Mycobiome of low maintenance iconic landscape plant boxwood under repeated treatments of contact and systemic fungicides

Boxwood, a low-maintenance landscape plant, has been plagued by diseases in recent years, and fungicide protection is now indispensable for its healthcare. The objective of present study was to determine how fungicide chemistry and repeated application may affect phyllosphere mycobiome. Three fungicides—Daconil (chlorothalonil, contact chemistry), Banner Maxx (propiconazole, systemic chemistry), and Concert II (a combination of both chemistries)—were first applied on April 12 then repeated at 2- and 3-week intervals, product dependent. Shoots from Buxus sempervirens 'Vardar Valley' were sampled immediately before, 1, 7 and 14 days after fungicide application on May 26 and August 25. As determined by amplicon sequencing, fungal community composition differed between shoot surface and internal tissue, with the former being dominated by Cladosporium and the latter by Shiraia species. Fungicide applications strongly affected epiphytic fungal community diversity, structure, and many functional groups. Daconil and Concert II suppressed greater numbers of epiphytes than Banner Maxx. Many epiphytic genera became less sensitive to Daconil treatment in August. This study provided the first mycobiome evidence supporting boxwood as a low-maintenance plant and demonstrating fungicide resistance to a multisite chemistry due to repeated applications. It also helped understand boxwood rising health issues associated with increasing fungicide use.