What is wrong with these shrubs? - Ask Extension
Hello, I have several shrubs, the same type that are almost dead and another one that is starting to die. Is this a disease or can anything be done to...
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What is wrong with these shrubs? #872741
Asked June 12, 2024, 8:20 AM EDT
Hello, I have several shrubs, the same type that are almost dead and another one that is starting to die. Is this a disease or can anything be done to help the remaining live shrubs that are in my garden bed?
Prince George's County Maryland
Expert Response
I have sprayed some Roundup in that bed and I initially thought that is what killed them.
Although an herbicide like Roundup can cause plant damage if it accidentally contacts the foliage of desirable plants, in this case, it is almost certainly not responsible for the type of dieback pictured. The shrubs look like a variety of Japanese Holly, which are widely grown, and the likely cause of dieback is also unfortunately fairly common. We think this is Black Root Rot, also called Thielaviopsis, and it's a widespread fungus that exists in many soils but which does not cause problems for healthy plants, instead taking advantage of root stress from conditions like over-watering or soil which is not acidic enough for the preferences of holly. We see these kinds of patchy branch dieback symptoms on Japanese Holly often, and this disease is the top suspect, in part because this type of holly happens to be among the more vulnerable to that infection.
Unfortunately there is no treatment in terms of curing existing damage or reversing dieback. Some fungicides are labeled for these kinds of root diseases, but they are not practical to use since they don't always eliminate the pathogen, cannot cure a plant, and don't address the core issue, which is stress making the plant vulnerable in the first place. If you want to see if the shrubs stabilize and recover, trim off all brown branches and carefully monitor the watering needs so they are not getting too wet from either irrigation or a nearby water source like a roof downspout outlet or leaky hose spigot. (The shrub with half or more of the branches brown is probably too weakened to try rehabilitating, but the other two with more limited dieback are in better shape.)
On a removed plant, indications of root rot tend to include either blackened roots (the namesake of this particular pathogen) or mushy roots that pull part easily, leaving behind a stringy inner core when tugged. Roots dead from other causes (or which are still alive) take more force to break apart, and tend to snap in half more cleanly.
If you want to investigate whether the soil is not acidic enough, you could have a laboratory soil test performed for that planting area. (They are more accurate than at-home test kits or soil probes.) These labs can't test for disease organisms, but the results of pH and nutrient levels can still be valuable. We can help to interpret the test results if desired.
Miri
Unfortunately there is no treatment in terms of curing existing damage or reversing dieback. Some fungicides are labeled for these kinds of root diseases, but they are not practical to use since they don't always eliminate the pathogen, cannot cure a plant, and don't address the core issue, which is stress making the plant vulnerable in the first place. If you want to see if the shrubs stabilize and recover, trim off all brown branches and carefully monitor the watering needs so they are not getting too wet from either irrigation or a nearby water source like a roof downspout outlet or leaky hose spigot. (The shrub with half or more of the branches brown is probably too weakened to try rehabilitating, but the other two with more limited dieback are in better shape.)
On a removed plant, indications of root rot tend to include either blackened roots (the namesake of this particular pathogen) or mushy roots that pull part easily, leaving behind a stringy inner core when tugged. Roots dead from other causes (or which are still alive) take more force to break apart, and tend to snap in half more cleanly.
If you want to investigate whether the soil is not acidic enough, you could have a laboratory soil test performed for that planting area. (They are more accurate than at-home test kits or soil probes.) These labs can't test for disease organisms, but the results of pH and nutrient levels can still be valuable. We can help to interpret the test results if desired.
Miri
