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Superintendent
David Davis
21262 Genoa Road
Linneus, MO 64653
Phone: 660 895-5121
FAX: 660 895=5122
Email: DavisDK@missouri.edu

Forage Systems Update
Vol 12, No. 2

We are interested in the interactions of potassium fertilization and genetic resistance to potato leafhoppers, an important insect pest, on yield and persistence of alfalfa. Our objectives are to 1) determine effects of K-fertilization on leafhopper tolerance, 2) measure effect of glandular hairs on leafhopper tolerance, and 3) evaluate interactions between K-fertilization and glandular hairs on leafhopper resistance and alfalfa persistence.

Plots were planted in fall 2001 at the Forage Systems Research Center on a Lagonda silt loam that slopes to an Armstrong silty clay loam after liming and applying P to soil test needs. Four replicates of two alfalfa varieties, one normal variety without glandular hairs, the other (PLH resistant) had glandular hairs that deter potato leafhoppers (Fig. 1). Alfalfa varieties were the main plots. Sub-plots were 1) no insecticide (control), 2) insecticide applied at economic threshold (IPM standard), and 3) insecticide applied regularly (scheduled spray). Insect treatment areas were subdivided into K treatments of 0, 125, and 250 lbs/acre, half applied after the first harvest and half applied after the fourth harvest in mid-September. All plots received P (75 lbs/acre) after the fourth harvest.

The alfalfa stands (Fig. 2) looked good for both varieties in spring, 2002 when the plant density was determined (Table 1). Plants within nine 0.1 x 0.1m quadrats placed on the ground were counted on April 1. We noted that plant density of the PLH-resistant variety was about 30% lower than the normal variety. The seed of the PLH-resistant variety was coated (adding 30% in weight) so the pure live seed planted per unit of "seed" weight was also 30% lower. One advertised advantage of coating is that germination and seedling survival are improved, so the same seeding rate should give a similar stand. That did not occur here, and the perceived value of the seed coating was not realized. This result should be tested further under Missouri conditions.

Plant density was determined again after the first and fourth harvests using a sod cutter to undercut strips from each plot (Fig. 3), after which plants were lifted from the strips and counted (Table 1). The densities after the first harvest were greater than in April suggesting that more plants had emerged, but this was not the case. Earlier we became aware of the difficulty in discerning if a "plant unit" as seen from above ground is, in fact, a single plant or consists of two or more closely spaced plants that form an intermingled crown.

Previous experiments at the Southwest Center had indicated that the above-ground counting method consistently underestimated the correct plant density as often two, three and, in some cases, up to five plants grew very close together to form a plant unit. This shows the value in the sod cutter methodology for this study (Fig. 4). We are further researching the relationship between above ground counts and those from the sod cutter.

Similar to the April count, plant densities after the first and fourth harvests did not differ due to the K or leafhopper treatments, but as in April, the PLH-resistant variety had about 30% fewer plants than the normal variety. We counted stems per plant allowing us to calculate stems per sq.ft. (Table 1). We calculated the weight (yield) per stem using yield data for the fourth harvest. Despite the 30% lower plant density, by mid-September the crowns of the PLH-resistant variety had expanded to have only 8% fewer stems per sq.ft., mainly because the lower plant density was offset by having 23% more stems per plant. In other studies we found that alfalfa plants in stands with low densities tend to develop larger crowns and support more stems. We expect stems per sq.ft. to stabilize at about 35 for both varieties until plant density is reduced to about four plants per sq.ft., after which the crown spread will not offset the continued plant loss, and stem density and yield will begin to decrease. Fertilizing with K generally increases crown spread.

We expected little effect on plant density due to K fertilization or leafhopper control this early in the experiment as the young plants are vigorous and are thinning mainly due to plant competition for light. As the stands thin, however, the ability of the crown to spread or compensate becomes compromised due to K deficiency, and detrimental effects from leafhoppers should become more evident.

Alfalfa Yields in 2002

Alfalfa yields were not affected by K fertilization rate in 2002 (Table 2), mainly because only half the K was applied after first harvest, and it did not have a major influence during the lower yielding periods of summer. The rest of the K was applied after the last (fourth) harvest. We expect to see the K response next year as nutrient removal continues to decrease soil K levels. Our earlier research shows that K stimulates crown development, especially over winter.

Leafhopper Populations

Leafhoppers had not arrived at time of the first harvest, but after cutting the population of adults and nymphs increased rapidly (Fig. 5). With no-spray (control) the PLH-resistant variety had only 35 to 62% as many leafhoppers as did the normal variety indicating the glandular hairs deter adults and their egg laying and interfere with mobility and feeding by the nymphs. Some oscillations occurred in the leafhopper populations, mainly due to weather events. Similar to most years few leafhoppers were found during regrowth after the third harvest. Most of the changes in population were due to adults that made up about 84% of the total in the sweeps.

The scheduled spray treatment did an excellent job of leafhopper control (Fig. 5). Leafhopper populations had increased to exceed the IPM threshold only 7 days after the first harvest (small plants have a low threshold), the time of year when there is most leafhopper influx and egg laying. The single insecticide application in the IPM treatment controlled the leafhoppers until the second harvest date. The IPM threshold for the control variety was reached 13 days after the second harvest and was high again at 24 days, but not above the threshold. Therefore, the number of sprays was reduced from four with the scheduled spray to only two when based on IPM.

The PLH-resistant and the normal variety differed in the no-spray treatment during the second harvest when populations were high. Yield of the second harvest was 0.67 tons/acre for the normal variety with no spray compared with 0.88 tons/acre for the PLH-resistant variety with no spray, showing the value of the glandular hairs. Leafhopper populations in the third growth developed slower and did not reach a high level or affect yield (both were 0.87 tons/acre).

Goals for the Future

We plan to continue the experiment because long-term persistence is the goal. The K effect will likely be expressed in 2003 if leafhopper populations are high. We will use the sod cutter to help count alfalfa plants in both spring and fall. Forage yield will be measured. Stems per plant will be counted after the first and fourth cuttings to monitor development and spread of the crowns.

Figure 1. Electron micrograph of a leafhopper among glandular hairs on an alfalfa leaf. The hairs are multi-cellular extensions of epidermal cells, and can be upright or flat. The upright hairs are the most effective in conferring resistance. The sticky exudates from the hair ends and the physical structure of the upright hairs are deterrents to egg laying and feeding by the leafhoppers. Some nymphs actually are entrapped by the sticky hairs and cannot move. Photo from Ranger and Hower. 2001. Crop Science 41:1427.

Figure 2. Generalized views of alfalfa seedlings at the experimental plot area in late October 2001 after seeding in early September 2001. Wheat was planted at a low seeding rate to help control erosion and to help reduce winter heaving of the alfalfa. Views are of plot area from the SE (upper left), SW (upper right). NW (lower left) and a close-up (lower right) showing a dime placed among the small, but established alfalfa plants interspersed among the thin stand of wheat plants. The wheat plants helped reduce soil erosion and modulate soil temperature to aid in over wintering of the alfalfa. Photo in lower right is of a thinner spot in the field to show individual plants. The stand averaged more than 19 plants per sq. ft. in June 2002, when counted after undercutting with a sod cutter and counting individual plants.

Figure 3. Plot area in late June 2002, just prior to the second harvest. Note the stand has filled in well. Bare strips in photo on left indicate where sod was cut for plant counts after the first harvest. The right photo shows a corner of a plot of the normal variety with no spray (left of flag) and the PLH-resistant variety with no spray (right of flag). Note color change.

Figure 4. Relationship between number of plant units (some units have more than one plant) counted from above ground compared with number of plants found after sod cutting and separating plants within a unit. The 1:1 ratio (diagonal line) is not followed because only 76% of the plant units counted from above actually consisted of one plant, whereas 19% consisted of two plants, 4% consisted of three plants, and 1% consisted of four plants that were coexisting in close proximity even though they appear as one plant from above.

Figure 5. Top panel shows populations of potato leafhopper between harvest dates in the no-spray treatment (control). The top panel also gives the percentage control provided by the PLH-resistant variety compared with the normal variety. Middle panel shows the populations in the no-spray treatment (same as top panel) compared to those in the scheduled spray treatment (arrows pointing up). Note the excellent control. The lower panel shows the populations in the scheduled spray treatment (same as middle panel) compared to the IPM treatment (arrows pointing down). No leafhoppers were found previous to the first harvest and only a few in the early regrowth of the fourth harvest.

Table 1. Effects of potato leafhopper control treatments, potassium fertilization rates, and varieties (PLH-resistant vs. normal) on plant density, stem density, stems per plant, and weight per stem on several dates in 2002.

Table 2. Effects of potato leafhopper control treatments, potassium fertilization rates, and varieties (PLH-resistant vs. normal) on yield of alfalfa at each of four harvests in 2002. Total annual yield is also shown.