Boxwoods, Boxwoods, Boxwoods, and not a Crop to Clip

—by Cristi Palmer, IR-4 Ornamental Horticulture Program Manager

Photo by Cristi Palmer

At least that is the way it seemed at the second Boxwood Summit held at the National Agriculture Library in Beltsville, MD. This gathering of boxwood researchers, extension personnel and boxwood aficionados offered an opportunity to present the latest and greatest research results and discuss the next important research directions to preserve boxwoods against leaf miners, Volutella and, of course, the devastating boxwood blight.

Welcomed by Dr. Joseph Spence, the USDA-ARS Area Director for Beltsville highlighted historical happenings and introduced us to some of the interesting sites in the area.

The stage was set with Bennett Saunders showcasing his family’s operation (Saunders Brothers, Inc.) which is about to celebrate 100 years in 2015. Over the years Saunders Brothers has grown many different crops including apples, peaches, annual vegetables and boxwoods. The secret to their success is diversity and flexibility, and a strong respect for family and community ties, as evidenced by Bennett’s slides peppered with family members through the ups and downs of growing crops for so many years.

Boxwood blight has shifted the Saunders Brothers operation again. While freely welcoming visitors in the past to browse their fields, now visitors must wear Tyvek suits and booties to prevent any accidental introduction of this devastating disease.

While boxwood blight dominated the conversations throughout the day, Richard Olsen (US National Arboretum) discussed the various breeding efforts for various traits including leafminer resistance. Mike Raupp (University of Maryland) presented biological characteristics of and management options for boxwood leafminer, and Beatrice Henricot (Royal Horticulture Society) conveyed information about Volutella blight, boxwood rust, Phytophthora, and the boxwood tree moth, an insect so far confined to portions of Europe.

Then, the focus became the research efforts on boxwood blight. Kurt Heungens (Institute for Agriculture and Fisheries Research) presented some of the key epidemiological findings from his laboratory including that clothing and utensils can carry the conidia and cause infections and that there are two distinct genetic types named G1 and G2, with G2 having been introduced later into Europe. Building on Heungens work, JoAnne Crouch (USDA-ARS) presented evidence that the two different genotypes may be two different species.

Photo by Gail Wisler

Len Coop (Oregon State University) demonstrated the power of using an epidemiological model to predict whether growers may need to consider preventative fungicide applications based on the length of time for adequate moisture and temperatures needed for conidia germination. Studying the laboratory diagnosis of boxwood blight using HIS3 and ITS PCR, Bob Marra (Connecticut Agricultural Experiment Station) examined early infection and determined the fungus can be detected as early as one day after inoculation, prior to development of visual symptoms, and in water and soil samples.

Norm Dart (Virginia Department of Agriculture and Consumer Services) presented part of his epidemiological research on conidia and microsclerotia. Conidia are usually short-lived surviving about one week in soil at 300% field capacity, while microsclerotia under the same soil conditions survived longer than 40 weeks. There are approximately 3,250 microsclerotia per boxwood leaf. If a boxwood plant has roughly 1,000 leaves, and that amount is typical of a small 2 – 3 foot shrub, an infected boxwood plant that has dropped its leaves has contributed more than 3 million microslerotia to about 9 sq ft of soil surface.

Complementing Dart’s studies, Nina Shishkoff (USDA-ARS) examined whether infected leaves held at different temperatures with either dry or wet moisture will sporulate. Conidia formed on leaves that were wet from 0 to 20C while fewer conidia formed on all the dry leaves and on wet leaves at -10 and 30C . Microslerotia survived at both moisture levels and up 20C.

Sanitizing agents were effective at killing conidia (Sharon Douglas, Connecticut Agricultural Experiment Station). Label rates of bleach, Lysol, ZeroTol, OxiDate, and X3 were effective after 5 minutes. For microslerotia (Shishkoff), ethanol and ZeroTol were effective at 15 minutes.

Kelly Ivors (California Polytechnic Institute) ended formal presentations with slides ranging from showing symptoms in situ in 110 year old plantings to potential animal and bird spore vectors to the diversity of host resistance and fungicide management options.

We then moved into breakout sessions where we discussed research gaps for management of pests and pathogens, breeding for boxwood tolerance, and boxwood blight biology and epidemiology. Across all three sessions, it quickly became clear there were more questions than answers.

For the Management session, one of the gaps is understanding efficacy of tank mixing fungicides and insecticides for reducing populations of leafminer while at the same time managing boxwood blight. The other key questions are whether mulch can be used to reduce splashing of inoculum from the soil to the lower leaves and whether altering pruning practices to have more open canopies can reduce disease development and spread.

For the Breeding session, there were a number of research gaps identified including clarification of techniques to assess host resistance or tolerance and even what those terms mean for the boxwood/boxwood blight system. There is also the need to establish test plots in locations where boxwood blight has already been diagnosed so that new cultivars and species can be examined over several years under field level inoculum loads.

For Biology and Epidemiology, understanding survival parameters and the role of the sticky matrix surrounding the conidia are gaps in knowledge as are gathering empirical data in the US for how boxwood blight is affected by relative humidity, rain events, temperature, and pathogen movement through monitoring specific weather and disease development.

By the end of the Boxwood Summit, we became convinced that while this disease has been well studied over the last few years, more efforts are needed to protect this valuable plant for generations to come.

Boxwood Summit Organizing Committee
Jo Anne Crouch, USDA-ARS Beltsville
Margery Daughtrey, Cornell University LIHREC
Sharon Douglas, CT Ag. Exp. Station
Kelly Ivors, CalPoly
Cristi Palmer,IR-4 @ Rutgers University
Nina Shishkoff, USDA-ARS Fort Detrick     On behalf of the Boxwood Blight Working Group