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© 2011 Plant Management Network.
Accepted for publication 27 July 2011. Published 26 September 2011.


Evaluation of Soybean Genotypes for Resistance to Charcoal Rot


Alemu Mengistu and P. A. Arelli, USDA-ARS, Jackson, TN 38301; J. P. Bond, Southern Illinois University, Carbondale, IL 62901; G. J. Shannon and A. J. Wrather, University of Missouri, Portageville, MO 63873; J. B. Rupe and P. Chen, University of Arkansas, Fayetteville, AR 72701; C. R. Little, Kansas State University, Manhattan, KS, 66506; C. H. Canaday and M. A. Newman, University of Tennessee, Jackson, TN, 38301; and V. R. Pantalone, University of Tennessee, Knoxville, TN, 37996


Corresponding author: Alemu Mengistu. alemu.mengistu@ars.usda.gov


Mengistu, A., Arelli, P. A., Bond, J. P., Shannon, G. J., Wrather, A. J., Rupe, J. B., Chen, P., Little, C. R., Canaday, C. H., Newman, M. A., and Pantalone, V. R. 2011. Evaluation of soybean genotypes for resistance to charcoal rot. Online. Plant Health Progress doi:10.1094/PHP-2010-0926-01-RS.


Abstract

Charcoal rot, caused by Macrophomina phaseolina, significantly reduces yield in soybean more than most other diseases in the midsouthern United States. There are no commercial genotypes marketed as resistant to charcoal rot. Reactions of 27 maturity group (MG) III, 29 Early MG IV, 34 Late MG IV, and 59 MG V genotypes were evaluated for M. phaseolina between 2006 and 2008 in a non-irrigated, no-till field that had been artificially infested for three years. There was significant variation in root colonization among genotypes and years, indicating the value of screening genotypes over multiple years. Based on CFUI there was no genotype that was consistently immune to charcoal rot each year. However, there were a total of six genotypes (one genotype in MG III, one in Late MG IV, and four in MG V) that were identified as moderately resistant. Some of the commercial and public genotypes were resistant to M. phaseolina at levels equal to or greater than the standard DT97-4290, a moderately resistant cultivar. The genotypes identified as having moderate resistance across the three years could be useful as sources for developing resistant soybean genotypes.


Introduction

Charcoal rot caused by Macrophomina phaseolina, has been reported to affect more than 500 cultivated and wild plant species (13), including economically important crops such as soybean, cotton, maize, sorghum (3,23), cassava (14), and sunflower (9). In the United States, the estimated soybean yield loss from charcoal rot was approximately 2 million metric tons in 2003 and more than 300,000 metric tons in 2005 (25). Commercially resistant soybean cultivars are not available (11). In sunflower, losses from charcoal rot can reach 60 to 90% if the conditions are favorable for infection (9). In soybean, charcoal rot can be severe when plants are stressed due to inadequate soil moisture. Specifically, severity of charcoal rot has been determined to be significant when air and soil temperatures are high (28 to 35°C), and when soil moisture is limiting (6,18,22), resulting in reduced yield and seed quality (22) (Fig. 1).


 

Fig. 1. Soybean plants killed by charcoal rot.

 

Drought is a persistent problem in non-irrigated soybean production in the United States. (19). Soybean stress due to insufficient moisture occurs most often due to inadequate rainfall, but it may also occur because of restricted root growth. Research is underway to determine the extent of severity of charcoal rot on cultivar performance in stress and non-stress environments and identify lines that may have some level of resistance to both drought and charcoal rot (Mengistu, unpublished data). Pastor-Corrales and Abawi (17) observed that resistance in common beans (Phaseolus vulgaris) to M. phaseolina was associated with drought tolerance.

Charcoal rot incidence has increased throughout the north central and southern regions of the United States. However, charcoal rot suppressed soybean yield more in Arkansas, Illinois, Indiana, Kansas, Kentucky, Missouri, Mississippi, and Tennessee than in the northern states with an estimated yield loss averaging 1.7 × 107 tons per year from 1996 to 2007 (25), making charcoal rot the most damaging soybean disease in the southern United States. Managing charcoal rot in soybean by adjusting planting dates, crop rotation, planting densities, and irrigation have all been suggested as means of control (1,5,7,15,24,26). Other control methods, including fungicide applications to seed and soil and biological control using hyperparasitism, have had limited success in reducing disease severity in other crops (7,20). Management through host plant resistance has been suggested as the only feasible method to prevent yield loss (1,2,21,22). Quantification of microsclerotia by CFUs (Fig. 2) in lower stems and taproot tissues of soybean has been reported to be a reliable method of rating host compatibility between soybean genotypes and M. phaseolina (11,16,21). However, soybean genotypes with high levels of resistance have not been identified.


 

Fig. 2. Colony forming units of Macrophomina phaseolina.

 

Moderate resistance to charcoal rot has been identified (11,16) with the genotypes DT97-4290, DT99-16864, DT99-17483, and DT99-17554, but it is not known if current commercial genotypes and breeding lines have resistance to charcoal rot. By combining genes for different traits, plant breeders have developed new genotypes and breeding lines to meet specific abiotic and biotic constraints. Since the resistance or susceptibility of these developed genotypes and breeding lines is unknown, it was important to compare the level of resistance introduced among genotypes developed by public and private breeders. The objective of this study was to evaluate the reaction of MG III through V genotypes to M. phaseolina and identify lines with high levels of resistance. Identification of soybean genotypes resistant to charcoal rot will be valuable to soybean breeders in planning future parental selections and in developing cultivars that benefit soybean producers.


Identifying Soybean Genotypes with High Charcoal Rot Resistance

Experimental field plots. Field plots were established from 2006 to 2008 at the West Tennessee Research and Education Center at University of Tennessee in Jackson, TN. The soil was a Dexter fine-silty loam (mixed, active, thermicultic hapludalfs). Soybean genotypes in MG III through V including six plant introductions were selected based on their commercial use, high yield in state variety trials, and performance in breeding trials. A total of 149 soybean genotypes consisting of 27 MG III, 29 Early MG IV, 34 Late MG IV, and 59 MG V were evaluated for reaction to M. phaseolina infection. Seeds were treated with 0.8 ml/kg of mefenoxam + fludioxinal + molybeenum (Apron Maxx + Moly, Syngenta, Greensboro, NC) prior to planting. The experiments were conducted in the same area each year and genotypes were planted in four 6.1-m rows spaced 0.7 m using an Almaco 4-row cone planter (model #AJ4RP2, Almaco, Nevada, IA). Within each MG the experimental design was a randomized complete block with three replications. Charcoal rot susceptible controls for each MG included Croton for MG III, LS98-0358 for Early MG IV, Merschman Dallas for Late MG IV, and Pharaoh for MG V (11,16). A moderately resistant genotype, DT97-4290 (11,16) (Late MG IV), was used as a resistant control for MGs Early IV, Late IV, and V tests. No known resistant soybean genotype is available for use as a control standard for MG III.

Planting dates were 13 May, 3 May, and 7 May in 2006, 2007, and 2008, respectively. Weeds were controlled pre-plant using Roundup Weathermax, pre-emergent using Canopy 75 DG and post-emergent using Reflex 2LC and Select 2E labeled herbicides. Plots were not irrigated, and the soil was not tilled. Harvest dates for MG III soybeans were 21 September in 2006 and 2007, and 9 September in 2008. Harvest dates for Early MG IV soybeans were 10 and 11 October and 9 September for 2006, 2007, and 2008, respectively. Harvest dates for Late MG IV soybeans were 6 October in 2006 and 25 September in 2007 and 2008. Harvest dates for MG V soybeans were 10, 11, and 2 October in 2006, 2007, and 2008, respectively.

Air temperature, precipitation, soil temperature, and water potential were measured during the growing season. The water potential measurements were taken with a Watermark soil moisture sensor and gypsum moisture block using a Watchdog Weather Station (Spectrum Technologies Inc., Plainfield, IL). The soil water potential sensors were placed within rows at 15-cm depth and attached to a micro-logger. Sensors were placed in a representative area of the field to record soil water potential on a daily basis. Temperature and precipitation sensors were also attached to a micro-logger and soil temperature was measured at the 5- and 10-cm depths.

Inoculum increase. Japanese millet (Echinochloa frumentaceae L.) seed (400 ml by volume) was soaked for 18 to 20 h in 4 liters of a solution containing distilled water and 40 g of table sugar, 0.5 g of yeast extract, and 0.25 g of tartaric acid per liter (11,12). The solution was decanted, and the millet seeds were divided equally into 12 autoclavable bags. A tube 5 cm in diameter and 10 cm long was inserted halfway into each bag and tied securely with the bag to facilitate the placement of culture plugs into the millet. Cotton plugs were inserted into each tube, and the samples were autoclaved at 121°C for 30 min. One-week-old culture plugs of M. phaseolina, grown on potato-dextrose agar (Difco Laboratories, Detroit, MI) acidified with 5% lactic acid was used to inoculate the millet. Thirty 0.5-cm-diameter plugs were placed into the opening of each bag and reclosed with the cotton plug. The bags were incubated at 30°C for 3 weeks, with periodic agitation to spread the inoculum within the bags. After 3 weeks, the millet was completely colonized and darkened with microsclerotia. The millet was allowed to air dry for ten days at 24°C on a laboratory bench, and applied with seed at planting at a rate of 1.5 g/m.

Determination of M. phaseolina population densities. Crop water deficits in the midsouthern United States usually begin to develop in June and may sometimes continue into September. The relationships among temperature and precipitation from planting to R8 for each year are indicated in Figure 3. At the plant growth stage of R7 (4), 10 randomly selected plants were carefully uprooted from the outside 2 rows of each plot to determine colony forming units (CFU) of the pathogen. Plant samples were excised just below the cotyledonary node. The lower stem sections and roots including lateral and fibrous roots of each plant were thoroughly washed and rinsed in water to remove soil and air dried. The combined root and stem sections from each plot were ground with a UDY cyclone sample mill (Model 3010, UDY Corp., Fort Collins, CO) and passed through a 28-mesh screen (600-μm openings). The mill was thoroughly cleaned between samples with air using a suction device. For each sample, 0.005 g of ground tissue was placed in a Waring blender with 100 ml of 0.525% NaOCl for 3 min and collected over a 45-μm-pore-size sieve. The triturate was washed with sterile distilled water and then added to 100 ml of autoclaved PDA amended with rifampicin (100 mg/liter) and tergitol (0.1 ml/liter) that had been cooled to 60°C (11). After 3 days of incubation at 30°C, M. phaseolina CFUs were counted and converted to CFU per gram of root and stem tissue. In previous studies (11,12), the correlation between disease severity and CFU was significant, so CFU in soybean tissue was used as a measure of disease severity. Each genotype was measured against the performance of each susceptible standard genotype for that maturity group. A colony forming unit index (CFUI) was determined where the CFU of each genotype was divided by the CFU of the check genotype for that maturity group. The genotypes were then classified based on the CFUI as resistant (0 to < 10%), moderately resistant (10 to ≤ 30%), moderately susceptible (> 31 to 60%), and susceptible (> 60%), calculated based on CFU of the susceptible checks from each year, in accordance with the classification system developed for charcoal rot (11).


 

Fig. 3. Total precipitation expressed below each graph and maximum air temperatures for the months of April through October for 2006 to 2008 at Jackson, TN.

 

Data analysis. The colony unit forming index data were analyzed using the analysis of variance (ANOVA) performed using SAS general linear mixed model procedure (SAS Procedures Guide, Version 8, 1999; SAS Institute Inc., Cary, NC), and comparisons made using Fisher’s least significant difference. Genotypes within each MG and year were analyzed separately.

Environmental conditions. The daily maximum air temperatures and precipitation in the 2006, 2007, and 2008 growing seasons at the Jackson, TN, location are shown in Figure 3. When comparing the August maximum air temperature for the three years, the average air temperature for 2007 was 37°C and was 4°C higher than the temperatures of the same month in 2006 and 2008. The 37°C air temperature for August in 2007 was also the highest recorded for any month in all the three years. Precipitation (mm) from May through August showed the differences between years where precipitation for 2007 fell short by 68% and 67% in 2006 and 2008, respectively. The average soil temperatures at 5.0-cm and 10-cm depths in 2007 were 8% and 1.6% higher than in 2006 and 2008, respectively (Table 1). In 2007, the maximum soil temperatures of 33 and 30°C were recorded for the month of August at 5- and 10-cm depths, and were 1 to 11°C and 1 to 10°C higher than in any other month during the three years of research, respectively. The maximum soil water potential, -200 kPa was also recorded in August of 2007. The average water potential during the most critical months of stress in June, July, and August showed the major differences in stress levels between the three years. The water potential for 2007 was 16.2% and 21.1% lower than in 2006 and 2008, respectively. The level of water potential began low and negatively increased as the season progressed making the conditions favorable for charcoal rot disease development (Table 1).


Table 1. Mean monthly soil temperature at 5- and 10-cm depth and soil water potential for May through October in 2006 to 2008 at Jackson, TN.

Year Month Soil temperatures (°C) at depths Soil water
potential

(kPa)
5 cm 10 cm
2006 May 23.4 21.5 -70.0        
June 28.2 26.3 -76.0        
July 30.0 28.0 -136.5        
August 31.2 29.4 -145.8        
September 25.9 25.0 -180.5        
October 20.0 19.1 -153.7        
2007 May 27.2 24.3 -80.5        
June 30.0 27.1 -91.1        
July 31.0 29.0 -136.1        
August 33.2 30.7 -200.0        
September 28.7 27.1 -111.0        
October 22.9 22.0 -54.6        
2008 May 24.5 22.1 -55.0        
June 30.5 28.1 -81.0        
July 32.2 29.9 -92.5        
August 30.6 28.8 -163.8        
September 27.4 26.0 -164.5        
October 25.0 22.9 -137.7        

Reaction of Soybean Genotypes to Charcoal Rot in MGs III Through V

Analysis of variance for CFUI indicated a genotype-by-year interaction (P < 0.05), and genotypes within each MG and year were analyzed separately. M. phaseolina was recovered from lower stems and roots of all genotypes, but there were genotypes with low colonization. Since 2007 was classified as a drought year and therefore most conducive to charcoal rot development, the CFUI for the genotypes are presented in Tables 2 to 5 in ascending order based on CFUI of 2007. The index for susceptible controls used for each MG was based on the mean CFUI across replications each year. The CFUI for susceptible standard controls were generated from: 5.7 × 105, 8.2 × 105, and 8.8 × 105 for Croton (MG III); 3.8 × 105, 7.9 × 105, and 6.4 × 105 for LS98-0358 (Early MG IV), and 4.9 × 105, 7.1 × 105, and 5.1 × 105 for Merschman Dallas (Late MG IV); and 5.3 × 105, 8 × 105, and 5.7 × 105 for Pharaoh (MG V) in 2006, 2007, and 2008, respectively.

Maturity Group III. The mean CFUI for the 27 MG III genotypes was 64, 98, and 62% in 2006, 2007, and 2008, respectively (Table 2). The mean CFUI for 2007 was 34 and 32% higher than the CFUI recorded in 2006 and 2008, respectively. There was no genotype that was considered resistant in all the three years. However, in 2006 and 2008, two genotypes were rated as moderately resistant (MR) while only one had MR reaction in 2007. The genotypes, DG3905 was the only genotype considered MR across the three years with a mean CFUI of 13.


Table 2. Reactions of soybean genotypes in MG III to M. phaseolina in 2006, 2007, and 2008. Colony Forming Unit Index (CFUI) values less than 10 were resistant (R), values between 10 and 30 were moderately resistant (MR), values between 31 and 60 were moderately susceptible (MS), values greater than 60 were susceptible (S).

Cultivar Year
2006 2007 2008
CFUI and Resistance
DG3950 11 MR 20 MR 8 MR
DK3964 43 MS 53 MS 43 MS
DKDK39T6 70 S 56 MS 61 S
DP3861 28 MR 58 MS 53 MS
DKB3652 51 MS 63 S 35 MS
Pioneer93M90 100 S 66 S 52 MS
DK3967 35 MS 69 S 64 S
Pioneer93M42 55 MS 70 S 40 MS
AGV6361 34 MS 72 S 52 MS
GX980609 74 S 72 S 69 S
Armor39P7 62 S 75 S 26 MR
GHH3945 88 S 75 S 58 MS
DKB3852 49 MS 78 S 52 MS
NKS37N4 100 S 81 S 59 MS
DK3968 48 MS 83 S 64 S
CPLRC3935 55 MS 84 S 38 MS
Excel8396 54 MS 84 S 55 MS
Progeny3900 63 S 84 S 74 S
LS980265 100 S 84 S 73 S
V39N4RR 100 S 91 S 56 MS
NKS39K6 58 MS 92 S 34 MS
Magellan 86 S 98 S 66 S
Crotonx 100 S 100 S 100 S
AG3906 53 MS 100 S 64 S
FFR3990 89 S 100 S 51 MS
GHH3606 66 S 100 S 61 S
AG3705 52 MS 100 S 38 MS
LSD (P = 0.05) 25 27 29
F-value   1.20 NS 1.04 NS 1.51*

 x Susceptible control.

 * = significance at P ≤ 0.05; ** = significance at P ≤ 0.01; NS = a non-significant difference.


Early Maturity Group IV. The mean CFUI within the 29 genotypes was 90, 83, and 81 in 2006, 2007, and 2008, respectively (Table 3). The resistant control, DT97-4290, had a mean CFUI of 23, and was the only genotype that had moderate resistance in all years. No other genotype showed resistance in 2006 and 2007 but there was one genotype, RO26185F that was MR in 2008. In 2006 52% were rated susceptible while 66% and 90% of the genotypes were rated susceptible in 2007 and in 2008, respectively.


Table 3. Reactions of soybean genotypes in Early MG IV to Macrophomina phaseolina in 2006, 2007, and 2008. Colony forming unit index (CFUI) values less than 10 were resistant; values less than 10 were resistant (R), values between 10 and 30 were moderately resistant (MR), values between 31 and 60 were moderately susceptible (MS), values greater than 60 were susceptible (S).

Cultivar Year
2006 2007 2008
CFUI and Resistance
DT97-4290x 28 MR 18 MR 22 MR
JTN-4607 92 S 35 MS 76 S
DP3478 79 S 42 MS 84 S
AG4103 97 S 43 MS 97 S
DynaGro3443 74 S 47 MS 81 S
Pioneer94M50 63 S 49 MS 100 S
USG74C36 66 S 52 MS 100 S
UA4805 45 MS 53 MS 96 S
FFR4545 61 S 57 MS 100 S
Excel8427 79 S 58 MS 97 S
DKB4251 39 MS 61 S 74 S
DG4460 50 MS 61 S 80 S
DPLDPX4112 53 MS 63 S 100 S
Armor45M1 58 MS 65 S 81 S
Trisler4254 37 MS 67 S 97 S
DC031175 39 MS 68 S 40 MS
GHH4534 66 S 70 S 74 S
DKB4451 58 MS 71 S 100 S
DC033218 89 S 71 S 100 S
Crows4444 42 MS 72 S 70 S
Progeny4401 55 MS 72 S 76 S
Stressland 66 S 72 S 96 S
V42N3 76 S 77 S 73 S
Merschman Rocky 58 MS 84 S 63 S
DC032033 55 MS 89 S 81 S
RO26185F 61 S 89 S 17 MR
DC032037 76 S 92 S 91 S
GHH4024 42 MS 97 S 83 S
LS980358y 100 S 100 S 100 S
LSD (P = 0.05) 21 50 29
F-value 0.81 NS 1.0 NS 1.83 NS  

 x Resistant control.

 y Susceptible control.

 * = significance at P ≤ 0.05; ** = significance at P ≤ 0.01; NS = a non-significant difference.


Late Maturity Group IV. The mean CFUI for the 34 genotypes was 36, 65 and 48 in 2006, 2007, and 2008 respectively (Table 4). There were 40%, 7% and 33% of the genotypes with MR reactions in 2006, 2007, and 2008 respectively. Over half of the genotypes (67%) in 2007 showed susceptible reaction but lower percentages exhibited susceptible reactions in 2006 (3%) and 2008 (38%). The genotype DC032121 had the lowest CFUI of 0% in 2008. However, only Manokin (CFUI = 18) and DT97-4290 (CFUI = 24) showed a consistent MR reaction in all three years.


Table 4. Reactions of soybean genotypes in Late MG IV to Macrophomina phaseolina in 2006, 2007, and 2008. Colony forming unit index (CFUI) values less than 10 were resistant; values less than 10 were resistant (R), values between 10 and 30 were moderately resistant (MR), values between 31 and 60 were moderately susceptible (MS), values greater than 60 were susceptible (S).

Genotype Year
2006 2007 2008
CFUI and Resistance
Manokin 30 MR 11 MR 13 MR
DT974290x 22 MR 30 MR 20 MR
DP4724 18 MR 32 MS 57 MS
AG4703 18 MR 34 MS 43 MS
NKS49Q9 27 MR 37 MS 52 MS
Progeny4804 29 MR 37 MS 33 MS
DC17879 35 MS 42 MS 13 MR
DC032121 16 MR 44 MS 0 R
Armor47G7 10 MR 48 MS 17 MR
AG4903 39 MS 52 MS 22 MR
Trisler4838 39 MS 58 MS 91 S
AGV46J5 49 MS 61 S 50 MS
USG7475 57 MS 62 S 100 S
V49N6 39 MS 63 S 23 MR
GHH4878 55 MS 63 S 61 S
FFR4886 14 MR 65 S 22 MR
DKxtj749 18 MR 66 S 54 MS
V44N6 55 MS 66 S 61 S
Pioneer94B73 37 MS 68 S 52 MS
DK49D64967 8 R 69 S 67 S
Progeny4805 27 MR 72 S 35 MS
DynaGro36Y48 37 MS 72 S 48 MS
DKDK4764 29 MS 76 S 85 S
DG4840 35 MS 76 S 26 MR
Schillinger495 41 MS 79 S 24 MR
DPLDP4919 35 MS 83 S 61 S
Pioneer94M80 33 MS 85 S 65 S
DK4866 57 MS 85 S 89 S
Excel8493 49 MS 86 S 24 MR
CPLRC4955 16 MR 87 S 26 MR
Stine4842 57 MS 93 S 61 S
Crows4817 31 MS 96 S 100 S
MPV4905n 47 MS 100 S 39 MS
Merschman Dallasy 100 S 100 S 100 S
LSD (P = 0.05) 13 34 19
F-value 1.99** 0.99 NS 1.72*

 x Resistant control.

 y Susceptible control.

 * = significance at P ≤ 0.05; ** = significance at P ≤ 0.01; NS = a non-significant difference.


Maturity Group V. The mean CFUI within the 59 genotypes was 55, 50, and 18% in 2006, 2007, and 2008, respectively (Table 5). There was no genotype that had resistant reaction in 2006, but 2% of the genotypes in 2007 and 51% in 2008 showed resistant reaction. Of particular attention are the genotypes with a CFUI of 0 in 2008. These were DC 3705, PI578441, DT99-16864, and DT99-17483. In addition, 15%, 34%, and 29% of the genotypes showed MR reactions in 2006, 2007, and 2008, respectively. There were four genotypes excluding the MR control DT97-4290 that had MR reaction across the three years and these were DT99-16864 (CFUI = 13), DT99-17483 (CFUI = 13), DT98-7553 (CFUI = 15), and DT99-17554 (CFUI = 15). These genotypes were also reported in an earlier research to have a moderate resistance (11).


Table 5. Reactions of soybean genotypes in MG V to Macrophomina phaseolina in 2006, 2007, and 2008. Colony forming unit index (CFUI) values less than 10 were resistant; values less than 10 were resistant (R), values between 10 and 30 were moderately resistant (MR), values between 31 and 60 were moderately susceptible (MS), values greater than 60 were susceptible (S).

Cultivar Year
2006 2007 2008
CFUI and Resistance
DC4012 31 MS 4 R 2 R
DT974290x 16 MR 10 MR 5 MR
RO3362 97 S 10 MR 2 R
DKxtj753 51 MS 12 MR 4 R
DC3705 57 MS 12 MR 0 R
PI578441 43 MS 13 MR 0 R
DC20293 66 S 15 MR 2 R
OZARK 66 S 15 MR 9 R
DT9916864 23 MR 17 MR 0 R
RO26476F 63 S 17 MR 2 R
DC4524 69 S 17 MR 7 R
DT9917554 22 MR 18 MR 5 R
DT9917483 19 MR 20 MR 0 R
DC20300 80 S 21 MR 2 R
Hornbeck5525 34 MS 23 MR 9 R
Pioneer95M30 60 S 23 MR 5 R
RO1332 69 S 23 MR 4 R
DC2973 31 MS 25 MR 2 R
Progeny5115 31 MS 27 MR 30 MR
DT987553 27 MR 17 MR 2 R
DynaGro33X55 77 S 29 MR 2 R
DC1158 54 MS 31 MS 2 R
FFR5663 37 MS 33 MS 25 MR
DK5567 63 S 35 MS 5 R
MPGExp7552n 46 MS 37 MS 16 MR
Jake 20 MR 38 MS 2 R
DynaGro33B52 49 MS 38 MS 25 MR
PI587585B 100 S 38 MS 47 MS
Excel8509 80 S 40 MS 42 MS
DPLDP5115 57 MS 44 MS 16 MR
DC7816 29 MR 46 MS 14 MR
TN02283 31 MS 46 MS 12 MR
RO1581F 43 MS 52 MS 26 MR
Armor5403 74 S 52 MS 5 R
CPLRC5222 40 MS 54 MS 23 MR
RO152F 69 S 54 MS 2 R
RO1888F 51 MS 60 S 9 R
DC4319 29 MR 63 S 16 MR
PI612609 51 MS 65 S 46 MS
MPV5206n 51 MS 67 S 12 MR
V52N3 74 S 73 S 7 R
USG7515nRS 66 S 77 S 46 MS
DC7726 57 MS 79 S 2 R
DC7844 46 MS 83 S 23 MR
RO26056 46 MS 83 S 14 MR
PI567187 69 S 83 S 96 S
PI594430C 89 S 83 S 5 R
DG5160 100 S 83 S 26 MR
RO21459 100 S 83 S 19 MR
RO1416F 37 MS 88 S 33 MS
GHH5053 60 S 88 S 32 MS
Progeny5250 66 S 94 S 9 R
FFR5033 40 MS 100 S 18 MR
Pharaohy 100 S 100 S 100 S
RO16232F 54 MS 100 S 5 R
PI578335B 100 S 100 S 32 MS
RO1769F 46 MS 100 S 60 S
Stine5142 57 MS 100 S 47 MS
Stoddard 17 MR 100 S 37 MS
LSD (P = 0.05) 16 20 12
F-value 1.49* 1.99** 5.22**

 x Resistant control.

 y Susceptible control.

 * = significance at P ≤ 0.05; ** = significance at P ≤ 0.01; NS = a non-significant difference.


Discussion

The non-irrigated environment was conducive for infection. However, there was a difference in the amount of precipitation received, soil water potential, and soil and air temperatures during the three years. In 2007, the growing conditions were dry and hot from April through August. Less available soil moisture likely contributed to a high level of M. phaseolina CFUs in root and stem tissues. This was in agreement with Kendig et al. (10), where the CFU level was higher in a treatment with soil water stress. The lack of irrigation in this test during the drought period profoundly affected the disease severity of the soybean genotypes from year to year. Genotypes that rated MR or R in all 3 years are obviously the most desirable. However, there are genotypes that were MR or R in two of the three years. Perhaps these genotypes can also be beneficial to growers in some years or environments. These genotypes could be better choices than those ranked as MS or S in all three years. Non-irrigated environments provide less soil moisture for soybean, especially at critical times in the growing season, resulting in an increase in the severity of charcoal rot (8). Faced with unreliable rainfall, producers should grow charcoal rot resistant soybean genotypes with reduced susceptibility to drought.

Cultivar selection for resistance is one of the most important decisions that producers make to maximize productivity. The data on CFUI indicate the significance of evaluating of soybean genotypes over several years. However, decisions made about a cultivar for its disease performance in one year may not indicate its performance in another year as indicated in this research. By comparing the level of resistance from a wide range of environments, growers may have the best opportunity of predicting the next year’s performance. Our results determined that there were six soybean genotypes (DG3905, Manokin, DT99-16864, DT99-17483, DT98-7553, and DT99-17554) that had moderate level of resistance across years under the non-irrigated and no-till environment and could be utilized by breeders as sources of resistance to charcoal rot.


Acknowledgment and Disclaimer

The authors thank D. Boykin for help with data analysis and J. Deffenbaugh and J. Jordan for technical assistance. This research was funded by the USDA, Agricultural Research Service, Project number 6401-21220-002-00D, The Tennessee Soybean Promotion Board, the United Soybean Board, the North Central Soybean Research Program and Project No. 11-287-J from the Kansas Agricultural Experiment Station, Manhattan, KS.

Mention of trade names or commercial products is solely for the purpose of providing specific information and does not imply recommendation or endorsement by the United States Department of Agriculture (USDA). USDA is an equal opportunity provider and employer.


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