Pathotype grouping of Cercospora sojina isolates on soybean and their sensitivity to QoI fungicides

Authors: Mengistu, Alemu; Ray, Jeffery - Jeff; KELLY, HEATHER - University Of Tennessee; LIN, BINBIN - University Of Tennessee; YU, HAO - University Of Tennessee; Smith, James - Rusty; Arelli, Prakash; Bellaloui, Nacer

Submitted to: Plant Disease
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 9/3/2019
Publication Date: 12/16/2019
Citation: Mengistu, A., Ray, J.D., Kelly, H.M., Lin, B., Yu, H., Smith, J.R., Arelli, P.R., Bellaloui, N. 2019. Pathotype grouping of Cercospora sojina isolates on soybean and their sensitivity to QoI fungicides. Plant Disease. 104:373-380. https://doi.org/10.1094/PDIS-02-19-0368-RE.
DOI: https://doi.org/10.1094/PDIS-02-19-0368-RE

Interpretive Summary: Frogeye leaf spot is a disease of soybean that causes significant yield loss. The objective of this research was to better understand the diversity among individuals of the fungus that causes frogeye leaf spot on the basis of their ability to cause disease (pathogenicity groups) and their reaction to fungicides. Disease severity ranged from 0 to 5, where 0 is very resistant and 5 is very susceptible. The results showed that the 83 individuals tested grouped into five pathogenicity groups, which varied by geographical locations and reaction to fungicide. The growth of 88 percent of the individuals was reduced significantly by the fungicide, suggesting that the fungicide was still effective against the majority of the individuals at the time they were acquired. The results indicated that testing soybean lines using individuals representing each pathogenicity group may lead to identifying soybean lines with useful genes for resistance to frogeye leaf spot. If the genes associated with resistance in each pathogenicity group are unique, they could be combined by soybean breeders to develop plants with resistance against individuals from multiple pathogenicity groups.

Technical Abstract: Frogeye Leaf Spot (FLS), caused by Cercospora sojina, is a common disease in the southern and northern US and causes significant yield loss. Multiple races and isolate diversity have been reported based on previous classification schemes. The objective of this research was to better understand the diversity among C. sojina isolates through the development and use of pathogenicity groups. In this study, 83 isolates acquired from 2006-2009 were screened using twelve soybean (Glycine max) differentials. Disease severity on the 12 differentials ranged from 0 to 9, where 0 is immune and 9 very susceptible. The severity for each isolate across differentials ranged from 1 to 7. The 83 isolates were grouped into five pathogenicity groups (PG): PG1, PG2, PG3, PG4 and PG5 reflecting the severity grouping. Using the 12 differentials, PG1 isolates were differentiated by the lack of infection on Davis, Peking, Kent, Palmetto Hood, CNS, Tracy and Richland. PG2 had a range of infections on a scale of 1-2 on all differentials except on Davis; PG3 isolates had severity ranging from 3-4 except on Davis. PG4 isolates caused no infection on Davis, a maximum disease severity of 5 on Peking, while the rest of differentials had severities from 5-6. PG5 isolates caused no infection on Davis, severity of 7 on CNS and severity of 8 on Kent, Hood, and Palmetto. The remaining seven differentials had severities of 9. Across the geographical locations, the predominant pathotypes were PG3 and PG4 and represented 84% of the tested isolates. Fungicide sensitivity tests showed that 88% of the isolates were sensitive and dominated the population, while only 6% had a high level of fungicide resistance, suggesting that FLS resistance was not yet completely developed and had not spread to other areas at the time when these isolates were acquired. The overall virulence profile of the isolates indicated that there was variation in disease severity suggesting that selection of resistance for each PG may produce lines with more precisely defined interactions to specific pathotypes of C. sojina. This may improve the screening and selection of useful resistance genes that could be pyramided for resistance to each pathogenicity group.