Performance of Potato Leafhopper Resistant Glandular-haired Alfalfa Varieties in Virginia

Crop and Soil Environmental News, February 1998

Curt Laub, research associate
Rod R. Youngman, associate professor and Extension specialist
Thomas P. Kuhar, graduate research assistant
Department of Entomology

Introduction
Five commercial alfalfa varieties which had been developed to express resistance to potato leafhopper (PLH) were evaluated in 1997. These varieties display, in addition to simple pubescence, erect glandular hairs (Figs. 1 & 2) which PLH resistance has been associated with. The actual mechanism of resistance is not known, but may involve toxic or repellent compounds released from the glandular hairs. These compounds either kill the leafhoppers or cause them to avoid glandular-haired alfalfa.

Summary
Throughout the growing season we measured potato leafhopper infestations, yield and forage quality, and development of glandular hairs. High numbers of nymphs were found on all varieties; however, by 31 July, fewer nymphs were present on most glandular-haired varieties compared to the non-glandular check. No significant differences in yield occurred. Crude protein differences may reflect differences in resistance to PLH feeding damage. Growers should be aware that these glandular-haired alfalfa varieties were, at least in the first year, susceptible to PLH damage. IPM scouting procedures and thresholds for PLH should still be used.

Methods and Materials
Entries: Arrest (Novartis), Dekalb variety DK 121HG, Pioneer variety 5347LH, Clean Sweep 1000 (Agway), Interceptor (Agripro)
Check: Pioneer variety 5454
Location: Northern Piedmont Agricultural Research & Extension Center, Orange, VA.
Plot Layout: Randomized block with 4 reps. Plots 4.6 x 6.1 m (15 x 20 ft).
Planting Date: 11 April, 1997,
PLH sample dates: 16 June, 8 July, 18 July, 31 July, 18 Aug. Sampled with 38 cm (15 in) diameter sweep net, 5 pendular sweeps per plot.
Insecticide spray dates: 19 June, 31 July. Material used: Cygon 400 at 0.9 l/ha (3/4 pt/a).
Harvest Dates: 8 July, 18 Aug., 24 Oct.

Wet weight measured in field. Subsample from each plot dried at 60 deg C (140 deg F) for 96 h to determine percent dry matter. Dry matter subsamples were analyzed at the Virginia Tech Forage Testing Laboratory for crude protein (CP) content.

Statistical Analyses: ANOVA and LSD were used to analyze differences among treatments in PLH sweep net counts after [x + 0.5]^1/2 transformation, yield, and CP.

Results and Discussion
Potato leafhopper populations: 1997 was a year of extremely high PLH activity. Sweep net counts exceeded the economic threshold in all plots on 16 June and 31 July, and all plots were sprayed on 19 June and 31 July. Adult counts on 18 July showed no significant differences between the check and 4 of the 5 glandular-haired varieties (Table 1). On 31 July significantly higher numbers (P < or = 0.05) of nymphs occurred in the check compared to 4 of the 5 glandular-haired varieties. This indicates that adults laid fewer eggs or fewer nymphs survived on the glandular-haired varieties.

Table 1. Potato leafhoppers, PLH resistant alfalfa trial, Orange Co., Va., 1997.

Variety	16 June 1	8 July	18 July	31 July	18 Aug.
	Adults per sweep 2
Arrest	0.8ab	1.1	1.3ab	1.3	0.0
Cleansweep	0.7b	1.2	0.8b	1.4	0.0
Dekalb DK 121 HG	0.6b	1.1	1.5ab	1.2	0.0
Interceptor	1.0ab	1.7	1.7a	1.7	0.0
Pioneer 5347LH	1.2ab	1.4	1.6a	1.3	0.0
Check (P 5454)	1.6a	1.2	2.2a	1.7	0.1
	Nymphs per sweep2
Arrest	0.4	0.5	0.3	3.2bc	0.0b
Cleansweep	0.8	0.4	0.7	2.7c	0.1b
Dekalb DK 121 HG	0.5	0.4	0.4	2.9bc	0.0b
Interceptor	0.7	0.6	0.4	3.2bc	0.0b
Pioneer 5347LH	0.5	0.5	0.5	4.8ab	0.0b
Check (P 5454)	0.5	0.5	0.7	6.3a	0.2a
1 Means within a column followed by the same letter were not significantly different (P > 0.05; LSD).
2 Statistical analyses were performed on (x + 0.5)1/2 transformed data, but actual treatment means are shown.

Yield: In 1997, three cuttings were made (Table 2). There were no significant differences in yield for any single cutting or for total yield, although numerically the check out-yielded all glandular-haired varieties. Table 2. Alfalfa dry matter yield, PLH resistant alfalfa trial, Orange Co., Va., 1997.

Variety	Dry matter yield (ton/a) 1
	Cutting 1 8 July	Cutting 2 18 Aug.	Cutting 3 24 Oct.	Total
Arrest	0.46	0.69	0.32	1.48
Cleansweep	0.47	0.75	0.31	1.54
Dekalb DK 121 HG	0.47	0.71	0.34	1.51
Interceptor	0.48	0.82	0.40	1.70
Pioneer 5347LH	0.53	0.72	0.35	1.59
Check (P 5454)	0.56	0.77	0.40	1.73
1 No significant differences were detected for any single cutting or for total yield (P > 0.05; LSD).

Forage quality: Treatment (variety) source of variation for crude protein (CP) was significant (P < or = 0.05) for the first and second cuttings (Table 3). These cuttings occurred during high PLH activity, and the lower CP percentage in the check compared to some glandular varieties may reflect higher PLH damage.

Table 3. Crude protein, PLH resistant alfalfa trial, Orange Co., Va., 1997.

Variety	% Crude Protein (CP)
	8 July1	18 Aug.	24 Oct.
Arrest	17.8bc	19.0a	22.7
Cleansweep	18.5ab	18.5ab	22.7
Dekalb DK 121 HG	17.9bc	18.9a	22.2
Interceptor	18.8a	18.5ab	22.9
Pioneer 5347LH	18.8a	18.4ab	22.5
Check (P 5454)	17.5c	17.9b	21.2
1 Means within a column followed by the same letter were not significantly different (P > 0.05; LSD).

Glandular hair development: Only 30-40% of the plants in these varieties express the glandular-hair trait. On plants that express the trait, we counted up to 37 and 58 glandular hairs per mm on abaxial leaflet veins and petioles, respectively, and up to 32 glandular hairs per mm2 on stems.

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