<i>Colletotrichum</i> Species Causing Glomerella Leaf Spot and Apple Bitter Rot in the Southeastern United States Exhibit Disparities in Relative Frequency, Morphological Phenotype, and Quinone Outside Inhibitor Sensitivity

Glomerella leaf spot (GLS), Glomerella fruit rot (GFR), and apple bitter rot (ABR), caused by <i>Colletotrichum</i> spp., are among the most devastating apple diseases in the southeastern United States. Although several species have been identified as causal pathogens of GLS, GFR, and ABR, their relative frequency and fungicide sensitivity status in the southeastern United States is unknown. In total, 381 <i>Colletotrichum</i> isolates were obtained from symptomatic leaves and fruit from 18 conventionally managed apple orchards and two baseline populations in western North Carolina and Georgia in 2016 and 2017. Multilocus DNA sequence analysis revealed that <i>C. chrysophilum</i> was the predominant cause of GLS and GFR and <i>C. fioriniae</i> was the causal agent of ABR. Baseline and commercial populations of <i>Colletotrichum</i> spp. were evaluated for sensitivity to pyraclostrobin and trifloxystrobin, and no statistical differences in sensitivity between the two species were observed for conidial germination. However, the effective concentration that inhibited growth by 50% values were significantly lower for <i>C. fioriniae</i> compared with <i>C. chrysophilum</i> for both fungicides regarding mycelial inhibition. Isolates recovered from commercial orchards revealed that five populations of <i>C. chrysophilum</i> and one population of <i>C. fioriniae</i> had reduced sensitivity to trifloxystrobin, and one <i>C. fioriniae</i> population had reduced sensitivity to pyraclostrobin via conidial germination assays. The <i>cytb</i> gene for 27 isolates of <i>C. fioriniae</i>, <i>C. chrysophilum</i>, and <i>C. fructicola</i> with different quinone outside inhibitor (QoI) sensitivities revealed the G143A mutation in a single isolate of <i>C. chrysophilum</i> with insensitivity to both fungicides. The results of these studies suggest that two <i>Colletotrichum</i> spp. predominantly cause GLS and ABR in the southeastern United States and that a reduction in sensitivity to some QoI fungicides may be responsible for control failures. This study also provides a basis for monitoring shifts in QoI sensitivity in <i>Colletotrichum</i> spp. causing disease on apple in the southeastern United States.