Breeding for citrus black spot resistance

Abstract

Citrus black spot (CBS) has caused problems in summer rainfall regions of Australia for more than 125 years with associated research focused on chemical and to a lesser extent cultural control strategies. This research has produced only partial and short-term solutions to the problem. More recently we shifted our attention to finding genetic resistance to this disease and were surprised to discover that pummelo (Citrus maxima) has robust resistance to CBS. Pummelo is a progenitor species of many cultivated citrus types, notably sweet orange and grapefruit, but is also present in lemon and many mandarins. This makes pummelo an obvious donor parent for the potential introgression of CBS resistance into new commercial cultivars. Our hybrids between pummelo and mandarin have shown evidence for segregation of CBS resistance and we are currently attempting to confirm this and to phenotype populations derived from backcrossing putatively resistant F1s to mandarin. There are many problems with using pummelo as a breeding parent, including poor peel colour, high furanocoumarins and strong segment walls, but techniques are being developed to reduce linkage drag on these traits. There are also significant challenges in phenotyping hybrid populations for disease reaction, including the need for hybrids to be fruiting, and the long latency period between fruit inoculation and symptom expression. These obstacles have made the identification of molecular markers a primary objective of our research, and for this reason fruit on more than 300 hybrids were recently inoculated. Accurate phenotyping of disease reaction represents the current bottleneck in efficient breeding for CBS resistance as we move toward the ultimate goal of producing commercial cultivars resistant to this important fungal disease.