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.

Characterization of the Race Structure of Leptosphaeria maculans Causing Blackleg of Winter Canola in Oklahoma and Kansas.

Blackleg, caused by the fungus Leptosphaeria maculans, is a widespread disease of winter canola (Brassica napus) in Oklahoma and Kansas. Deployment of genetic resistance is the primary strategy for managing blackleg. Resistance genes (Rlm) in canola interact with avirulence genes in the fungus (AvrLm) in a gene-for-gene manner. Little is known about the diversity and frequency of avirulence genes and the race structure in the region. Isolates of Leptosphaeria spp. were collected from diseased leaves in nine counties in Oklahoma and one county in Kansas from 2009 to 2013. Based on pathogenicity and PCR amplification of mating type and species-specific internal transcribed spacer loci, most isolates (n = 90) were L. maculans. The presence of avirulence genes was evaluated using phenotypic interactions on cotyledons of differential cultivars with Rlm1, Rlm2, Rlm3, and Rlm4 and amplification of AvrLm1, AvrLm4-7, and AvrLm6 by PCR. The avirulence alleles AvrLm6 and AvrLm7 were present in the entire L. maculans population. AvrLm1 was found in 34% of the population, AvrLm2 in 4%, and AvrLm4 in only 1%. A total of five races, defined as combinations of avirulence alleles, were identified that included AvrLm1-2-6-7, AvrLm2-6-7, AvrLm4-6-7, AvrLm1-6-7, and AvrLm6-7. Races virulent on the most Rlm genes, AvrLm1-6-7 at 32% and AvrLm6-7 at 62%, were predominant. Defining the avirulence allele frequency and race structure of L. maculans should be useful for the identification and development of resistant cultivars and hybrids for blackleg management in the region. The results suggest that Rlm6 and Rlm7 would be effective, although their deployment should be integrated with quantitative resistance and cultural practices, such as crop rotation, that limit selection pressure on Rlm genes.