Agronomy Journal 92:721-725 (2000)
© 2000 American Society of Agronomy
ALFALFA
Symptoms and Growth of Potato Leafhopper-Tolerant Alfalfa in Response to Potato Leafhopper Feeding
Stephen A. Lefkoa,
Larry P. Pedigob and
Marlin E. Riceb
a Monsanto Co., 3100 Sycamore Rd., DeKalb, IL 60115 USA
b Dep. of Entomology, Iowa State Univ., Insectary Bldg., Ames, IA 50011-3140 USA
stephen.a.lefko{at}monsanto.com
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ABSTRACT
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Alfalfa (Medicago sativa L.) cultivars marketed as resistant, or tolerant, of the potato leafhopper (Empoasca fabae [Harris]) are increasingly available, yet their mechanism of resistance is poorly understood. Our objective was to determine if growth characteristics differ between potato leafhopper–tolerant and susceptible alfalfa cultivars under different pest pressures, and how differences may contribute to a greater yield potential in tolerant cultivars. Two field experiments, one near Ames, IA, and one near Chariton, IA, were planted with four potato leafhopper–tolerant alfalfa cultivars and one susceptible cultivar during the spring of 1996. Estimates of alfalfa dry matter, stem length, number of nodes, and hopperburn were taken on each harvest date, and adult potato leafhopper density in each cultivar was determined regularly over the 3-yr study. Potato leafhopper density was significantly greater in the susceptible cultivar compared with one or more tolerant cultivars in only two of the 15 growth intervals sampled. Although there was only one harvest when dry matter differed significantly between alfalfa types, there was a trend for tolerant alfalfa to produce more dry matter than the susceptible alfalfa when the leafhopper pressure was high. Tolerant cultivars had more nodes, longer internodes, longer stems, and less hopperburn than the susceptible cultivar when the leafhopper number was high. Alfalfa cultivars tolerant of potato leafhopper feeding were not immune to this pest; however, they had favorable growth characteristics that could translate to greater yields when the leafhopper density is high.
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INTRODUCTION
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ALFALFA SERVES AS HOST to an abundance of phytophagous insect species. One of these, the potato leafhopper, is an economically important pest of alfalfa in the North-Central and northeastern United States. This ubiquitous pest is an assimilate remover, and its symptoms in alfalfa are stem stunting and leaf chlorosis, commonly called hopperburn (Manglitz and Ratcliffe, 1998).
Economic loss from potato leafhopper is best linked to reductions in alfalfa biomass, but less so to forage quality (Hutchins et al., 1989). Reductions in biomass caused by the leafhopper result from shorter stems, not reduced stem density or reduced leaf mass (Faris et al., 1981; Hower and Flinn, 1986; Oloumi-Sadeghi et al., 1988; Hutchins and Pedigo, 1989). Hutchins and Pedigo (1989) found that high leafhopper density reduced the number of nodes per stem, and this reduction may have contributed to smaller leaf area indices in leafhopper-stressed alfalfa. They also suggested that reduced stem length may provide less opportunity for alfalfa leaves to compete with weeds for photosynthetically active radiation, and that this may favor the growth of weed species and reduce the life of an alfalfa stand.
In 1997, several seed companies sold alfalfa that was marketed as resistant to, or tolerant of, the potato leafhopper. Leafhopper-resistant alfalfa gained attention in the 1980s when Sorensen et al. (1985, 1986) and Shade and Kitch (1986) released perennial alfalfa germplasm with multiple pest resistance. Releases were referred to as glandular-haired alfalfa; however, the mechanism of resistance has not thoroughly been described. Some studies have shown that potato leafhopper densities are usually similar in tolerant and susceptible alfalfa types (Lefko et al., 1997; Hogg et al., 1998). Lefko (1999) described this result using a stand-tolerance concept as follows: The density of potato leafhopper is similar between tolerant and susceptible alfalfa because a similar number of leafhoppers can survive on a fraction of the alfalfa stand that is most preferable. Biomass may be similar between stand-tolerant alfalfa and susceptible alfalfa because injury is uneven and limited to fewer plants in the stand. Compensation by less favorable plants (Hutchins and Pedigo, 1989; Hutchins et al., 1990) and an inverse relationship between injury per individual and number of individuals per stem (Womack, 1984; Hower and Flinn, 1986) are two explanations for reduced loss in stand-tolerant alfalfa. This type of tolerance differs from Painter's (1951) definition that dealt with an individual plant's response to insect feeding. In the same study, it was described how tolerant alfalfa could suffer a yield loss from potato leafhopper; however, the potential for loss was significantly less than in susceptible alfalfa. Besides this study, there are few reports of yield benefits from leafhopper-tolerant alfalfa. Hogg et al. (1998) showed that tolerant cultivars had slightly lower yields than susceptible cultivars when the leafhopper density was low, and a yield advantage when the leafhopper density was high and populations were left unmanaged.
To date, there are no reports of how the growth and developmental characteristics of leafhopper-tolerant alfalfa differ from susceptible alfalfa when under stress from potato leafhopper. An explanation of these characteristics may help researchers understand the potential for these new cultivars to outperform susceptible alfalfa under high potato leafhopper stress. Our objectives were to (i) compare the symptoms of potato leafhopper injury in tolerant and susceptible alfalfa cultivars, and (ii) determine if alfalfa developmental characteristics affected by the leafhopper differ between tolerant and susceptible cultivars.
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Materials and methods
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Two study areas, one near Ames, IA (41°59' N, 93°38' W), and one near Chariton, IA (41°00' N, 93°18' W), were planted with four potato leafhopper–tolerant (glandular-haired) alfalfa cultivars and one susceptible cultivar. These locations were chosen based on environmental differences and histories with different pest populations (Lefko, 1999). Tolerant cultivars included AmeriGuard 301, Trailblazer, 5347LH, and XAE49, an experimental line. The susceptible control was `645', an alfalfa that, historically, has produced high yield test results in Iowa. The five cultivars were arranged in a randomized complete block design with four replications. Both locations were planted during the fourth week of April 1996 and treated with EPTC (S-ethyl dipropylthiocarbamate) before planting to reduce competition from weeds. Each plot measured 7.6 by 6.1 m and was planted using a single-row planter. Plot dimensions were determined by the limited availability of seed. Rows were spaced 20 cm apart, and the planter was calibrated to deliver 0.19 g seed m-1 (9.5 kg ha-1). The plots at both sites were located within 0.6-ha fields of `Defiant', an alfalfa cultivar that is susceptible to potato leafhopper.
Alfalfa was harvested twice during 1996 and three times during 1997 at each location. Alfalfa was harvested three times during 1998 at Chariton; however, inclement weather delayed the second harvest and prevented a third harvest at the Ames location. Alfalfa was harvested at the 0.1-bloom stage of development based on a visual estimate, and the cutting height was approximately 5 cm. Dates of alfalfa harvest are given in Table 1
.
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Table 1 Average dry matter estimates ±SE (grams) and corresponding average density ±SE (no. per 12 sweeps) of adult potato leafhopper (PLH) in alfalfa treatments. Results are arranged by location and harvest date
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Estimates of alfalfa dry matter, stem length, number of nodes, and hopperburn were taken on each harvest date. Dry matter sampling programs were different between 1996 and subsequent years. Rows of alfalfa were distinct during 1996; therefore, dry matter was estimated by sampling a randomly selected 0.5 m of row in each plot. In 1997 and 1998, rows were less distinct, so dry matter was estimated from four 0.35-m2 quadrats randomly placed in each plot. After stem length, node number, and hopperburn measurements were taken, the samples were bagged and dried at 60°C for 72 h for dry matter determination.
In 1996, stem length, node number, and hopperburn measurements were taken from all stems collected for dry matter estimates. In 1997 and 1998, the same data were collected from five randomly selected stems in each quadrat (20 stems per plot, 80 stems per cultivar). Stem length was measured from the clipped end to the terminal end, which was usually a bud. Nodes were counted from the clipped end to the last node with a fully expanded trifoliolate leaf. Internodal distance was calculated for each stem by dividing length by the node number. A 1 to 5 hopperburn scale was used to visually estimate the severity of this symptom. The scale was: 1, no hopperburn; 2, no more than 1/3 of leaves with yellowing; 3, between 1/3 and 2/3 of leaves with yellowing; 4, more than 2/3 of leaves with yellowing; and 5, more than 2/3 of leaves with yellowing and some with necrotic tissue.
Insect sampling methods were similar in all three years. Potato leafhopper adults were sampled using a 38-cm-diam. muslin sweepnet. A sampling unit consisted of 12 pendulum sweeps of the alfalfa canopy per plot. Sweep samples were taken lengthwise through plots, with sampling lanes within plots alternating with sampling date. Insect samples were bagged and kept frozen until they were processed. The number of sampling dates during each regrowth interval at each location is given in Table 1.
The density of potato leafhopper adults and nymphs was compared among cultivars according to growth intervals. Data from all insect sampling dates during the growth interval were pooled in the analysis of variance tests. Means were separated using Fisher's protected
(SAS Inst., 1990).
Stem length, node number, internodal distance, and hopperburn were compared among cultivars under two different pest density regimes. Potato leafhopper density data from Ames on 8 July 1996, Chariton on 10 July 1996, and Ames on 1 July 1997 represented the high pest density regime (Table 1). The number of sampling units (stems) for cultivars used in the high-density regime were: AmeriGuard 301,
; Trailblazer,
; 5347LH,
; XAE49,
; and 645,
. The average pest density ranged from 2.2 to 5.0 potato leafhoppers per sweep for all cultivars during these growth intervals. Stem data from these three growth intervals were pooled for each cultivar and used for the high pest density regime.
The potato leafhopper density data from Ames on 22 May 1997, Chariton on 29 May 1997, and Chariton on 19 May 1998 represented the low pest density regime (Table 1). Two hundred forty stems were sampled for each cultivar on each of these dates, and stem data were then pooled among dates for each cultivar. These pooled data sets were used as the low pest density regime. Separate analysis of variance tests were used to determine differences in stem characteristics among cultivars within the high and low pest density regimes. If the analysis of variance was significant
, means were separated using
. Alfalfa yield typically declines in a 7:5:3 ratio with each harvest of a three-cutting system (Fick et al., 1988). The differences in stem growth associated with this decline precluded comparisons of stem variables between pest density regimes (growth periods) for individual cultivars. This was due to the low pest density regime consisting of combined data from initial spring growth and the high regime consisting of data from summer growth intervals (Nelson and Smith, 1968).
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Results and discussion
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Overall, the density of adult potato leafhopper was similar among all alfalfa cultivars throughout the 3-yr study at both locations (Table 1). Of the 60 comparisons among the susceptible cultivar and tolerant cultivars, there were only five comparisons with significant differences. The susceptible cultivar (645) had a density significantly greater than all four tolerant cultivars during the second growth interval of 1998 at Ames, and the density in the susceptible cultivar was significantly larger than in XAE49 during the second growth interval of 1997 in Chariton (Table 1). There was only one instance when the quantity of dry matter differed significantly between alfalfa types. More dry matter was produced in 645 compared with AmeriGuard 301, Trailblazer, and 5347LH during the growth period at Chariton, IA, that ended 2 July 1997. The leafhopper density was moderate in all alfalfa cultivars during that interval. Although the difference was not significant, estimates of dry matter in the susceptible cultivar often exceeded the dry matter produced in tolerant cultivars when the leafhopper number was low. Dry matter results should not be used to extrapolate estimates of alfalfa yield per production area, because the sampling units were small.
Findings reported by Hogg et al. (1998) showed that the potential for tolerant alfalfa to produce more dry matter per unit area than susceptible alfalfa increases as the leafhopper density increases. Differences between alfalfa types in the number of nodes, stem length, internodal distance, and hopperburn may explain most of this relationship. In both pest density regimes in this study, there was no growth interval during which the number of leafhoppers differed significantly among cultivars. There were differences in the average number of nodes among alfalfa cultivars under both high and low pest density regimes (Fig. 1)
. Under the low-density regime, there were significantly fewer nodes in Trailblazer, 5347LH, and the susceptible 645 compared with AmeriGuard 301 and XAE49. Although statistically significant, these differences are all less than one node and are probably biologically unimportant. Under the high pest density regime, there were significantly fewer nodes in the susceptible alfalfa compared with all tolerant cultivars. Tolerant cultivars had as many as 1.3 more nodes per stem compared with the susceptible control. The ability of potato leafhopper–induced injury to reduce the number of nodes was demonstrated earlier by Hutchins and Pedigo (1989). They found as many as 2.2 more nodes on alfalfa not stressed by the leafhopper compared with alfalfa that was stressed by 200 leafhoppers m-2 1 d after harvest. Additional nodes could increase the trifoliolates per plant over the course of development. These changes would increase the cumulative leaf area and perhaps increase the plant's growth rate, dry matter production, or forage quality.
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Fig. 1 Average number of nodes ±SE in cultivars under low and high potato leafhopper density regimes. Within pest density regimes, means labeled with the same letter are not significantly different according to Fisher's protected
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Internodal distance is a function of stem length and number of nodes, and there were differences among cultivars under both pest density regimes. Under the low-density regime, 645 and 5347LH had internodal distances significantly greater than the other three cultivars (Fig. 2)
. Under the high-density regime, 5347LH and XAE49 had internodal distances greater than all other cultivars. These differences are small, but meaningful because the number of nodes per stem ranged from 8 to 10, and as already described, all tolerant cultivars produced significantly more nodes under the heavy pest density regime. The ability of tolerant alfalfa to maintain longer internodal distances (resist stunting) compared with the susceptible alfalfa contributed to its dry matter advantage when the leafhopper number was high.
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Fig. 2 Average length of internodes ±SE in cultivars under low and high potato leafhopper density regimes. Within pest density regimes, means labeled with the same letter are not significantly different according to Fisher's protected
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The additional nodes and longer internodal distances likely were the primary causes for large differences in stem length among tolerant and susceptible cultivars under both pest density regimes. At low pest densities, 645 had an average stem length significantly greater than three of the four tolerant alfalfa cultivars (Fig. 3)
. Again, this helps explain why dry matter tended to be higher in the susceptible alfalfa when the leafhopper number was low (Table 1). Conversely, under the high-density regime, three tolerant alfalfa cultivars had average stem lengths greater than the susceptible alfalfa (Fig. 3). Several studies have shown that leafhopper density directly affects alfalfa stem length and that stem length, which is the plant component most responsible for variation in dry matter production (Oloumi-Sadeghi et al., 1988; Hutchins et al., 1989; Hutchins and Pedigo, 1989). Considering the density of potato leafhopper was similar among alfalfa cultivars, tolerant alfalfa has the potential to grow longer stems than susceptible alfalfa when both are under high and similar leafhopper stress. This characteristic explains some of the increased yield potential of tolerant compared with susceptible alfalfa when under leafhopper stress (Hogg et al., 1998; Lefko, 1999).
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Fig. 3 Average stem length ±SE for cultivars under high and low potato leafhopper density regimes. Within pest density regimes, means labeled with the same letter are not significantly different according to Fisher's protected
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Comparisons of hopperburn under the high pest density regime showed the hopperburn score in the susceptible alfalfa was dramatically greater than scores of all tolerant cultivars (Fig. 4)
. There were no significant differences in the average hopperburn score among tolerant cultivars; however, it is important to note that tolerant alfalfa did show the hopperburn symptom. It is apparent that tolerant cultivars resisted the hopperburn symptom more than the susceptible cultivar, because the potato leafhopper density was similar among alfalfa types during these regrowth intervals. This trait was identified earlier by Jarvis and Kehr (1966), who screened 75 alfalfa clones for resistance to potato leafhopper symptoms. They found that equal numbers of nymphs did not cause the same degree of hopperburn on all clones. Similarly, Kindler and Kehr (1973) screened alfalfa clones and found variable levels of alfalfa resistance to hopperburn and a poor relationship between hopperburn and final alfalfa yield.
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Fig. 4 Average hopperburn score ±SE for cultivars under the high potato leafhopper density regime. Means labeled with the same letter are not significantly different according to Fisher's protected
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Reducing the potential for hopperburn could increase the yield and the forage value of tolerant alfalfa compared with susceptible alfalfa under the same leafhopper stress. Fuess and Tesar (1968) found that leaf-drop due to natural leaf senescence was increased by 1.2 Mg ha-1 when the harvest maturity was delayed from 10 to 100% bloom. The hopperburn symptom looks similar to natural leaf senescence, and we observed that hopperburned leaves tended to drop from stems before healthy-looking leaves. Leaf-drop increases greatly after alfalfa is cut and the moisture content begins to decline. Therefore, alfalfa that resists hopperburn may maintain more leaves and increase total crop yield and forage value.
Data on alfalfa phenology was not collected during this study; however, observations were made on maturity (days to harvest) for both alfalfa types. Tolerant alfalfa matured 7 to 10 d earlier than susceptible alfalfa after the initial seeding year and during growth intervals when the leafhopper density was particularly high (Table 1, 1 July 1997, Ames). It is conceivable that this resistance trait could shorten the alfalfa growth interval enough in areas where the leafhopper is a perennial pest that some producers normally limited to three cuttings could obtain four cuttings.
In summary, the tolerant cultivars used in this study were not immune to the potato leafhopper; however, they had advantages over susceptible alfalfa when the leafhopper density was high. Tolerant cultivars showed less hopperburn and stunting compared with a leafhopper-susceptible cultivar. Alfalfa's ability to resist these symptoms could increase forage productivity through higher forage yield and quality and decreased developmental time. Moreover, the advantages detected in these first commercial releases likely will increase as alfalfa breeders increase the level of tolerance to potato leafhopper.SAS Institute 1990
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ACKNOWLEDGMENTS
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We thank Pioneer Hi-Bred International, Inc., for funding this research, and Forage Genetics and America's Alfalfa (ABI) for providing alfalfa seed and loan of equipment. This work could not have been completed without the technical help from Anna Dierickx, Kendra Dvorak, Ryan Gesner, Rene Hoffmann, Kelli Lotz, Eng Han Low, Michael Nagel, Brad Russel, and Robert Yaklich. We would also like to thank Jim Secor and Dave Starrett of Iowa State University for their help with alfalfa production.
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NOTES
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Iowa Agric. and Home Economics Exp. Stn. paper no. J-18256.
Received for publication March 15, 1999.
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REFERENCES
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TOP
ABSTRACT
INTRODUCTION
