Population genetic characteristics and mating type frequency of Venturia effusa from pecan in South America

Authors: Bock, Clive; FRUSSO, ENRIQUE - National Agricultural Research Institute(INIA); ZOPPOLO, ROBERTO - National Institute For Agricultural Research (INIAP); ORTIZ, EDSON - Noble Research Institute; SHILLER, JASON - Noble Research Institute; CHARLSTON, NIKKI - Noble Research Institute; YOUNG, CAROLYN - Noble Research Institute; RANDALL, JENIFER - New Mexico State University

Submitted to: Phytopathology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 5/20/2022
Publication Date: 9/26/2022
Citation: Bock, C.H., Frusso, E., Zoppolo, R., Ortiz, E.R., Shiller, J., Charlston, N.D., Young, C., Randall, J.I. 2022. Population genetic characteristics and mating type frequency of Venturia effusa from pecan in South America. Phytopathology. Vol 112:2224-2235. https://doi.org/10.1094/PHYTO-01-22-0031-R.
DOI: https://doi.org/10.1094/PHYTO-01-22-0031-R

Interpretive Summary: Scab is the major disease of pecan in the southeastern U.S.A. and is important in South America. The disease results in yield loss and loss in nut quality. The pathogen population genetics have been characterized in the U.S.A., but are unknown in South America. The aim of this study was to determine the genetic diversity and structure of populations of the scab fungus on pecan in South America, and to determine the mating types and frequencies present. The four sampled orchards populations from Argentina, Brazil (2 orchards) and Uruguay had moderate to high genetic diversity. Most of the genetic diversity was at the scale of a tree, with some at the level of the orchard, indicating population structure. But differentiation was limited among the four populations. Migration was evident between all pairs of populations. Both the mating types of the fungus occurred in all four orchards and were at equilibrium in all orchards suggesting sexual reproduction. The genetic diversity and structure between the two mating type populations indicated a free exchange of genetic material. The characteristics of the population genetics of South America have many similarities to those previously described for populations in the U.S.A. Characterizing the genetic diversity, genetic structure and reproductive systems of the fungus are important to establish its evolutionary potential, and thus its adaptability – and can provide a basis for informed approaches to utilizing available host resistance and guiding phytosanitary decisions.

Technical Abstract: Scab, caused by the plant pathogenic fungus Venturia effusa, is a major disease of pecan in South America, and can result in serious loss in quantity and quality of nut yield. Although the pathogen population genetics have been studied in the U.S.A., the characteristics of the population genetics and population structure of V. effusa in South America are unknown. Furthermore, mating types of the pathogen generally exist in equilibrium in the U.S.A. It is not known if both mating types exist in South America, and if they do, whether they exhibit equilibrium. The aim of this study was to describe the genetic diversity and structure of populations of V. effusa on pecan in South America using 15 microsatellite markers, and to determine the mating type status in those populations. The four hierarchically sampled orchard populations from Argentina (AR), Brazil (BRC and BRS) and Uruguay (UR) had moderate to high genotypic (G = 2.84 to 4.74) and gene (Hexp = 0.178 to 0.524) diversity. Structure to the populations was detectable: an analysis of molecular variance indicated that 77.3% and 19.6% of the genetic variation were within tree and between orchard levels, respectively. A minimum spanning network and discriminant analysis of principal components also indicated population structure, but also a varying admixture among populations. Genetic differentiation was minimal between the UR, BRC and BRS populations (D = 0.052 to 0.085), but these three populations were more clearly differentiated from the AR population (D = 0.245 to 0.348). Linkage disequilibrium was observed in three of the four populations. Although there was no relationship between geographic distance and genetic differentiation, migration was evident between all pairs of populations. Both the MAT1-1 and MAT1-2 mating types occurred in all four orchards and were at equilibrium in all orchards. The genetic diversity and structure between the two mating type populations indicated a free exchange of genetic material. The characteristics of the population genetics of South America populations of V. effusa have many similarities to those previously described for populations in the U.S.A. Characterizing the genetic diversity, genetic structure and reproductive systems of V. effusa are important to establish the evolutionary potential of the pathogen, and thus its adaptability – and can provide a basis for informed approaches to utilizing available host resistance.