Screening Method for Distinguishing Soybean Resistance to Fusarium virguliforme in Resistant × Resistant Crosses

Abstract

Current greenhouse screening methods for soybean [Glycine max (L.) Merr.] sudden death syndrome (SDS) are not sensitive enough to evaluate progeny of resistant x resistant crosses, which can possess resistance alíeles at up to 12 quantitative trait Iooi (QTL). The objective of the study was to modify the current greenhouse screening method, the layer method, to increase its sensitivity in distinguishing SDS-reslstant lines. Three experiments were conducted using infested sorghum [Sorghum bicolor (L.) Moench] as inoculum. In Exp. 1, five different inoculum densities (1:1, 1:5, 1:10, 1:15, and 1:20 inoculum:soil ratio) were compared, and the optimum density to distinguish genotypes was identified. In Exp. 2, the association between SDS field ratings and greenhouse severity scores was determined. In Exp. 3, the inoculum layer method was compared to the proposed modified mixed method. Inoculum ratios 1:15 and 1:20 showed the largest difference in SDS ratings between soybean genotypes. The best time to assess disease severity was 30 to 36 d after planting (DAP), with the highest correlation with field ratings at 36 DAP {r = 0.82, p < 0.05). The proposed modified method siiifted the frequency distribution of SDS scores toward a normal curve. The findings indicate that the modified screening method using a 1:20 inoculum:soil ratio correlated well with field data and provided adequate screening of lines possessing up to 12 SDS resistant QTL without negatively impacting germination.

Sudden death syndrome - a growing threat of losses in soybeans.

“…These include dip inoculation of sprouted seed in F. virguliforme spore suspension and planting in greenhouse mix [80,81,85], temperature-controlled water bath method [38], exposing cut stem to cell free culture filtrate [42], molecular marker-assisted breeding for resistance [60], toothpick inoculation and soil infestation methods [111], planting seeds in a mixture of 1 part inoculum with 20 part soil [64], using cones filled with steam-treated soil mix (2:1 sand: soil) and topped with 3 g of fungus infested white sorghum grains [25]. Navi and Yang (unpublished), modified greenhouse screening technique [80,81,85] to identify resistance sources in germplasm and explore efficacy tests of seed treatments with biocontrol agents (BCA) and or chemicals against SDS (Fig.…”

Quantitative Phenotyping of Vascular Damage Caused by Fusarium Wilt Disease in Cowpea

“…Phenotype-based screening for resistance donors and selection of superior breeding lines requires a sensitive screening method with reliable metrics to accurately assess the severity of disease. Reliable disease-phenotyping metrics allow for accurate identification of true phenotypic disease responses (resistant, moderately resistant and susceptible) in screening assays (Luckew et al, 2012). Fusarium wilt resistant and susceptible cowpea genotypes can be easily identified in a germplasm collection or in a segregating population when the trait is under control by one or two genetic factors.…”

“…However, under quantitative inheritance effective selection for resistant genotypes can be compromised by the lack of an objective screening method; therefore, in these cases quantitative metrics can provide accurate and precise quantification of the severity of disease (Ndeve, 2017). In resistant and moderately resistant genotypes, disease severity expressed as the extent of plant wilting can be weakly associated with length of vascular discoloration which is an indicator of the extent of vascular colonization and the damage caused by the fungus (Talboys, 1972; Luckew et al, 2012; Ndeve, 2017). Less objective disease assessment is error prone and can lead to inaccurate association between plant wilting and the extent of vascular discoloration and damage, which could compromise the individual usefulness of each metric in Fusarium wilt disease severity assessment and the translation of the results into practical use.…”

“…As in cowpea, susceptible common bean and cotton cultivars can display wilt symptoms 5 to 9 days after inoculation, and severe or complete plant wilting and defoliation, which indicates the severity of damage to the vascular system, can occur 14 days after inoculation (Pastor Corrales and Abawi, 1987; Ulloa et al, 2006). Luckew et al (2012) reported that in lines of soybean from resistant × resistant crosses, the severity of wilt caused by Fusarium virguliforme was not correlated with the severity of damage in the root system. These reports indicate that reliance on plant wilting to quantify severity of Fusarium wilt and the use of qualitative and semi-quantitative metrics can compromise the accuracy of wilt disease severity estimation, especially if plant wilt phenotypes are not highly associated with vascular discoloration symptoms as the two traits are under control by distinct mechanisms (Talboys,1972; Luckew et al, 2012)…”

“…Luckew et al (2012) reported that in lines of soybean from resistant × resistant crosses, the severity of wilt caused by Fusarium virguliforme was not correlated with the severity of damage in the root system. These reports indicate that reliance on plant wilting to quantify severity of Fusarium wilt and the use of qualitative and semi-quantitative metrics can compromise the accuracy of wilt disease severity estimation, especially if plant wilt phenotypes are not highly associated with vascular discoloration symptoms as the two traits are under control by distinct mechanisms (Talboys,1972; Luckew et al, 2012)…”