Genotypic Diversity of Common Phytophthora Species in Maryland Nurseries and Characterization of Fungicide Efficacy

Abstract

The genetic diversity of P. plurivora, P. cinnamomi, P. pini, P. multivora, and P. citrophthora, five of the most common species found in Maryland ornamental nurseries and mid-Atlantic forests, was characterized using amplified fragment length polymorphism (AFLP). Representative isolates of genotypic clusters were then screened against five fungicides commonly used to manage Phytophthora. Three to six populations were identified for each species investigated with P. plurivora being the most diverse and P. cinnamomi the least. Clonal groups that originated from forest or different nurseries suggest an ongoing pathway of introduction. In addition, significant molecular variation existed for some species among nurseries an indication that unique genotypes being present in different nurseries. Insensitive isolates to fungicides were detected with P. plurivora (13), P. cinnamomi (3), and P. multivora (2). Interestingly, insensitive isolates primarily belonged to the least common genotypic clusters. Because all but two isolates were sensitive to dimethomorph and ametoctradin, the ability of these chemicals to manage Phytophthora is promising. Nevertheless, the presence of two insensitive isolates could portend general insensitivity to these chemicals as well. Results from this study provide a foundation to future population determination and fungicide sensitivity of the plant pathogenic genus Phytophthora in Maryland's ornamental nursery industry.