Vol. 29, No. 4, 1996

Soybean cyst nematode (SCN) is widely distributed in Kentucky's soybean growing areas. The use of SCN-resistant varieties has long been a recommended production practice for infested fields. However, continuous use of such varieties can result in a shift to a race of SCN which is able to vigorously attack previously resistant varieties. For his reason, many states have long recommended that producers periodically grow a crop of SCN-susceptible soybeans within a crop rotation when SCN populations are too low to cause yield reductions. In Kentucky, the UK Plant Pathology Department recommends a four year rotation in SCN-infested fields [PPA3; "Wanted: Soybean Cyst Nematode" (video)]. Year one should be a nonhost crop (like corn), followed by an SCN resistant soybean variety, then another nonhost crop (corn or milo). Producers would grow an SCN-susceptible soybean variety in the fourth year of this rotation. This is, of course, provided that the three previous years have brought SCN populations down to a safe level. In the 1990s, some Kentucky producers reported good yields from SCN-susceptible Maturity Group (MG) II varieties in heavily infested fields. If predictable, the use of SCN-susceptible MG II varieties would give soybean producers another option in their effort to manage SCN. Thus, the goal of this research was to determine if SCN-susceptible MG II varieties can produce better yields than MG IV, SCN-susceptible varieties under SCN pressure in Kentucky fields. MATERIALS AND METHODS We planted four high-yielding varieties from each of the following classes: 1) MG II, SCN-resistant 2) MG II, SCN-susceptible 3) MG IV, SCN-resistant, and 4) MG IV, SCN-susceptible. These 16 varieties were planted on May 31 and June 20 on the Darren Luttrell farm in Ohio County in 1995. The soil was a Melvin/Newark intergrade. The site was in corn in 1994, but had shown moderate-to-high SCN levels after susceptible soybean in 1993. Conventional tillage was done prior to each planting date. Weed control was accomplished using a post emergence treatment of bentazon, fluazifop, and fomesafen. Plots were six 15 inch rows wide by 20 feet long. Initial SCN egg counts were determined from seven samples of the surface six inches of soil taken between the two middle rows of each plot immediately after planting. The nematicide aldicarb (Temik) was applied on the date of planting at a broadcast rate of 20 pounds/A on one-half of the SCN-susceptible plots. Thus, each susceptible variety was present in each of the four replications both with and without Temik. This treatment is labelled for band application in both Kentucky and Ohio. We broadcasted Temik in an effort to document that SCN was, in fact, responsible for measurable yield losses of SCN-susceptible varieties. Note: we did not apply Temik to any plots of SCN-resisant varieties. We measured canopy closure at both R1 (beginning flowering) and R5 (beginning seed fill), mature plant height, and lodging. The four central rows of each plot were harvested with a small plot combine as each MG dried down. After harvest, we took a final SCN egg count (using the same techniques as for the initial count). Egg count data are shown below as final:initial ratios to make clear the change in SCN activity as the growing season progressed (Table 1). A final:initial egg count ratio greater than 1 indicates that SCN were multiplying during the season. In contrast, a finial:initial egg count ratio less than one indicates that SCN numbers actually declined during the season. All data were statistically analyzed as follows: we first compared all 16 resistant and susceptible varieties without Temik (using a split plot analysis with planting dates as whole plots and varieties as split plots). Then, we also compared the 8 susceptible varieties, both with and without Temik (using a split plot analysis with planting dates as whole plots and combinations of susceptible varieties and Temik treatments as split plots). RESULTS Data shown in Table 1 were averaged across planting dates. The averages of the 4 varieties of each class are included to more easily compare these classes. Initial egg counts were somewhat variable and on the low side of what we desired to force yield reductions (data not shown). Final egg counts were generally lower for resistant varieties than for susceptible varieties in both MGs (data not shown). For all eight resistant varieties, the final:initial egg count ratio was less than one, indicating a decline in egg numbers over the season; conversely, the susceptible varieties all showed increased egg numbers by the end of the season (Table 1). As expected, Temik applications did not significantly reduce the final:initial egg count ratio for susceptible MG II or IV varieties (Table 1). It is well documented that Temik is active only for the first six weeks after planting. The lack of significance was due to great variability in initial egg counts from plot to plot. However, careful examination of the data in Table 1 shows a trend for Temik to reduce final:initial egg count ratios of susceptible MG II varieties more than that of susceptible MG IV varieties. Perhaps the longer growing season of the MG IV varieties allowed the Temik to degrade more fully and the nematodes to thus replenish their numbers. In general, best yields (49.0 bu/A) were produced by MG IV varieties with SCN resistance (Table 1). Susceptible MG IV varieties averaged 44.4 bu/A as did resistant MG II varieties. Susceptible MG II varieties had the lowest average yield at 41.9 bu/A. Thus, growing susceptible MG IV varieties instead of resistant MG IV varieties cost 4.6 bu/A and using susceptible MG II varieties instead of resistant MG II varieties cost 2.5 bu/A. In spite of overall lower performance by MG II varieties, two of the individual varieties, Jack (SCN resistant) and Pioneer 9273 (SCN susceptible), still looked quite competitive with MG IV resistant and MG IV susceptible varieties, respectively. For example, Jack yielded 50.9 bu/A as compared to an average of 49.0 bu/A for resistant MG IVs. Pioneer 9273 yielded 47.2 bu/A as compared to an average of 44.4 bu/A for susceptible MG IIs. Furthermore, in the case of Pioneer 9273, growers could use an SCN-susceptible MG II variety to reduce the chances of an SCN race shift without reducing yield potential from that of susceptible MG IV varieties . Thus, careful variety selection may allow Kentucky growers to utilize MG II SCN-resistant or susceptible varieties in their soybean cropping systems. We also compared the response of SCN-susceptible MG II and IV varieties to a single Temik application applied just after planting (Table 1). Seven of the eight SCN-susceptible varieties showed increased yields when treated with Temik. We cannot explain why the lone exception (Pioneer 9273) had lower yields with Temik than without Temik. The remaining varieties gained from 1 to 4 bushels/A when treated with Temik, indicating that SCN losses were modest. There appears to have been a slight trend for Temik to help susceptible MG IV varieties more than susceptible MG II varieties in terms of yield (Table 1). Canopy closure at R1 did not differ among MG II varieties (Table 2). Since flowering naturally occurs later for MG IV varieties than for MG II varieties, the MG IVs had greater canopy closure ratings. Pioneer 9451 and Caverndale Farms 492 both lagged behind the other MG IV's in canopy closure: however, that did not seem to hurt their yield performance. Temik aplications did not significantly increase canopy closure for MG II varieties, but did significantly increase canopy closure for MG IV varieties (Table 2). Temik apparently acts as a growth promoter, even in the absence of nematode pressure, according to its vendor (Rhone-Poulenc). Thus, Temik appears to have helped vegetative growth of the MG IV susceptible varieties more than that of MG II susceptible varieties. Plant height was quite variable within MG (Table 2). The shortest MG IIs (for example, MWS 210 CN) may cause concerns about low podding heights. In contrast, the shortest MG IV (Caverndale Farms 492) had only modest harvest losses as a check variety in our 1995 preliminary MG II variety trials. Finally, taller plants do not necessarily produce higher yields. For example, Pioneer 9273, one of the shorter MG II varieties, recorded one of the highest yields (Table 2). By maturity, Temik had only a modest influence on plant height of susceptible varieties (Table 2). As with canopy closure, the increase in plant height tended to be more for MG IV than for MG II varieties. CONCLUSIONS On average, resistant MG IV varieties outyielded their susceptible MG IV counterparts (49.0 to 44.4 bu/A), and resistant MG II varieties outyielded their susceptible MG II counterparts (44.4 to 41.9 bu/A). However, the top resistant MG II variety (Jack) and the top susceptible MG II variety (Pioner 9273) produced yields similar to the best MG IV varieties tested. Thus, carefully selected MG II varieties may give Kentucky growers another option to consider in their fight against the nematode. The Temik treatment of susceptible varieties was included to assess how much loss the nematodes were inflicting on such susceptible varieties. Yield gains of susceptible varieties were small (1 to 4 bu/A) under Temik treatment, indicating that, overall, SCN pressure was moderate in this Ohio County field. This Temik rate was of course, not an economically viable SCN control option. In 1996, we plan to identify a more heavily infested nematode site to more effectively test how well MG IIs can perform as compared to MG IVs in an on-farm situation. Table 1. Response of SCN-resistant and SCN-susceptible MG II and IV soybean varieties to nematode pressure and Temik applications on the Darren Luttrell farm in Ohio County in 1995. SCN Ratio (f/i) Yield (bu/A) Variety name MG rxn w/oTemik w/Temik w/oTemik w/Temik MWS 210 CN II rest. 0.43 -- 41.1 -- Wilken 2571 II rest. 0.37 -- 42.6 -- Callahan 892311-04N II rest. 0.16 -- 42.9 -- Jack II rest. 0.23 -- 50.9 -- Average of 4 MG II resistant varieties 0.30 -- 44.4 -- ---------------------------------------------------------------------------- Asgrow A2396 II susc. 5.53 2.21b 38.6 39.5c Ciba 3253 II susc. 4.55 3.39 39.8 43.8 Pioneer 9273 II susc. 1.94 1.84 47.2 44.1 Lynks 5298 II susc. 6.99 1.30 42.1 43.1 Average of 4 MG II susceptible varieties 4.75 2.19 41.9 42.6 ---------------------------------------------------------------------------- Delsoy 4210 IV rest. 0.50 -- 45.4 -- Pioneer 9451 IV rest. 0.22 -- 49.4 -- Asgrow A4715 IV rest. 0.83 -- 48.9 -- Pioneer 9481 IV rest. 0.73 -- 52.1 -- Average of 4 MG IV resistant varieties 0.57 -- 49.0 -- ---------------------------------------------------------------------------- SS FFR-439 IV susc. 2.39 4.82 42.3 44.6 So. Cross Jacob IV susc. 7.10 3.05 45.1 46.1 So. Cross Joshua IV susc. 5.33 6.40 44.3 48.4 Caverndale Farm. 492 IV susc. 3.93 7.53 46.1 49.4 Average of 4 MG IV susceptible varieties 4.69 5.45 44.4 47.1 ---------------------------------------------------------------------------- LSD(0.10)a 3.04 3.12 5.0 3.3 aFor comparing varieties within a Temik treatment. bThe LSD(0.10) for comparing variety ratio response to Temik application was not significant. cThe LSD(0.10) for comparing variety yield response to Temik application was 4.7. Table 2. Canopy closure at R1 and mature plant height for MG II and IV soybean varieties as influenced by Temik aplication on the Darren Luttrell farm in Ohio county in 1995. SCN R1 Canopy closure (%) Mature plant ht. (in.) Variety name MG rxn w/o Temik w/Temik w/o Temik w/Temik MWS 210 CN II rest. 26 -- 20.5 -- Wilken 2571 II rest. 23 -- 29.0 -- Callahan 892311-04N II rest. 26 -- 27.1 -- Jack II rest. 23 -- 33.7 -- ----------------------------------------------------------------------------------- Asgrow A2396 II susc. 23 28a 23.0 25.8b CIBA 3253 II susc. 24 30 22.4 24.9 Pioneer 9273 II susc. 24 29 23.3 22.7 Lynks 5298 II susc. 24 26 25.1 26.6 ----------------------------------------------------------------------------------- Delsoy 4210 IV rest. 42 -- 36.3 -- Pioneer 9451 IV rest. 29 -- 31.7 -- Asgrow A4715 IV rest. 37 -- 34.5 -- Pioneer 9481 IV rest. 47 -- 39.4 -- ----------------------------------------------------------------------------------- S. States FFR-439 IV susc. 41 52 34.4 37.1 South. Cross Jacob IV susc. 38 46 32.1 34.7 South. Cross Joshua IV susc. 43 54 36.3 38.7 Caverndale Farm. 492 IV susc. 33 54 20.4 24.1 ------------------------------------------------------------------------------------ LSD(0.10)a 6 4 2.8 1.6 aFor comparing varieties within a Temik treatment. bThe LSD(0.10) for comparing variety canopy closure response to Temik application was 6. cThe LSD(0.10) for comparing variety plant height response to Temik application was 2.3.