Link to home

Megan M. Dewdney

Megan M. Dewdney was born in Penticton, British Columbia, Canada. She attended the University of British Columbia and earned her B.Sc. Agr. Hons from the Faculty of Agriculture specializing in Plant Science where her interest in plant pathology began. She received an M.Sc. in Plant Sciences, specializing in Plant Pathology at McGill University in Montreal, Quebec, Canada. She obtained her PhD in plant pathology from Cornell University in 2008. Her dissertation focused on the epidemiology of Erwinia amylovora on apple flower surfaces under the guidance of Herb Aldwinckle. In 2008, Dewdney joined the University of Florida Plant Pathology Department where she is now an associate professor.

At the Citrus Research and Education Center in Lake Alfred, Dewdney developed a program in citrus pathology with an emphasis on foliar fungal pathogens and huanglongbing (HLB). One of the challenges upon arrival was to balance the need for on-going research and extension on fungal diseases with that of HLB. Citrus has historically been a $9 billion industry in Florida and is a major part of the Central Florida ethos. This iconic industry is now critically threatened by HLB but losses from fungal diseases contribute to the economic malaise. Although her program started with HLB, she was called to concentrate on fungal diseases with discovery of citrus black spot in Florida, an exotic disease, and the development of fungicide resistance with Alternaria brown spot, an important tangerine disease.

Citrus black spot, caused by Phyllosticta citricarpa (syn. Guignardia citricarpa), is an invasive disease first discovered in 2010 in Hendry and Collier Counties, Florida (Plant Disease 96:1225). Black spot is the most serious fungal disease of citrus, so it presented a new and significant disease management challenge, as well as a quarantine issue, for Florida. At the time of discovery, Dewdney assumed research and extension leadership for citrus black spot and made significant contributions to understand the distribution and etiology of this important pathogen in Florida. A significant complication in dissecting the biology and epidemiology of P. citricarpa was the existence of a morphologically indistinguishable fungus, P. capitalensis that inhabits the same ecological niches as the pathogen. Dewdney and colleagues developed a qPCR assay to detect, quantify, and distinguish between these fungi in environmental samples. This research supported further investigation into the biology and epidemiology of P. citricarpa (Plant Disease 98:112). To optimize fungicide recommendations, Dewdney and colleagues characterized the sensitivity of P. citricarpa isolates to quinone-outside inhibitors (Qols) (Plant Disease 98:780) and continue to evaluate other modes of action including DMI and SDHI fungicides. Their data demonstrated that the pathogen population in Florida was highly sensitive to all three classes of fungicides and supported their use in the management of this critical citrus disease. After its discovery in Collier County in 2010, P. citricarpa spread into other citrus producing counties in Florida, but no sexual reproductive structures could be identified as belonging to this fungus, raising epidemiological questions about the disease. Further investigation revealed the presence of only P. capitalensis pseudothecia in leaf litter. To understand this phenomenon, they investigated the mating-type (MAT) loci of P. citricarpa and P. capitalensis. Dewdney and colleagues determined that P. citricarpa has an idiomorphic, heterothallic MAT locus structure whereas P. capitalensis was homothallic (Phytopathology 106:1300). A survey of P. citricarpa isolates from Florida revealed that only the MAT1-2 idiomorph existed in Florida and there was only one genetic individual, explaining why no P. citricarpa pseudothecia were identified. In contrast, isolates collected from Australia, where sexual reproduction is documented, exhibited a 1:1 ratio of MAT1-1 and MAT1-2 isolates. These results indicate that P. citricarpa isolates from Florida are likely descendent from a single clonal lineage and confirmed that, uniquely, the population is reproducing asexually (Phytopathology 106:1300) and that the conidia were a more important part of the disease cycle in high summer rainfall areas than previously described. This work demonstrated that disease management in Florida should concentrate on the production and dispersal of pycnidiospores, an ongoing focus of her program and international collaborations.

In 2008, Alternaria brown spot re-emerged as a serious problem on tangerines growing in Florida. Fungicide resistance was suspected to cause the disease control failures. Dewdney, her student, Byron Vega, and colleagues (Plant Disease 96:1262), developed a rapid fungicide sensitivity screening method and conducted a survey of tangerine groves for QoI resistance. They found that 58% of Alternaria alternata isolates were resistant to QoIs and 75% of the blocks surveyed had greater than 20% resistant isolates (Plant Disease 98:67).

Vega and Dewdney (Plant Disease 98:1371) also investigated the phenotypic stability, fitness components, and disease-causing ability of A. alternata tangerine pathotype isolates. QoI sensitivity was found to be stable and resistant isolates were more virulent than the sensitive isolates. Laboratory results were confirmed in the greenhouse where applications of a commercial QoI at full rate, failed to control disease caused by QoI-resistant isolates.

Boscalid, a succinate dehydrogenase inhibitor, was registered in 2011 to control Alternaria brown spot. In a baseline study, A. alternata isolates collected between 1996 and 2012, were found to be sensitive to boscalid and no mutations in the conserved histadine-residues of the succinate dehydrogenase (SDHI) genes were described (Plant Disease 99:231). These data demonstrated that A. alternata populations from Florida are sensitive to SDHI fungicides, making them valuable additions to disease and fungicide resistance management.

Information on the functionality and stability of fungicide resistance in A. alternata was incorporated into disease management recommendations delivered in extension presentations, the Florida Citrus Pest Management Guide and articles to citrus growers in Florida, and have resulted in more sustainable patterns of fungicide use.

Currently, Dewdney is involved in collaborative projects on novel methods to manage huanglongbing, including thermotherapy and novel antimicrobial compounds. Dewdney’s research projects have led to 111 extension publications as part of her large citrus pathology extension program. She has given 17 invited talks and on average gives 15 grower talks yearly. Her research on citrus diseases has resulted in eight papers published in APS journals from 2012 through 2016.