test in the area they are growing (if they are sharing a planting bed) may reveal a pH level needing adjustment. Such tests don't screen for pathogens but may help inform future fertilization efforts (not that they are typically needed for shrubs) as well as how much sulfur to add, if needed, to lower pH. As an optional test add-on, the lab might also be able to sample for salt contamination (sodium levels) in case saltwater intrusion or prior storm surges have increased the sodium levels in the soil to levels some plants cannot tolerate. Japanese Holly is reportedly salt-tolerant but we do not know to what extent, and a tolerance still doesn't mean the condition won't stress the plant to the point of making it more vulnerable to other issues.  The leafless holly branches will not regrow, so you can trim them off. Sometimes the progression of root rot is unstoppable but might take more months or even a year or longer to cause enough damage to make the plant succumb fully, so it's a matter of wait-and-see at this point. It's possible the damage was caused by something else like a stem breakage or root drought stress, or the root infection was minor and short-lived, so the plant might not decline further and isn't necessarily doomed.  Check to make sure any nearby roof downspout outlets, if applicable, empty away from the planting bed so roots are not inundated during heavy rain with too much water. Although your county is not in the current state of drought most of the state is experiencing, it still might need some supplemental watering. Feel the soil about six inches deep next to the root zone and water well if it's becoming somewhat dry to the touch at that depth. (If damp when probed, it's probably fine without intervention.)  Feel free to send us images of the Azalea's symptoms, as you did mention something is affecting them as well but we can't see them in the photos. Most of the above information will apply to them as well, from how to monitor them for watering needs, soil acidity preferences, possible salt intolerance, and the fact that treatment with a fungicide is likely impractical and ineffective. While Japanese Hollies thrive in full sun but tolerate a fair amount of shade, Azaleas grow best in a mix of sun and shade where they receive some dappled light or indirect light only during the hottest part of the day; they are woodland plants by nature, accustomed to receiving light moving through the tree canopy and not saturating their leaves for long periods. If not in soil that's acidic enough, or if repeatedly drought-stressed or over-watered, they too can have branches die back or foliage that becomes yellower overall.  Miri

responder: The Question Asker Replied June 08, 2023, 8:14 PM EDT
response: Miri,Thank you for the background and detailed explanation of the potential causes affecting the Japanese hollies.  I’ve added several images below of the azalea, which are adjacent to the Japanese holly.  The azaleas actually have more damage and the issue started much sooner (prior to the Japanese holly). I've also attached an image of an emerald green(?) which is experiencing symptoms (3 or 4 have this condition along the property), and are not in close proximity to the azalea or Japanese hollies.  Could these issues all be related?  Thank you!Kyle On Jun 8, 2023, at 2:55 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 June 09, 2023, 1:50 PM EDT
response: Hi Kyle,  Thank you for the additional photos. Since Azaleas are sensitive to more conditions than Japanese Holly (drought, wet soil, full sun) because they have different preferences for where they're grown, they probably became stressed first or more severely, hence the branch dieback as the roots struggled. Here too, just trim off the dead branches and monitor their root zone moisture for watering needs. If that nearby roof downspout outlet that we see in one of the photos empties into that bed, see if you can extend it further away so excess rainwater (when we get regular rains again someday) doesn't saturate the plant roots. It's unlikely a pest or disease (aside from some root rot, possibly) is behind the branch dieback on the Azaleas.  It's difficult to determine what is bothering the Arborvitae, though they too are touchy about drought and over-watering, or soils which don't drain well enough. It's normal for these and many other evergreens to shed some of their oldest foliage (in this case, it will dry and turn brown and then fall off) in the interior of the plant where the leaves are oldest and most shaded, but when leaf loss extends to the branch tips, something else is the culprit. Do deer visit the yard? We are assuming not since they also willingly eat Roses and Azaleas, but wanted to check since they do consume this type of Arborvitae and will eventually strip the lower portions bare of foliage, where it will not regrow. Otherwise, for now, just trim this dieback off too (any stems fully brown, not those with brown interiors but green tips) and monitor the plant for watering needs. Sometimes wood-boring insects or a fungal pathogen can infest/infect the main stem, but this is less likely here as more drastic and widespread dieback would be associated with that event. If the plant's root ball wasn't loosened enough or any burlap removed upon planting, that could also explain why it may be struggling to establish, even if planting was a few years ago. No treatment is recommended for now, but feel free to send us photos of new or worsening symptoms if they appear.  Miri