Soybean Improvement Through Plant Breeding: Pest Resistance
The goal of plant breeding programs is to improve plant performance through genetic gain, and it is widely acknowledged that breeding gains in soybean have made important contributions to increased production.
A study was conducted in three field environments in central Illinois in 2010 and 2011 to assess pest severity on 40 Maturity Group (MG) III old and newer soybean varieties in 2010 and 59 MG III old and newer varieties in 2011. Varieties included those from both the public and private sectors that were released from 1923 through 2008. 22% of the 59 varieties were released before 1970, 25% were released between 1970 and 1990, and 53% were released after 1990.
At growth stages R1 and R6, plants of each variety were rated for foliar disease presence and insect feeding. Disease ratings were made for damage resulting from bacterial diseases, brown spot, Cercospora leaf blight, downy mildew, frogeye leaf spot, stem canker, and sudden death syndrome (SDS). Insect ratings were made for foliar feeding damage done primarily by bean leaf beetles and Japanese beetles. At growth stage R8, ratings were made for the stem diseases charcoal rot, Cercospora blight on stems, pod and stem blight, and anthracnose. Green stem syndrome, stem canker, soybean aphid, and SDS were infrequently observed; thus, their results are not reported.
Results are presented in an Agronomy Journal article entitled “Evaluation of disease and pest damage on soybean cultivars released from 1923 through 2008 under field conditions in central Illinois” by Hartman, Bowen, Haudenshield, Fox, Cary, and Diers.
A summary of the results follows, along with some conclusions indicated by the results.
● Severity ratings for most diseases evaluated in the study were generally greater in older vs. newer soybean varieties. However, none of the diseases were absent on the newer varieties.
● A majority of the disease ratings were negatively correlated with variety release year; i.e., newer varieties had lower infestations by diseases.
● The authors concluded that conventional breeding efforts in the past have modestly reduced the severity of some soybean diseases as a result of incorporating genes for resistance to those diseases, as well as from non-targeted breeding efforts that have eliminated susceptible plants in breeding populations.
● Major genes that impart resistance to stem pathogens have not been incorporated into soybean varieties. Thus, the negative relationship between stem pathogens and variety release date measured in this study likely indicates the gradual improvement over time in resistance to these diseases that has resulted from direct selection for plants with less disease in breeding programs.
● A trend in the relationship between insect foliar feeding and variety release year was not found in this study. This is not surprising since agronomically acceptable soybean varieties with resistance to insect pests have not been developed.
● The relationship between severity ratings for most of the pests in this study and release year of soybean varieties appeared to depend on both location and year. This indicates that inconsistent natural pest pressure across years and environments may not be sufficient to impart consistently significant damage to any soybean variety, regardless of its release date. This is the reason that many breeding programs geared toward developing pest resistant soybean varieties use artificial inoculation techniques to ensure consistent selection pressure in all populations and environments.
Composed by Larry G. Heatherly, Mar. 2016, larryheatherly@bellsouth.net