Glandular-haired varieties developed for resistance to PLH were compared with standard (without glandular hairs) varieties in small plot trials at Western and Northwest Branches. The trial established in 1996 at Western Branch was continued, which is part of a four-state evaluation of PLH-resistance in alfalfa. Three new small plot trials were seeded in 1997 at these two sites. At Western and Northwest Branches, variety trials were established which were not and will never be treated with insecticides. At Western Branch, another experiment was established to determine what level of resistance is needed in alfalfa varieties to prevent economic losses of forage yield and quality. This experiment is one of three locations in cooperation with Forage Genetics and Pioneer Hi-Bred Int'l, Inc. Fourteen varieties were seeded which differ in resistance to PLH, varying from 0 to approximately 50% resistant plants. Insecticide treated control plots of each variety are included in the trial, along with the untreated plots. Data from this trial has not yet been analyzed and summarized.
At Wooster and at six on-farm sites, a resistant glandular-haired variety is being compared with a standard alfalfa variety in both insecticide treated and untreated plots. Plot size is about 40 x 40 ft at most of the on-farm sites, and 100 x 100 ft. at Wooster. In these trials, plots were scouted weekly for PLH population levels, but data have not been summarized. During the growing season, large differences in PLH yellowing and PLH population levels were observed among varieties at certain times. The glandular-haired varieties showed greater resistance to PLH than the standard varieties, although all varieties were damaged when PLH levels were very high. At several on-farm sites, weed competition and slow establishment were a problem, particularly with the glandular-haired varieties. However, excellent establishment of all varieties was observed at other sites.
Potato leafhopper populations reached extremely high levels across much of the state during the summer of 1997. In the established stand at Western Branch, significant and dramatic differences in yield were observed among the glandular-haired and standard susceptible alfalfa varieties (Table 1). The glandular-haired varieties were superior in yield in the absence of insecticide treatment, demonstrating improved resistance to PLH. Although insecticide treatment increased yield of all varieties across the three summer harvests, it would not have provided a positive net return for the resistant varieties at the second harvest, but did for the standard varieties. It is also important to note that the resistant varieties were similar in yield to the standard varieties in treated plots (Table 1). The glandular-haired varieties are more dormant, i.e. they begin growth later in the spring and regrow more slowly after harvest. However, this did not translate into reduced yields for the glandular-haired varieties where PLH was controlled.
Table 1. Forage yield of resistant and commercial check varieties grown without (Untrt) and with (Trt) insecticide treatment for PLH at South Charleston, OH in 1996-97.
| Variety | Cut 1, 1997 | Summer, 1997 | Total 1997 | Total 1996-97 | ||||
|---|---|---|---|---|---|---|---|---|
| type | Untrt | Trt | Untrt | Trt | Untrt | Trt | Untrt | Trt |
| Resistant | 2.12 | 2.32 | 3.13** | 3.53 | 5.25** | 5.86 | 6.79** | 7.95 |
| Standard | 2.21+ | 2.34 | 2.53 | 3.46 | 4.74 | 5.81 | 5.49 | 8.15 | +, ** Indicates statistically significant difference at P=0.10 and P=0.01, respectively, between resistant and check varieties within a treatment. |
The glandular-haired varieties included in the 1996 seeding at Western have at best 35% PLH-resistant plants. Newer experimental varieties being developed by plant breeders have over 50% resistant plants. Several of these newer experimental varieties (Exp.1-Exp.4) were included in the new unsprayed trials established at Western and Northwest Branches in 1997. They were superior in yield (according to LSD at P=0.05) to the first commercially released glandular-haired varieties and the standard check variety included in the trials (Fig 1). Visual ratings of PLH yellowing (not shown) and PLH nymphal counts reflected the higher resistance level of these newer experimental lines (Fig. 2).
PLH resistant varieties represent a significant new tool in alfalfa pest management. Although PLH is recognized as an important pest of alfalfa in Ohio and other midwestern states, growers often fail to regularly scout alfalfa and apply insecticides in a timely manner when PLH activity warrants treatment. These data confirm 1996 findings demonstrating that glandular-haired varieties are dramatically superior to standard alfalfa under heavy PLH pressure. Although the first glandular-haired varieties to be released will benefit from timely insecticide treatment when PLH populations are high, this may not be the case as varieties with higher levels of resistance are developed. Our results in 1997 demonstrate that plant breeders are still making significant improvement in PLH resistance. Varieties may soon be available which will derive no economic benefit from insecticide treatment for PLH, especially if they are combined with other methods in an integrated control program. Further research is needed to develop such integrated control programs. These PLH-resistant varieties and associated management practices in an integrated control program will provide growers with significant economic benefits, not to mention the environmental benefits of reduced pesticide use.
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