Biology and Management of Bacterial Spot of Peppers in Oklahoma.

Bacterial spot, an important disease of bell and chili peppers grown in Oklahoma, is caused by Xanthomonas euvesicatoria. We evaluated isolates from 1995 to 2015 (n = 72) for avirulence alleles and race by assessing hypersensitive responses (HRs) on differentials with resistance genes Bs1, Bs2, Bs3, or Bs4. Most isolates (96%) expressed AvrBs2 (races 1, 3, 7, 8), and only three were virulent on Bs2 (race 6). Chili cultivars, grown locally for capsaicin production, were susceptible to all races. Copper-based spray programs were evaluated on bell pepper hybrids with and without Bs2 resistance from 2008 to 2010 and on bell and jalapeño hybrids with Bs2 or Bs1-2-3 from 2017 to 2018. Bs2 and Bs1-2-3 hybrids generally had lower disease and higher yields (≤21 t/ha) than susceptible entries. Copper reduced disease by 20 to 40% and increased yield by an average of 4 t/ha, but yield responses were not always significant (P = 0.05). In August 2018, disease increased to >50% on Bs2 hybrids but remained low on the Bs1-2-3 hybrid. Despite the breakdown of Bs2 resistance, yields of the Bs2 hybrids were not reduced. Avirulence alleles and race of isolates from susceptible, Bs2, and Bs1-2-3 hybrids at the end of the 2018 trial depended on source plant genetics. AvrBs2 was expressed in 86% of isolates from a susceptible hybrid but not in any isolates from the Bs2 and Bs1-2-3 hybrids. HR resistances effectively protected yield, but their deployment may not be sustainable without cultural practices such as crop rotation that limit pathogen survival and transmission to subsequent crops.

Host Genotype and Hypersensitive Reaction Influence Population Levels of Xanthomonas campestris pv. vitians in Lettuce.

Dynamics of population sizes of Xanthomonas campestris pv. vitians inoculated onto or into lettuce leaves were monitored on susceptible and resistant cultivars. In general, population growth was greater for susceptible (Clemente, Salinas 88, Vista Verde) than resistant (Batavia Reine des Glaces, Iceberg, Little Gem) cultivars. When spray-inoculated or infiltrated, population levels of X. campestris pv. vitians were consistently significantly lower on Little Gem than on susceptible cultivars, while differences in the other resistant cultivars were not consistently statistically significant. Populations increased at an intermediate rate on cultivars Iceberg and Batavia Reine des Glaces. There were significant positive correlations between bacterial concentration applied and disease severity for all cultivars, but bacterial titer had a significantly greater influence on disease severity in the susceptible cultivars than in Little Gem and an intermediate influence in Iceberg and Batavia Reine des Glaces. Infiltration of X. campestris pv. vitians strains into leaves of Little Gem resulted in an incompatible reaction, whereas compatible reactions were observed in all other cultivars. It appears that the differences in the relationship between population dynamics for Little Gem and the other cultivars tested were due to the hypersensitive response in cultivar Little Gem. These findings have implications for disease management and lettuce breeding because X. campestris pv. vitians interacts differently with cultivars that differ for resistance mechanisms.

The inheritance of resistance to bacterial leaf spot of lettuce caused by Xanthomonas campestris pv. vitians in three lettuce cultivars.

Lettuce yields can be reduced by the disease bacterial leaf spot (BLS) caused by the pathogen Xanthomonas campestris pv. vitians (Xcv) and host resistance is the most feasible method to reduce disease losses. The cultivars La Brillante, Pavane and Little Gem express an incompatible host-pathogen interaction as a hypersensitive response (HR) to California strains of Xcv resulting in resistance. Little was known about the inheritance of resistance; however, resistance to other lettuce pathogens is often determined by resistance gene candidates (RGCs) encoding nucleotide-binding leucine-rich repeat (NB-LRR) proteins. Therefore, we determined the inheritance of BLS resistance in the cultivars La Brillante, Little Gem and Pavane and mapped it relative to RGCs. The reaction to Xcv was analyzed in nine F1, F2 and recombinant inbred line populations of lettuce from HR×compatible or HR×HR crosses. The HR in La Brillante, Pavane and Little Gem is conditioned by single dominant genes, which are either allelic or closely linked genes. The resistance gene in La Brillante was designated Xanthomonas resistance 1 (Xar1) and mapped to lettuce linkage group 2. Xar1 is present in a genomic region that contains numerous NB-LRR encoding RGCs and functional pathogen resistance loci in the RGC2 family. The Xar1 gene confers a high level of BLS resistance in the greenhouse and field that can be introgressed into commercial lettuce cultivars to reduce BLS losses using molecular markers.