By Keith Mickler, County Coordinator and Agriculture Agent
Man oh man, do we have a beaucoup of problems going on with our ever-charming hoity-toity boxwoods this spring. Boxwoods are by far the majority of plants that I have seen this spring with problems. Just this past week, I had four people stop by my office with boxwood problems.
Buxus is the genus for boxwood and most boxwoods are struggling just to exist. Even though boxwoods are adapted to a wide range of light requirements one thing they are not adapted to is waterlogged, sopping wet, mucky soil. Mucky soil is exactly what we had here in Rome most all of fall, winter and into early spring; thus boxwood problems abound with vengeance. Boxwoods prefer a fertile, well-drained soil in order to look their best.
Mother Nature dumped almost twice as much rainfall as normal (October through March) onto our beloved landscapes, thus our soil became a waterlogged, sopping wet, mucky mess that even the toughest of boxwoods struggle to exist.
My condolences to all if you were once the proud parent of a prized, award winning display of boxwood majesty, you just might be singing the Buxus Blues.
Boxwood problems associate with plant pathogens
Decline (a catch all of problems): Boxwood decline is a poorly understood complex involving the fungi Paecilomyces, Volutella, Macrophoma and Phytophthora, as well as cold injury, drought stress (brought on by root rot), and nematodes (microscopic round worms). This phenomenon is also closely related to cultural problems associated with boxwoods, such as improper pH and nutritional status, improper irrigation, poor drainage, and improper mulch management.
Root rots by Phytophthora are more of a problem in wet soils.
Macrophoma can cause leaf blight, but usually acts as a weak pathogen following root diseases or environmental stresses.
Volutella may cause a dieback or stem blight and often follows winter injury.
Symptoms consist of weak and spindly plants, along with dead or dying branches occurring randomly on the boxwood. Older leaves drop prematurely with the remaining foliage turning yellow in color. Leaves and stems often have pink eruptions of spores on black fruiting bodies. Dead areas or cankers develop along branches and/or near the crown.
Prevention & Treatment: A thorough diagnosis is important before taking corrective action. This should include a nematode analysis, soil analysis, and evaluation of drainage in the area and the degree of plant rooting in litter under the plant. Samples for root disease should be submitted to the University of Georgia Plant Disease Diagnostic Clinic (through the county extension office) for analysis.
Crowded growth and dead leaves in the branch crotches maintain high levels of humidity in the canopy, making for conditions favorable to dieback diseases. Prune dead stems back to healthy tissue. You must disinfect your pruning shears frequently using household bleach diluted 1:9 with water or use rubbing (isopropyl) alcohol for 10 seconds. Removing dead branches and leaves from the crotches of the boxwood, and yearly renewal of mulch, will help in control.
Proper cultural practices, such as providing water when necessary, avoiding over-watering over fertilizing, and thinning shrubs for better air circulation are essential in maintaining a vigorous and healthy boxwood.
Canker/Stem Blight: caused by the fungus Volutella buxi. The first noticeable symptom is certain branches or plants in a group do not show new growth as early in the spring as do others, nor is the new growth as vigorous as that on healthy specimens. The leaves turn from green, to a light green, to various shades of tan. Infected leaves turn up and lie close to the stem instead of spreading out like leaves found on healthy stems. Diseased leaves and branches will have small, rose/pink colored, waxy fruiting bodies containing the fungus. The bark at the base of an infected branch is loose and will peel off easily revealing a gray to black discolored wood beneath.
Prevention & Treatment: Remove all dead branches as soon as they are noticeable. Annual removal and destruction of all leaves that have lodged in crotches is recommended. The use of copper-based fungicides have shown to be very effective in preventing canker. Make the first application right after all the dead leaves and dying branches have been removed but before new growth begins.
Boxwood Blight: Boxwood blight has been detected in Georgia and in Floyd County. Boxwood blight is caused by the fungal pathogen Calonectria pseudonaviculata which causes leaf spots, stem cankers, defoliation, and death of boxwoods.
There are no known resistant boxwoods; a few have been tested and found to be more tolerant than others. B. sinica var. insularis ‘Nana’ and B. microphylla var. japonica ‘Green Beauty’ are worth a try.
As boxwood blight spreads additional leaf spots form and combine until the all leaf surfaces are infected, and the blighted leaves drop from the plant. Infected stems have dark brown to black lesions or cankers on them. Once the cankers encircle a stem, water supply is cut off from that point outward, and the stem dies.
Boxwood Blight can survive for at least five years on blighted and fallen foliage, as well as on stem lesions found on dying or dead plants. Rainfall and overhead irrigation can splash the spores to adjacent plants thus infecting them.
Prevention & Treatment: Boxwood blight may be confused with other boxwood diseases, such as boxwood decline or other stem blight diseases. If boxwood blight is suspected, have the disease identified. Samples should be submitted to the University of Georgia Plant Disease Clinic (through your local county extension office) for analysis.
Prune diseased stems, rake up fallen foliage, and dispose of both. Apply fresh mulch beneath the plants to reduce the chances of reinfection from spores that could splash from the soil onto foliage. The most effective fungicides for the control of boxwood blight are chlorothalonil or chlorothalonil mixed with thiophanate methyl.
There are no quick cures for the many issues facing our boxwoods. You can be part of boxwood preservation by looking often for the problems describe above. Secondly, make sure you have healthy well drained, fertile soil, and if you find a problem attack it now not later.
The current state of boxwood existence can be a tough and painful event for the ever loving owner. Some boxwoods will not make it, some will struggle for a while, and others may go on to regain their composure and once again be majestic specimens.
Lastly, you will need be patient for this could be a long process; plants and soils are living organisms and often have an agenda of their own.
Keith Mickler is the County Coordinator and agriculture agent for The University of Georgia/Floyd County Cooperative Extension. Located at 12 E. Fourth Ave., Rome, GA 30161 (706) 295-6210. Office hours are Monday-Friday from 8 a.m. to 5 p.m.