Tolerance Studies
Tolerance Study S. graminum
Statistically significant differences were detected among the grasses for the FPLI index based on plant biomass with S. graminum, for both infestation levels (Fig. 1; five aphids: F = 8.13; df = 3, 72; P < 0.0001; ten aphids: F = 8.53; df = 3, 72; P < 0.0001). The FPLI based on plant biomass was highest for the susceptible sorghum check at both infestation levels; however, K×S was not significantly different from sorghum at either the 5- or 10-aphid infestation level. Summer had significantly lower FPLI values at both infestation levels than sorghum, as well as K×S at the higher infestation level. Kanlow had significantly lower FPLI values than any of the other grass treatments; however, as demonstrated by the results for antibiosis, that can likely be attributed to its strong antibiotic response. Thus, FPLI values for Kanlow were deemed to be skewed, and are not presented.
Significant differences in the FPLI index based on plant height were also detected among the grasses evaluated for the two S. graminum infestation levels (Fig. 2; five aphids: F = 6.65; df = 2, 54; P < 0.003; ten aphids: F = 7.88; df = 2, 54; P = 0.001). Similar to the FPLIs for biomass, FPLI values based on plant height were highest for the susceptible sorghum check and K×S, with no significant differences detected between sorghum and K×S at either infestation level. Summer had significantly lower FPLI values at both infestation levels than sorghum and K×S. Collectively, the FPLI values based on plant biomass and plant height indicate the presence of tolerance in Summer to S. graminum.
Tolerance Study S. flava
Significant differences in the FPLI index based on plant biomass for S. flava were also detected among grasses for both levels of infestation (Fig. 3; five aphids: F = 9.80; df = 3, 71; P < 0.0001; ten aphids: F = 12.09; df = 3, 71; P < 0.0001). The susceptible sorghum had the highest FPLI values for S. flava at both infestation levels. The FPLI values for Summer were not significantly different from sorghum at either infestation level, indicating a lack of tolerance. The mean FPLI value for K×S at the 5-aphid infestation level was significantly lower than both Summer and sorghum. For the 10-aphid infestation level, the FPLI value for K×S was significantly lower than sorghum; however, it was not significantly different from Summer. Again, Kanlow had the lowest FPLI values among grasses; however, it is not presented since the low FPLI values were considered to be a product of its strong antibiotic effect.
Significant differences in the FPLI based on plant height were also detected among treatments for both S. flava infestation levels (Fig. 4; five aphids: F = 16.59; df = 3, 72; P < 0.0001; ten aphids: F = 21.40; df = 3, 72; P < 0.0001). At the 5-aphid infestation level, sorghum had a mean FPLI value significantly higher than both Summer and K×S. However, the mean FPLI was significantly lower for K×S than Summer at the lower infestation level. Sorghum also produced the highest FPLI value at the 10-aphid infestation level; however, it was not significantly different from Summer at that infestation level. No significant difference was detected between K×S and Summer for FPLI at the high infestation level, however the FPLI value for K×S was significantly lower than sorghum.
Antibiosis Studies
Antibiosis Study S. graminum
Significant differences were detected among the three switchgrass populations and sorghum for the mean number of S. graminum at both infestation levels (Table 1; five aphids: F = 12.23; df = 3, 72; P < 0.0001; ten aphids: F = 7.05; df = 3, 72; P = 0.0003). The susceptible sorghum cultivar BCK60, included in this evaluation as a check, consistently had the highest mean number of S. graminum among the grasses tested at all time points and infestation combinations. However, at the 14-day and 10-aphid infestation level, no significant differences were detected among any of the switchgrasses and sorghum for mean aphid numbers, despite sorghum supporting at least twice as many aphids as any switchgrass population. This was likely the result of the large variation among replications for that treatment combination. Also, aphid counts were generally skewed among the susceptible grasses within the 14-day 10-aphid infestation level due to aphid populations overwhelming the susceptible plants, resulting in the reduction of plant quality and subsequent decline in aphid numbers. Thus, that treatment combination was found to be less informative than others.
Table 1 Mean ± SEMa number of S. graminum among switchgrass populations and sorghum at 7 and 14 days after initial introduction of five or ten aphids
At the 7-day time point and 5-aphid infestation level, K×S was not significantly different from the susceptible sorghum, with mean aphid numbers (±SEM) of 34.2 ± 5.2 and 45.2 ± 4.6, respectively. Further, K×S consistently supported the most S. graminum among the three switchgrass populations tested at all treatment combinations, and had significantly more aphids than both Kanlow and Summer at both 7 and 14 days for the 5-aphid infestation level. The mean S. graminum (±SEM) for K×S was 38.3 ± 7.6 at the 7-day 10-aphid infestation level; however, it did not support aphid numbers that were significantly higher than Summer, which had 28.5 ± 6.8 aphids.
The mean number of S. graminum among switchgrass populations was consistently the lowest for Kanlow at all treatment combinations. Kanlow supported significantly fewer aphids than Summer and K×S within both the 5-aphid and 10-aphid infestation levels at the 7-day evaluation, with mean aphid numbers (±SEM) of 7.3 ± 3.6 and 8.2 ± 1.8, respectively. Kanlow also had significantly fewer aphids than K×S at the 14-day 5-aphid infestation level, with mean aphid numbers (±SEM) of 4.4 ± 2.5 and 51.8 ± 24.3, respectively, for the two populations. Although no significant differences were detected among any of the grasses at the 14-day 10-aphid infestation level, Kanlow supported a mean aphid number (±SEM) of 1.3 ± 0.7; less than one-tenth of the aphids supported by the next lowest population, Summer, with 14.7 ± 6.8 mean aphids.
Antibiosis Study S. flava
Significant differences were also detected among switchgrass populations and sorghum for the mean number of S. flava at both infestation treatment levels (Table 2; five aphids: F = 14.63; df = 3, 72; P < 0.0001; ten aphids: F = 9.95; df = 3, 72; P < 0.0001). A significant interaction between treatment and infestation level (F = 3.03; df = 3, 72; P < 0.035), and treatment and time (evaluation date after infestation; F = 6.13; df = 3, 72; P < 0.001) was also detected. Results for the mean aphid numbers at 7 and 14 days after infestation were similar between the S. graminum and S. flava evaluations; however, the relative rank of K×S and Summer was generally exchanged between the two studies. The susceptible sorghum check was consistently among the highest of all grasses for the mean number of S. flava at most time points and infestation combinations. At 7 days after aphid introduction, sorghum had significantly higher mean aphid numbers at both the 5-aphid and 10-aphid infestation levels than all switchgrass populations. However, at the 14-day mark, sorghum was not significantly different from Summer for the 5-aphid infestation level, or Summer and K×S for the 10-aphid infestation level.
Table 2 Mean ± SEMa number of S. flava among switchgrass populations and sorghum at 7 and 14 days after initial introduction of five or ten aphids
When considering the 5-aphid infestation level, Summer had significantly more S. flava than all other switchgrass populations at both the 7-day and 14-day evaluations, with mean aphid numbers of 28.1 ± 6.1 and 110.9 ± 24.5, respectively. For the 10-aphid infestation level, Summer was not significantly different from K×S at either time point; however, both had significantly greater mean aphid numbers than Kanlow at both the 7-day and 14-day evaluations.
Similar to the results for S. graminum, Kanlow consistently had the lowest mean aphid numbers for S. flava as well. Although Kanlow was not significantly different from K×S for the 5-aphid infestation level at either time point, K×S had nearly a twofold higher mean aphid number (±SEM) than Kanlow at the 7-day evaluation (11.2 ± 2.7 and 6.3 ± 3.2, respectively), and over a threefold difference at the 14-day mark (33.4 ± 7.8 and 10.7 ± 5.3, respectively). For the 10-aphid infestation level, Kanlow produced significantly fewer aphids at both evaluation dates than Summer and K×S. Further, for the 10-aphid infestation level and both evaluation dates, the mean number of aphids for Kanlow was less than one-sixth of those for either of the other populations of switchgrass.
Cumulative Aphid Days
Cumulative aphid days were also significant at both 5-aphid and 10-aphid infestation levels with S. graminum (Table 3; five aphids: F = 27.19; df = 3, 72; P < 0.0001; ten aphids: F = 17.20; df = 3, 72; P < 0.0001). Generally, CAD for S. graminum supported the results for mean aphid numbers at 7 and 14 days after aphid introduction. At both the low and high infestation level, the susceptible sorghum check was significantly higher than any of the switchgrasses with mean CADs (±SEM) of 998.9 ± 133.4 and 883.8 ± 116.5, respectively. Although not significantly different from Summer at the 10-aphid infestation level, K×S produced the largest response among the three switchgrass populations, with mean CADs (±SEM) of 614.0 ± 170.7 and 412.7 ± 76.8 at the 5-aphid and 10-aphid infestation levels, respectively. The mean CAD response for Kanlow was significantly lower than both Summer and K×S at both 5-aphid and 10-aphid infestation levels. Kanlow had a mean CAD (±SEM) of 73.0 ± 31.6 at the 5-aphid infestation level and 70.1 ± 10.7 at the 10-aphid infestation level. Overall, the mean CADs for Kanlow were less than one-half of those for Summer and K×S at both infestation levels. No significant differences were detected between infestation levels within the switchgrass populations and sorghum (F = 0.98; df = 3, 72; P = 0.41).
Table 3 Mean ± SEMa cumulative aphid days (CAD) over duration of the study for switchgrass populations and sorghum infested with S. graminum and S. flava (5- and 10-aphid infest levels)
Significant differences were also detected among the grasses for CADs at both the 5-aphid and 10-aphid infestation levels with S. flava (Table 3; five aphids: F = 14.26; df = 3, 72; P < 0.0001; ten aphids: F = 20.99; df = 3, 72; P < 0.0001). Over the duration of the experiment, Summer sustained the highest number of S. flava for both the 5-aphid and 10-aphid infestation levels among switchgrasses. For the 5-aphid infestation level, Summer had a CAD (±SEM) of 1,694.2 ± 310.5, which was significantly higher than any other switchgrass. Summer also had the highest CAD for the 10-aphid infestation level, with a CAD (±SEM) of 2,471.7 ± 268.8; however, that was not significantly different from K×S within the same level, which had a CAD (±SEM) of 1,763.4 ± 266.2. Kanlow produced the lowest CADs for both the 5-aphid and 10-aphid infestation levels, with mean CADs (±SEM) of 198.5 ± 86.5 and 283.9 ± 132.0, respectively. At both infestation levels, CADs for Kanlow were significantly less than both K×S and Summer. Further, Summer sustained CADs that were more than eightfold higher than the CADs for Kanlow within both infestation levels.