Rapid Decline of Calonectria pseudonaviculata Soil Population in Selected Gardens Across the United States

Abstract

Calonectria pseudonaviculata (Cps) poses a serious threat to boxwood, an iconic landscape plant in American and European gardens. Under the mild climatic conditions of the United Kingdom, Cps remained recoverable in infected leaf debris after being left on the soil surface or buried for 5 years. The primary objective of this study was to determine how this fungus may weather the warmer summers and colder winters in the United States by sampling and baiting soil with boxwood cuttings, and by on-site testing with sentinel plants. Soil sampling started in a Virginia garden in January 2016 and was extended to California, Illinois, New York, and South Carolina in early summer of 2017 through late fall of 2018. The Cps soil population as measured by the percentage of infected bait leaves declined sharply within the first year of blighted boxwood removal and fell to an almost undetectable level at the end of this study. To validate these baiting results, the Virginia garden was tested on-site four times with container-grown boxwood plants while the South Carolina garden and three New York gardens were tested once. Each test began with sentinel plants set out for field exposure, followed by evaluation(s) on-site then in the laboratory, after plants were retrieved from these gardens and incubated under conducive environments for 2 weeks. Cps was not observed on any sentinel boxwood plant on-site or in the laboratory with one exception. These observations indicate that Cps did not survive in the United States garden soil over time as well as it did in the United Kingdom. These results have important practical implications while challenging the notion that fungi producing microsclerotia will always survive in the soil for many years.

Characterization of the Soil Bacterial Community from Selected Boxwood Gardens across the United States

“…This study characterized the bacterial communities in urban garden soils where the boxwood blight pathogen Calonectria pseudonaviculata ( Cps ) population declined rapidly [ 33 ]. We reported two important discoveries.…”

“…The bacterial orders and BCA candidates observed resemble those in the suppressive soils from other environments [ 21 , 59 , 60 , 61 , 62 ]. They are important components of a healthy soil microbiome [ 3 , 4 , 63 , 64 ], and likely have contributed to the decline of the Cps population in these garden soils [ 33 ]. These results highlight the importance of microbiome components in garden soil health and provide a new perspective from which to undertake ornamental plant disease management in gardens and other public spaces in the future.…”

“…As described previously [ 33 ], private gardens included in this study represented five geographic regions of the United States: California (San Mateo County) for Pacific West, Illinois (Cook County) for North Central, New York (Long Island) for Northeast, South Carolina (City of Florence) for Southeast, and Virginia (Powhatan County) for the Mid-Atlantic region. Five gardens, one in each state, were sampled four times, twice per year in 2017 and 2018.…”

“…Soil samples were collected in three replicates from individual gardens in early summer (ES) and late fall (LF) of 2017 and 2018 using the same protocol in a coordinated fashion [ 33 ]. Briefly, the top 6 cm of soil including limited leaf debris was taken using a soil sampler and placed in a new Ziploc ® bag (Bay City, MI, USA).…”

“…Although this work was done in the absence of boxwood disease, it demonstrated the potential for improving boxwood health by planting different species or cultivars in the landscape to manipulate the soil microbiome. In a more recent study, we demonstrated that the soil population of Calonectria pseudonaviculata ( Cps ), a destructive fungal pathogen of boxwood crops and plantings [ 31 , 32 ], declined sharply within the first year of blighted boxwood removal and fell to an almost undetectable level at the end of year 2 or 3 in all selected gardens across the United States [ 33 ]. We speculated that soil bacteria in these gardens may have contributed to the Cps decline, as has been shown for other plant pathogens [ 34 ].…”