First report: Co-infection of Sarcococca hookeriana (sweetbox) by Coccinonectria pachysandricola and Calonectria pseudonaviculata causes a foliar disease of sweetbox in Pennsylvania

Authors: CASTROAGUDIN, VANINA - Orise Fellow; Shishkoff, Nina; STANLEY, OLIVIA - Hood College; WHITESELL, REESE - Hood College; OLSON, TRACEY - Pennsylvania Department Of Agriculture; Crouch, Jo Anne

Submitted to: Plant Disease
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 11/27/2020
Publication Date: 12/1/2020
Citation: Castroagudin, V.L., Shishkoff, N., Stanley, O., Whitesell, R., Olson, T., Crouch, J.A. 2020. First report: Co-infection of Sarcococca hookeriana (sweetbox) by Coccinonectria pachysandricola and Calonectria pseudonaviculata causes a foliar disease of sweetbox in Pennsylvania. Plant Disease. 105(5):1568. https://doi.org/10.1094/PDIS-06-20-1198-PDN.
DOI: https://doi.org/10.1094/PDIS-06-20-1198-PDN

Interpretive Summary: Plants such as boxwood, pachysandra and sweet box are high-value broadleaf evergreen shrubs and groundcovers widely planted in landscapes worldwide. Recently several diseased sweetbox plants were found in Pennsylvania. The diseased sweetbox were growing next to a stand of boxwood with boxwood blight disease. The goal of this study was to identify what was causing the sweetbox disease. Two fungi were isolated and identified from the sweetbox plants based on their appearance and their DNA fingerprints, namely the boxwood blight pathogen and a fungus called Coccinonectria pachysandricola that has only been seen from sweetbox once before. Infectivity trials showed that both fungi could cause disease in sweetbox. Wounded plants had higher levels of disease than unwounded plants. Knowledge of this new disease and the conditions that allow the disease to occur will be useful to growers, horticulturalists, plant pathologists, disease diagnostic clinics, and regulatory agencies working to control diseases of these important landscape plants.

Technical Abstract: Sweetbox (Sarcococca hookeriana) are high value ornamental shrubs susceptible to only two diseases, caused by Calonectria pseudonaviculata (Cps) and Coccinonectria pachysandricola (Cpa). In July 2018, 18-month old sweetbox with leaf spots and defoliation were observed in a residential landscape in Lancaster County, Pennsylvania. Small tan leaf spots grew to cover half of the leaf, developing a concentric banding with dark brown rings and a yellow halo. The symptoms agreed with those of Cpa disease of sweetbox reported from Washington D.C. Diseased plants were located in front of Buxus sempervirens with boxwood blight. Isolation and morphological, genetic and pathogenic characterization of the fungi recovered from diseased plants followed Salgado-Salazar et al.(2019). White to salmon pink spore masses developed on the abaxial leaf surface after humid chamber incubation. Two distinct fungal cultures were recovered (JAC 18-61, JAC 18-79) on PDA (Fisher Scientific, Pittsburg, PA). JAC 18-61 presented cultural and morphological characteristics as described for Cps. JAC 18-79 produced flat, filamentous, light salmon colonies with ochre centers and white filiform borders containing pale pink sporodochia, verticillate and simple conidiophores (x¯: 61.8 ± 20.12 µm, N = 20) with lateral, cylindrical phialides (x¯ = 18.1 ± 5.83 x 2.4 ± 0.7 µm, N = 20), and ellipsoid, hyaline conidia without septa (x¯ = 15.2 ± 1.9 x 3.3 ± 0.7 µm, N = 20). Sexual structures and chlamydospores were not observed. The characteristics of JAC 18-79 agree with those reported for Cpa. Bidirectional sequencing of the ITS, beta-TUB, and RPB1 and RPB2 regions were performed as described. BLASTn comparisons against NCBI GenBank revealed JAC 19-61 sequences (MT318150 and MT328399) shared 100% identity with Cps sequences (JX535321 and JX535307 from isolate CB002). Sequences from JAC 18-79 (MT318151, MT341237 to MT341239) were 100% identical to Cpa sequences (MH892596, MH936775, MH936703 from isolate JAC 16-20, and JF832909, isolate CBS 128674). The genome of JAC 18-79 was sequenced and yielded an assembly of 26.3 Mb (204 contigs > 1000 bases, N50 = 264.3 kb, 92x coverage, JABAHV0000000000) that contained the MAT1-2 mating-type idiomorph and shared 98.9% similarity with Cpa BPI910731. Pathogenicity tests showed JAC 19-61 (Cps) caused lesions on wounded and unwounded sweetbox and boxwood leaves. In general, JAC 18-79 (Cpa) infected wounded leaves of both hosts, however, one unwounded sweetbox and two unwounded boxwood plants developed lesions possibly due to the presence of natural wounds. Control plants did not develop symptoms. These results diverge with previous reports of Cpa infecting unwounded sweetbox leaves and indicate that virulence variation among Cpa isolates should be further explored. Plating of symptomatic tissue and examination of spores massed from the lesions fulfilled Koch’s postulates. To our knowledge, this is the first occurrence of Cpa blight on sweetbox in Pennsylvania, and the second incidence of the disease in the U.S. This is also the first report of co-infection of Cpa and Cps from diseased sweetbox foliage. Given the capacity of Cpa to infect both sweetbox and boxwood, inspection for Cpa causing foliar blight on boxwood is advisable.