Progress in understanding the genetics of resistance from the grape Muscadinia rotundifolia to the bacterial pathogen Xylella fastidiosa
Pierce's disease (PD) of grape is caused by the bacterial pathogen Xylella fastidiosa (Xf) and is a persistent threat to California's viticulture industry. Development of PD resistant varieties can provide a long-term solution to the disease and a thorough understanding of the genetics controlling Xf resistance will be required to produce such varieties. Resistance is known to exist in the American grape species Muscadinia rotundifolia and a hybrid family between M. rotundifolia and Vitis rupestris (8909) was available for initial screening. Disease evaluation of the 8909 family was based on PD symptoms and direct detection of Xf growth in stem tissue via ELISA. Results showed a 10:13 segregation of resistant to susceptible genotypes. The ratio did not significantly deviate from 1:1 suggesting that a primary gene controls this resistance. Xf resistance mapping steps were initiated on a second-generation family (9621) derived from the sibling cross of two Xf resistant genotypes 8909-15 X 8909-17. 145 genotypes from the 9621 family were screened in a similar manner as the parents. The segregation ratio of resistant to susceptible genotypes did not significantly deviate from 3:1, which also suggests that a primary gene controls Xf resistance. The presence of susceptible genotypes in the family derived from a R X R cross suggests Xf resistance in this population is a dominant trait. AFLP marker data were available on 70 of 145 individuals in the family. Statistical associations among the AFLP makers and between markers and resistance genes were calculated using the computer program 'Joinmap 3.0'. Results based on this preliminary data suggest that Xf resistance maps to a similar position on a linkage group when using either ELISA or disease phenotype data (LOD scores of 9 and 4 respectively).