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CROP BREEDING, GENETICS & CYTOLOGY

Comparison of Potato Leafhopper-Resistant and Susceptible Alfalfa in New York

^a Dep. of Plant Breeding, 252 Emerson Hall, Cornell Univ., Ithaca, NY 14853-1902
^b New York State Integrated Pest Management Program, Cornell Univ., Ithaca, NY 14853-1902
^c Dep. Of Agronomy and Plant Genetics, Univ. of Minnesota, 420 Borlaug Hall, 1991 Upper Buford Circle, St. Paul, MN 55108

* Corresponding author (jlh17{at}cornell.edu)

	ABSTRACT

TOP NOTES ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS AND DISCUSSION CONCLUSIONS REFERENCES

 
Potato leafhopper [PLH; Empoasca fabae (Harris)] is the most damaging pest of alfalfa (Medicago sativa L.) in midwestern and eastern states. Our objective was to compare alfalfa populations with glandular-hairs (PLH-resistant) and susceptible populations for forage yield and quality, PLH damage, and PLH nymph and adult populations. Potato leafhopper-resistant and susceptible alfalfa populations were planted in New York (NY) plot trials in 1997, 1998, and 1999, and compared seeding year through second production year. When PLH populations exceeded the action threshold for NY, PLH-resistant populations had lower PLH damage scores (2.3 vs. 3.7), fewer number of nymphs per 10 stems (3.5 vs. 6.2), and fewer adult PLHs per five sweeps (9.1 vs. 18.1) than susceptible populations. However, PLH-resistant populations yielded more than susceptible populations only in one of four trials. Irrespective of PLH damage level, PLH-resistant populations had higher crude protein (CP) concentration (203 vs. 194 g kg^-1) and matured earlier than susceptible populations. Seeding year trials were severely damaged by PLH. The following year at first harvest, PLH-resistant populations yielded more than susceptible populations (0.24 Mg ha^-1 more in 1997 trial, and 0.61 Mg ha^-1 more in 1999 trial), even though PLHs were not causing economic damage. Resistant populations that were not treated with insecticide averaged lower total season yields than insecticide-treated, PLH-susceptible populations (0.29 Mg ha^-1 less in seeding year, 0.95 Mg ha^-1 less in production yrs). Potato leafhopper-resistant populations provide benefits to growers such as higher CP concentration and less hopperburn, but do not provide complete protection against yield loss from PLH damage.

Abbreviations: ADF, acid detergent fiber • CNY, central New York • CP, crude protein • NDF, neutral detergent fiber • NY, New York • NIRS, Near infrared reflectance spectroscopy • PLH, potato leafhopper • WNY, western New York

	INTRODUCTION

TOP NOTES ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS AND DISCUSSION CONCLUSIONS REFERENCES

 
POTATO LEAFHOPPER is the most damaging insect pest of alfalfa in midwestern and eastern states (Manglitz and Ratcliffe, 1988). Potato leafhopper feeding injury symptoms include yellowing of leaf tips (known as hopperburn), stunting, reduced biomass, and decreased leaf protein concentration (Hutchins and Pedigo, 1989; Hutchins et al., 1989; Ecale and Backus, 1995; Lefko et al., 2000a).

Potato leafhopper is a migratory insect, and source populations develop in the Gulf Coast and southeastern states (Taylor and Shields, 1995). Factors affecting the arrival time and development rate of PLH in the northeastern USA include weather patterns, temperature, and host plant species availability (Taylor and Shields, 1995). In NY, PLH damage on alfalfa can be expected annually. However, severity of infestations is variable across years and counties. From 1988 through 2000, 3 to 83% (mean 30%) of acreage monitored in County Cooperative Extension and Crop Management Association IPM programs reached the action threshold for PLH (J.K. Waldron, personal communication, 2000). Weekly monitoring of fields is recommended from early June through late August because forage yield and quality losses from PLH damage occur before yellowing is visible on foliage (Undersander et al., 1991). Insecticide treatment is recommended when PLH densities exceed action thresholds (Cornell Cooperative Extension, 2001). Potato leafhopper-resistant alfalfa cultivars that minimize yield loss from PLH injury would reduce insecticide costs. Hutchins et al. (1989) reported that PLH feeding does not reduce nutrient levels in alfalfa forage, but rather results in large decreases in biomass and total nutrient yield.

Potato leafhopper-resistant alfalfa cultivars with glandular hairs on leaves and stems became commercially available in 1997. Glandular-haired cultivars have been studied to determine the resistance mechanism. Elden and McCaslin (1997) reported that the glandular hairs were associated with a substance antibiotic to PLH. Furthermore, Hogg et al. (1998) found that nymph populations were reduced by 40 to 50% in both large and small plots of PLH-resistant cultivars. However, PLH produced nymphs when caged on heterogeneous populations of both resistant and susceptible cultivars (Lefko et al., 2000b). Thus, an antibiotic resistance mechanism was not important under field conditions when PLHs were caged on resistant alfalfa. Hogg et al. (1998) and Lefko (1999) concluded that in a production field, nonpreference is not likely the resistance mechanism since the number of adult PLHs on resistant alfalfa was similar to that on susceptible alfalfa. Rather, PLH-resistant cultivars tolerated high levels of PLH by having more nodes, longer internodes, longer stems, and less hopperburn than the PLH-susceptible cultivars (Lefko et al., 2000a). Additionally, PLH-resistant alfalfa matured 7 to 10 d earlier than susceptible alfalfa.

The PLH-resistant cultivars released in 1997 yielded 10 to 20% more than PLH-susceptible cultivars when PLH infestations were high, and yielded 2 to 8% less than PLH-susceptible cultivars when PLH infestations were low or controlled with insecticides (McCaslin, 1998; Miller, 1998). Lefko et al. (2000b) found lower yield loss rates from PLH for resistant cultivars than for susceptible cultivars in the seeding year, second cutting, and later years and harvests, but not in the seeding year, first cutting when PLHs were caged on alfalfa in the field. Hogg et al. (1998) reported that PLH-resistant cultivars in trials in Ohio and Indiana (southern locations) yielded significantly more than susceptible cultivars, whereas in Wisconsin and Minnesota (northern locations) the yield difference was close to zero or was significantly less. Yield differences between PLH-resistant and susceptible cultivars sometimes were less than expected based on visual ratings of PLH damage (Miller, 1998). Cultivars resistant to PLH tended to have lower regrowth and fall growth, lower persistance, and higher relative feed value (McCaslin, 1998). Other researchers have not compared PLH-resistant and susceptible cultivars for forage quality components. Much of the research on PLH-resistant alfalfa has focused on deciphering the resistance mechanism and developing management recommendations for midwestern states. Our objective was to compare alfalfa populations with glandular-hairs (PLH-resistant) and susceptible populations for forage yield and quality, PLH damage, and PLH nymph and adult populations in NY.

	MATERIALS AND METHODS

TOP NOTES ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS AND DISCUSSION CONCLUSIONS REFERENCES

 
Alfalfa populations were compared under natural levels of PLH populations and insecticide-controlled levels of PLH. Alfalfa trials were planted in western NY (WNY) at Clarendon in 1997, and in central NY (CNY) at Ithaca in 1997, 1998, and 1999. Across these trials, there were a total of 22 alfalfa populations that had glandular hairs and were claimed to have some resistance to PLH, and 17 populations were susceptible to PLH and did not have glandular hairs (Table 1). The PLH-resistant populations were ones submitted for testing in the unsprayed alfalfa trials in the Cornell Forage Yield Testing Program. Companies that submitted PLH-resistant populations in the 1997 trials usually submitted more recently developed PLH-resistant populations in the 1998 and 1999 trials. In general, the PLH-resistant populations seeded in 1998 and 1999 were bred for higher levels of PLH resistance than those seeded in 1997 (first released populations). The seeding rate was 20.2 kg ha^-1. The trial in WNY was planted on 24 Apr. 1997, and the field was prepared by the producer. This trial was planted on a Middlebury gravelly loam (coarse-loamy, mixed, active, mesic Fluvaquentic Eutrudepts). The trial sites in CNY were fertilized with 336 kg ha^-1 10-20-20 after plowing, and were planted on 7 May 1997, 30 Apr. 1998, and 3 May 1999. The 1997 CNY trial was planted on a Hilton loam (fine-loamy, mixed, active, mesic Glossic Hapludalfs) and the 1998 and 1999 CNY trials were planted on a Dalton Channery silt loam (coarse-silty, mixed, active, mesic Aeric Fragiaquepts). Weeds were controlled with herbicides as needed.

Yield was determined by harvesting the plots with a flail harvester. Plot size harvested was 1 m by 4 m. Approximately 25% of the plots were subsampled for dry matter determinations. When plots were damaged by PLH, dry matter determinations from PLH-resistant populations were applied to surrounding PLH-resistant populations, and dry matter determinations from susceptible populations were applied to surrounding susceptible populations. Plots were harvested 12 wk after seeding and then at 6- to 7-wk intervals thereafter (Table 2). The 1997 CNY trial was not harvested for a second production year due to variable forage growth caused by factors unrelated to PLH. The 1998 CNY trial was not harvested in the seeding year because PLH damage was not evident, and volunteer buckwheat (Fagopyrum esculentum Moench) was not controlled by herbicides and grew throughout the trial.

Before each harvest where PLH damage was present, plots were rated for PLH damage using the recommended rating scale of 1 to 5, where 1 is no apparent injury, 2 is very minor stunting and yellowing, 3 is moderate stunting with yellowing evident on 20 to 40% of leaves, 4 is significant stunting and yellowing on 40 to 60% of leaves, and 5 is severe stunting and yellowing on 60 to 100% of leaves (McCaslin and Miller, 1998). Samples for forage quality and PLH nymph counts were taken six times. Samples were taken on the same day that the plots were harvested for yield (Table 2). Nymph counts were determined by carefully cutting 10 stems per plot 90 mm above the ground, and placing them into plastic bags. The bags were transported to a 5°C cooler. Nymph counts were determined 2 d later by counting the total number of nymphs per 10 stems. In addition, populations were evaluated for adult PLHs by taking five net sweepings per plot from three randomly selected, unsprayed replicates. In 1998, adult PLH count per five sweeps data were collected 4 wk after second harvest in CNY and 10 d before third harvest in WNY. In 1999, adult PLH count per five sweeps data were collected 5 d before harvest in seeding year and first production year trials (Tables 3 and 4).

For forage quality analyses, samples of 300 to 500 g fresh weight were hand-harvested from each plot just before harvest and dried for 7 d at 55°C. The dried samples were weighed, ground through a 3-mm screen, subsampled, and then reground through a 1-mm screen. Near infrared reflectance spectroscopy (NIRS) data were collected. Random samples were chosen for wet-lab analyses for NIRS calibration in 1997 and 1998. In 1999 and 2000, calibration samples for wet-lab analyses were chosen with the WINISI II (FOSS NIRSystems, Silver Spring, MD)¹ software package. Calibration samples were analyzed for neutral detergent fiber (NDF), acid detergent fiber (ADF), and lignin using the Van Soest Sequential Fiber Method (Van Soest et al., 1991), and for CP as Kjeldahl N x 6.25. Regression equations relating near infrared wavelengths and laboratory data were developed for the 1997 and 1998 data and had R² values for CP, NDF, ADF, and lignin of 0.98, 0.97, 0.98, and 0.95, respectively. Regression equations developed for the 1999 data had R-squared values for those same components of 0.98, 0.95, 0.93, and 0.83, respectively. The correlation coefficients between quality data using the equation developed in 1999 and those for the legume hay equation purchased with the NIRS were: 0.95 for CP, 0.90 for NDF, and 0.91 for ADF. Thus, in 2000, quality data were predicted using the legume hay equation.

Analyses of variance and contrast estimates were computed for each harvest using PROC MIXED in SAS with replicates as random effects and populations as fixed effects (Littell et al., 1996). The contrasts computed were the average of PLH-resistant populations vs. the average of susceptible populations for each harvest/insecticide treatment. Additionally, for the trials planted in 1998 and 1999, the mean of the PLH-resistant, untreated populations was compared with the mean of the susceptible, insecticide-treated populations. Seeding and production year data for each of four trials were not combined across years because the insect pressure was variable across years. The trials were not combined because the populations varied across trials.

	RESULTS AND DISCUSSION

TOP NOTES ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS AND DISCUSSION CONCLUSIONS REFERENCES

 
In CNY, the precipitation as a percentage of the historical mean was 84% in 1997, 90% in 1998, 58% in 1999, and 89% in 2000. In both 1998 and 2000, just more than 50% of the total precipitation for June, July, and August fell in June. In addition, PLH damage to alfalfa was less severe in 1998 and 2000 than in 1997 and 1999. Weather conditions in WNY were similar to those in CNY. A drought in 1999 resulted in low alfalfa yields.

Seeding Year Data
Potato leafhopper populations in 1997 went above the action threshold at first harvest in CNY (Table 3). The PLH-susceptible populations were stunted and the leaves were chlorotic. The PLH-resistant populations averaged 1.7 units or 34% lower PLH damage score, 0.50 Mg ha^-1 higher yield, and 3.5 fewer PLH nymphs per 10 stems than susceptible populations (P < 0.01). Similarly, Miller (1998) reported a yield advantage from a PLH-resistant cultivar of 0.4 to 2.0 Mg ha^-1 across susceptible cultivars in the seeding year. Yield and PLH damage score were negatively but moderately correlated (r = -0.34, n = 100, P < 0.01). The PLH-resistant populations averaged 5 g kg^-1 higher CP and higher fiber concentrations (ADF, NDF, and lignin), and matured earlier than susceptible populations (P < 0.01). Forage regrowth was reduced during a late summer drought and was not harvested.

At WNY, PLH populations went above the action threshold during the second growth, but not before first harvest. The PLH population was above the action threshold 3 wk after first harvest on 31 July, but then dropped below threshold by 13 August (2 wk before second harvest). The PLH-resistant populations averaged 0.1 unit or 2% lower PLH damage score than the susceptible populations at first harvest (P < 0.05) and 2.4 units or 48% lower PLH damage score at second harvest (P < 0.01). Yield differences between the PLH-resistant and susceptible populations were not statistically significant at both first and second harvest. Yield differences between PLH-resistant and susceptible populations may have been detected at second harvest if the trial had been harvested earlier, before the plants had time to recover from the PLH damage earlier in the growth cycle.

The CNY trial planted in 1999 was harvested once in the seeding year, and yields were low because of drought. The insecticide-treated plots had a mean PLH damage score of 2.4 (Table 3). Thus, the treated plots were only partially protected and there were PLH on the trial before or between insecticide treatments. The insecticide-treated plots were sprayed 7 wk after seeding and again 3 wk later.

The insecticide-treated plots averaged 1.2 units or 24% lower PLH damage score, 4.3 fewer nymphs, 30.7 fewer adult PLH per five sweeps (P < 0.01), and 0.27 Mg ha^-1 more yield (not statistically significant) than the untreated plots (Table 3). So even though the untreated plots were damaged from PLH feeding, these plots did not yield significantly less than the treated plots. Thus, it seems that the drought had a larger effect on reducing yields than did the PLH. Schroeder et al. (1988) reported that both drought stress and PLH feeding on alfalfa increased proline concentration, and they suggested that the plant response to both stresses may be related and partially additive. In our research, the most visible difference between the insecticide-treated and untreated plants was that the former averaged 44 ± 16% of stems with seed pods per plot compared with an average of 5 ± 7% for the untreated plants (P < 0.01). It seems the untreated plants were more water stressed than the insecticide-treated plants, and the flowers probably aborted.

Within the untreated plots, the PLH-resistant populations had 1.1 units or 22% lower PLH damage score, 3.6 fewer nymphs, and 18.1 fewer adult PLHs per five sweeps than the susceptible populations (P < 0.01, Table 3). Within both insecticide-treated and untreated plots, the PLH-resistant populations did not yield significantly more than the susceptible populations. Within insecticide-treated plots, the PLH-resistant populations averaged 18% (P < 0.01) more stems with seed pods per plot than the susceptible populations (52 ± 11% vs. 34 ± 15%). Within untreated plots, the PLH-resistant populations averaged 8% more stems with seed pods per plot than susceptible populations (9 ± 7% vs. 1 ± 1%, P < 0.01). The PLH-resistant populations probably were earlier to mature than the susceptible populations, and in the untreated plots, were less damaged by PLH and could set more seed pods than the susceptible populations.

Within the insecticide-treated plots, the PLH-resistant populations averaged 9 g kg^-1 higher CP concentration (P < 0.01), 7 g kg^-1 lower NDF, 7 g kg^-1 lower ADF, and 2 g kg^-1 lower lignin (P < 0.05) concentration than the susceptible populations (Table 3). Within the untreated plots, the PLH-resistant populations averaged 21 g kg^-1 higher CP, 13 g kg^-1 lower NDF, and 3 g kg^-1 lower lignin concentration than the susceptible populations (P < 0.01). Within an insecticide treatment, the PLH-resistant populations had higher CP and lower fiber than did the susceptible populations, similar to findings by McCaslin (1998). The differences between PLH-resistant and susceptible populations were larger in the untreated plots probably because the susceptible populations were stunted and chlorotic from PLH injury.

Production Year Data
When PLH levels were above the action threshold, the PLH-resistant populations averaged from 0.5 units or 10% lower PLH damage score than the susceptible populations to 1.4 units or 28% lower damage score (P < 0.01) (Table 4). Similar to the 1999 seeding year trial, the PLH-resistant populations in the insecticide-treated plots at second harvest in 1999 had significantly lower PLH damage scores than the susceptible populations. Ideally, these scores should not have differed. Apparently, insecticide applications only partially protected plots from PLHs.

For the second harvest in the untreated plots, PLH-resistant populations had significantly fewer nymphs and adult PLHs than susceptible populations in two out of three comparisons [0.5 fewer nymphs per 10 stems (P < 0.01) and 0.5 fewer adult PLHs per five sweeps (P < 0.05) in the 1997 CNY trial in the first production year, and 4.2 fewer nymphs per 10 stems and 17.1 fewer adult PLHs per five sweeps in the 1998 CNY trial in the first production year (P < 0.01)]. In the second production year of the 1998 trial, nymph differences between PLH-resistant and susceptible populations were not observed even though resistant populations had 0.8 units or 16% lower PLH damage score than susceptible populations. This may have been due to a heavy rain (60 mm) in the middle of July, which could have reduced nymph populations before sampling. Adult PLH count data were not collected in this trial.

In trials and main plots planted in CNY in 1997 and 1999 that were not treated with insecticide, the resistant populations yielded more than the susceptible populations at first harvest in the first production year following severe PLH damage in the seeding year [0.24 Mg ha^-1 more in 1997 trial (Table 4) and 0.61 Mg ha^-1 more in 1999 trial (Table 5)]. At first harvest (17 June for 1997 trial and 19 June for 1999 trial), adult PLHs were not found in the plots. The yield difference was likely a carryover effect from PLH damage in the seeding year, similar to results reported by Hogg et al. (1998). Furthermore, plot color notes (Score 1 = light green, Score 2 = medium green, Score 3 = dark green) taken in spring 1998 on the 1997 CNY trial indicated that PLH-resistant populations were significantly darker green (average 2.0 ± 0.1, range 1.7–2.0) than susceptible ones (average 1.4 ± 0.2, range 1.2–1.9) (P < 0.01). The PLH damage score in 1997 was negatively correlated with spring color score in 1998, whereby populations with lower PLH damage scores in 1997 were darker green in 1998 (r = -0.87, n = 100, P < 0.01). Color differences were not observed in spring 2000 in the 1999 CNY trial.

For 1999 in CNY, PLH damage scores for PLH-resistant and susceptible populations not treated with insecticide averaged 3.4 units, whereas for 1998 and 2000 averaged 1.6 (Table 4). Similar to the seeding year trials, all the plots in 1999 suffered from lack of soil moisture, and the PLH-resistant and susceptible populations did not differ statistically for yield at any harvest.

When insects were controlled in 2000, no statistically significant differences in yield were found between the more recently developed PLH-resistant populations and PLH-susceptible populations at first and second harvest and across the total season (Table 5). Thus, it seems that yield potential of the more recently developed PLH-resistant populations has improved compared with that of the first PLH-resistant populations released in 1997. In 2000, the third harvest was taken late in the growing season, on 22 September. The PLH-resistant populations yielded less than did the susceptible populations at third harvest in the insecticide-treated plots in both the 1998 CNY trial (second production year) and the 1999 CNY trial (first production year) (Tables 4 and 5). Because the third harvest was delayed until much later than usual, the third harvest yield differences may be a fall dormancy response where the resistant populations are more dormant than the susceptible populations.

Within both insecticide-treated and untreated CNY plots, the resistant populations consistently had higher CP concentrations at second harvest than the susceptible populations [range from 4 g kg^-1, (P < 0.05) to 13 g kg^-1 (P < 0.01), Table 6]. However, fiber concentrations at second harvest were not significantly different between resistant and susceptible populations. Furthermore, the resistant populations generally matured earlier than the susceptible populations, a trait which is usually associated with lower forage quality. Further research is necessary to determine plant architecture features that result in higher CP concentration in PLH-resistant populations. The 1997 WNY trial was sampled at third harvest in 1998, and PLH-resistant populations had significantly lower fiber concentrations and were earlier in maturity than the susceptible populations (P < 0.01).

	CONCLUSIONS

TOP NOTES ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS AND DISCUSSION CONCLUSIONS REFERENCES

 
Alfalfa populations with glandular hairs and some resistance to PLH sustained significantly less visible PLH damage, supported fewer nymphs and adult PLHs, had higher CP concentration, and matured earlier than susceptible populations across a series of years, weather conditions, and varying PLH populations in NY. Despite these advantages in forage appearance and quality when PLH populations were above the action threshold, the PLH-resistant populations yielded significantly more than the susceptible populations in only one out of seven comparisons. Furthermore, PLH-susceptible populations that were treated with insecticide yielded significantly more than PLH-resistant, untreated populations at harvest when there was moderate to severe PLH damage. Results for total season yields of PLH-resistant populations compared with susceptible populations were not consistent. Factors other than PLH that likely influenced total season yields were drought, fall dormancy, and year of PLH-resistant population release. These results indicate that the current PLH-resistant cultivars do not provide complete protection against yield loss due to PLH. However, following severe PLH damage in the seeding year, yields of PLH-resistant populations were higher than susceptible populations at first harvest in the first production year. Agronomically acceptable PLH-resistant cultivars for NY need to be developed that can withstand yield loss associated with moderate to severe PLH feeding damage. When insecticides are not used to control PLH, currently available PLH-resistant cultivars provide benefits to growers such as higher CP concentration and less hopperburn.

	ACKNOWLEDGMENTS

 
We thank Nate Herendeen for arranging the WNY trial site on the Christensen family farm, and for gathering sweep data in WNY.

	NOTES

TOP NOTES ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS AND DISCUSSION CONCLUSIONS REFERENCES

 
This research was supported in part by the New York State Integrated Pest Management Program and by seed companies who paid fees to enter germplasm in Cornell yield trials.

¹ Use of specific products does not constitute an endorsement or recommendation by Cornell University and does not imply approval to the exclusion of other suitable ones.

Received for publication June 14, 2001.

	REFERENCES

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