A case of the boxwood blues Fungal disease damaging plants

This boxwood shows damage from Colletotrichum theobromicola, or “Colletotrichum dieback,” as the disease is now commonly called. It had only been reported in Louisiana, North Carolina, Alabama and a few other states.

Boxwoods form the evergreen backbone of historic and traditional garden designs in the United States and abroad.

These favored shrubs tolerate the severe pruning that is necessary to create crisp English gardens. Their slow growth habit makes them perfect for creating French parterre gardens, designed for viewing from upper story windows and balconies.

While garden designers and landscape architects find a myriad of uses for their formal shapes, home gardeners and even professional landscapers may have to manage diseases and insects on boxwoods planted in less-than-ideal spots.

(See Clemson Home & Garden Factsheet #2052 “Boxwood Diseases and Insect Pests” at http://bit.ly/1ASxup6)

Because boxwoods have been cultivated for centuries, it might seem like boxwood diseases and insects are easy to recognize, but a new boxwood disease was discovered for the first time in Charleston late last year.

A colleague, who is a landscape professional, brought a boxwood sample into the office for help identifying an alarming issue in a client’s yard.

We were both perplexed by the random pattern of dead limbs scattered throughout the boxwoods in the landscape.

With more than 200 Korean boxwoods (Buxus sinica var. insularis ‘Wintergreen’) at the site, it was extremely important to discover the reason the plants were dying and develop a plan to save the rest.

The dead foliage on the affected limbs was straw-colored, with no other visible symptoms. There were lesions on the bark, which peeled away easily.

After examining the roots, which were healthy and white, we dismissed the possibility of Phytophthora root rot, which is a common issue with boxwoods in areas with poor drainage.

Stumped, we decided to submit the sample to the Clemson University Plant Problem Clinic, where plant pathologists use laboratory techniques to isolate and identify disease-causing organisms.

Armed with a proper identification, recommendations for treatment can then be made.

We were shocked when we received the final report, stating that a fungal disease that had never before been found in South Carolina was found on the branch of our specimen.

Prior to submitting this sample, “Colletotrichum dieback,” as the disease is now commonly called, had only been reported in Louisiana, North Carolina, Alabama and a few other states.

According to Dr. Raj Singh, a plant pathologist at the Louisiana State University Plant Diagnostic Center, a fungus known as Colletotrichum theobromicola causes the dieback disease, which at first glance may appear to be common Phytophthora root rot.

Just like our specimen, the roots and crown of the plant appear healthy, but when the bark is peeled back, the wood underneath is a distinctive black color.

Gardeners can control this and other fungal diseases using cultural practices that support healthy plants and minimize the spread of pathogens. First, remove and destroy affected branches and twigs, and completely remove plants that are severely affected.

This local landscape professional was forced to remove several plants that were beyond saving, and pruned others to remove the diseased areas.

Irrigation practices can exacerbate fungal disease problems. Water only when absolutely necessary to reduce the amount of moisture on the foliage. If you must irrigate, do so early in the day to allow plants plenty of time to dry out.

Avoid overhead irrigation that splashes water on leaf surfaces. Drip irrigation is preferred, but in closely spaced plantings like boxwoods, even drip irrigation increases the humidity within the plant canopy, creating the perfect environment for fungal plant diseases to flourish.

Increase air circulation by pruning and spacing plants further apart to help reduce humidity.

Finally, plants that are over-fertilized are more susceptible to disease and insect damage. Apply fertilizer only as needed based on soil test results. In general, boxwoods need little fertilizer.

After applying recommended fungicides, such as chlorothalonil, mancozeb and thiophanate-methyl at the proper rates, the landscaper was able to gain control over the disease affecting his client’s boxwood-laden landscape.

He noted that regular maintenance will be the key to suppressing this issue in the future.

Join Clemson Extension and the Tri County Master Gardeners for the 2015 Carolina Yards Gardening School-Spring Edition at 8:30 a.m.-12:30 p.m. March 14 at Trident Technical College.The Carolina Yards Gardening School is designed to help home gardeners create low-maintenance, beautiful landscapes that work with nature and protect natural resources.

The school features classes on healthy soils, soil amendments, mulches, hardscapes and landscape design. Participants will receive a complimentary soil test, discount shopping at local garden centers and nurseries, plus chances to win garden-inspired door prizes.

Deadline for regular registration, which is $55, is March 1. Late registration will be taken until March 13 and the cost rises to $70. To register https://www.regonline.com/CYGS or email adabbs@clemson.edu

The Lowcountry Chapter of the S.C. Native Plant Society, in partnership with Charles Towne Landing, will hold a Native Plant Sale from 9 a.m.-noon March 14 in the parking lot of Charles Towne Landing, 1500 Old Towne Road, Charleston.

The sale will include perennials, shrubs, trees, ferns and native grasses. Admission to the plant sale is free. If exploring Charles Towne Landing, pay entrance fee at the visitor center. Learn more at www.scnps.org.

The Tri County Master Gardeners will be accepting soil samples and hosting “Ask a Master Gardener” clinics from 10 a.m.-1 p.m. March 6, 7, 13 and 14 at Ace Hardware on Trolley Road in Summerville and 10 a.m.-1 p.m. March 20, 21, 27 and 28 at Flowertown Garden Center, 410 E 5th North St. in Summerville.