Host-pathogen interactions in the pine-Fusarium circinatum pathosystem and the potential for resistance deployment in the field

Abstract

Pine pitch canker (PPC) emerged as a significant problem in 1945 in Southeastern USA. The causal agent, Fusarium circinatum, has spread widely and now occurs in pine forests and plantations worldwide. Fusarium circinatum causes damping off, shoot and tip die-back, and the death of seedlings in nurseries. Infection of mature trees leads to excessive resin bleeding on branches and main stems, sunken cankers with bark remaining attached, and finally, tree death. Arguably, F. circinatum is the most important pathogen of pine seedlings in many areas of the world. At least 67 species of Pinus, 18 Pinus hybrids, and 6 other non-pine tree species are susceptible to PPC. The selection and development of tree germplasm resistant to pathogens is considered the most robust approach to reduce losses to diseases. Genetic variation in resistance to F. circinatum certainly exists, even in generally very susceptible hosts, such as P. radiata. Exploiting genetic resistance as a tool to manage PPC requires screening large numbers of tree genotypes and fine-tuning efficient phenotyping protocols. The greater the number of genetic lines and replications, the higher the selection differentials and accuracy of the genetic parameter estimates. Recent advances in physiological and molecular techniques exploring the plant-pathogen interaction have expanded our understanding of genetic resistance mechanisms in pines to PPC and represent an added-value tool to support phenotypic selection. This review provides an overview of current knowledge on the molecular, physiological, and genetic basis of resistance to F. circinatum in pines and considers possibilities to improve the resistance of pines against the pathogen through breeding and selection.