Characterizations of an Emerging Disease: Apple Blotch Caused by Diplocarpon coronariae (syn. Marssonina coronaria) in the Mid-Atlantic United States

Fatemeh Khodadadi, Phillip L. Martin, Daniel J. Donahue, Kari A. Peter, Srd An G. Acimovic

Research output: Contribution to journalArticlepeer-review

6 Scopus citations

Abstract

Apple orchards with minimal or reduced fungicide inputs in the MidAtlantic region of the United States have experienced outbreaks of severe premature defoliation with symptoms that matched those of apple blotch disease (ABD) caused by Diplocarpon coronariae. Fungal isolates obtained from symptomatic apple leaves and fruit produced uniform slow-growing, dark-gray colonies on peptone potato dextrose agar and had conidia. Internal transcribed spacer DNA sequences matched with D. coronariae and Koch's postulates were fulfilled when typical ABD symptoms occurred when reinoculated onto apple leaves and fruit. Spore dispersal in nonfungicide-treated orchards detected with quantitative PCR was low in early spring and dropped to undetectable levels in late May and early June before rising exponentially to highs in July and August, which coincided with symptom development. Only low spore numbers were detected in fungicide-treated orchards and nearby forests. In preliminary fungicide tests, fluxapyroxad, thiophanate methyl, and difenoconazole effectively inhibited mycelial growth of isolates in vitro. When apple cultivars Fuji and Honeycrisp were inoculated with D. coronariae, Honeycrisp showed delayed onset of symptoms and lower disease severity, and the transcription profile of seven host defense-related genes showed that PR-2, PR-8, LYK4, and CERK1 were highly induced in Honeycrisp at 2 and 5 days postinoculation. This is the first report of ABD in the MidAtlantic United States, which includes studies of seasonal D. coronariae spore dispersal patterns, preliminary fungicide efficacy, and host defense-related gene expression to assist development of best ABD management practices.

Original languageEnglish (US)
Pages (from-to)1803-1817
Number of pages15
JournalPlant disease
Volume106
Issue number7
DOIs
StatePublished - Jul 2022

All Science Journal Classification (ASJC) codes

  • Agronomy and Crop Science
  • Plant Science

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