© 2009 Plant Management Network.
Evaluation of Resistance to Chinch Bug in Pearl Millet in Temperate and Subtropical Environments
John A. Rajewski, Department of Agronomy & Horticulture, University of Nebraska, Lincoln, NE 68583-0817; Xinzhi Ni, Crop Genetics and Breeding Research Unit, USDA-ARS, University of Georgia, Tifton, GA 31793-0748; Jeffrey P. Wilson, Crop Genetics and Breeding Research Unit, USDA-ARS, University of Georgia, Tifton, GA 31793-0748; Ismail Dweikat, Department of Agronomy & Horticulture, University of Nebraska, Lincoln, NE 68583-0817; and G. David Buntin, Department of Entomology, University of Georgia, Griffin, GA 30223-1797
Rajewski, J. A., Ni, X., Wilson, J. P., Dweikat, I., and Buntin, G. D. 2009. Evaluation of resistance to chinch bug in pearl millet in temperate and subtropical environments. Online. Plant Health Progress doi:10.1094/PHP-2009-0112-01-RS.
Pearl millet inbreds and hybrids were evaluated for resistance to chinch bug at Lincoln, NE, and Tifton, GA, in 2003 and 2004. Insect feeding damage expressed as plant mortality, stunting, leaf sheath necrosis, and tiller mortality was observed over time. Resistance was expressed by a slower rate of plant damage to resistant genotypes compared to susceptible ones. Across experiments, inbreds 59464B and 59668M-1 were most frequently identified as resistant. Inbred Tift 99B was susceptible in every experiment and by every assessment criterion. The experiments revealed information on the genetic expression of resistance. When insect damage among hybrids made with Tift 454 was compared, resistance tended to be dominant or overdominant in expression. Evidence for location-specific resistance caused by environmental conditions or genetic differences in the insect populations between the two locations was observed. Inbreds 03GH707 and Tift 454, developed at Tifton, were resistant only in some assessments at Tifton, but not at Lincoln. 16RmR1, developed at Lincoln, was susceptible in both Lincoln experiments, but not at Tifton. 03GH706 was susceptible in some Tifton assessments, but was not among the most susceptible inbreds in the Lincoln experiments. Evaluations are needed at multiple locations to effectively identify resistance to damage caused by chinch bug feeding in pearl millet.
The chinch bug, Blissus leucopterus leucopterus (Say) (Hemiptera: Blissidae), and false chinch bug, Nysius raphanus Howard (Hemiptera: Lygaeidae), can cause significant injury and even the loss of stands of pearl millet, Pennisetum glaucum (L.) R. Br. (2,10). Hudson (5) and Kennedy (7) reported that the chinch bug is the most important insect pest for pearl millet forage and grain production in the Southeast, Midwest, and Midsouth regions of the United States (Fig. 1). Economic losses caused by chinch bug on pearl millet are not well documented, but it has been an important pest on other grass species throughout the southern and middle Great Plains states. Annual losses caused by chinch bugs in sorghum have been estimated as $10 million to $11.3 million in Nebraska, and $19 million in Kansas (11). Blissus spp. also cause significant damage to turf grasses, such as St. Augustine grass and buffalo grass (4,8).
Although insecticides can be used to manage chinch bugs (1), genetic resistance in pearl millet to chinch bug may be a more cost-effective strategy. Genetic variation in resistance to chinch bug exists in other grass species. Wilde and Morgan (13) reported that ‘Early Sumac’ sorghum is significantly more resistant to chinch bug than ‘Honey,’ ‘Redlan,’ and ‘Spanish Broomcorn.’ Davis et al. (3) evaluated maize (Zea mays L.) for resistance to chinch bug in the greenhouse, and identified ‘Tx601’ as the most resistant, while ‘Mp420’ was the most susceptible inbred line assessed.
Resistance has been identified in some pearl millets (9,10,12,14), however, all evaluations to date have been conducted on a limited, local scale in spite of the widespread geographic importance of the insect pest. The objective of these experiments was to evaluate pearl millet germplasm for chinch bug resistance in both temperate and subtropical environments.
Pearl millet germplasms developed by the University of Nebraska, Lincoln, NE, and the USDA-ARS Crop Genetics and Breeding Research Unit at Tifton, GA, were evaluated in experiments at Lincoln and Tifton in 2003 and 2004. In the Lincoln experiments, 16 pearl millet genotypes were planted on 17 June 2003 (Table 1), and 27 genotypes were planted on 19 June 2004 (Table 2). Plots were single rows, 7 m long and planted with 4 replications in a randomized complete block design. At Tifton, GA, 12 pearl millet genotypes were planted on 8 July 2003, and 18 genotypes were planted on 18 June 2004. Single-row plots were 4 m long, and planted in 4 replications in a randomized complete block design. Inbreds and hybrids were evaluated in each experiment. Hybrids were made either with some of the inbreds evaluated, or alloplasmic variants of the inbreds. These alloplasmic variants were derived from backcrossing an inbred into diverse cytoplasms that include the B, A1, A4, or M cytoplasms. The nuclear genetic composition of these alloplasmic variants are indicated by the numeric designator, and the cytoplasm is indicated by the alpha suffix. We assumed that cytoplasm had no effect on plant response to chinch bug feeding.
Table 1. Pearl millet mortality resulting from chinch bug feeding at Lincoln, NE, 2003.
AUIDC = area under insect damage curve, calculated with data collected at 5-day intervals. Data presented are from 10-day intervals.
Table 2. Pearl millet mortality resulting from chinch bug feeding at Lincoln, NE, in 2004.
AUIDC = area under insect damage curve, calculated with data collected at 5-day intervals. Data presented are from 10-day intervals.
Pearl millet plots were planted adjacent to rows of proso millet (Panicum miliaceum L.) at both locations. Proso millet was used as a trap crop to initiate chinch bug infestations and increase the pest population. The natural infestation on proso millet provides more uniform infestations on the experimental plots of pearl millet. The variety ‘Sunup’ was used at Lincoln, and the variety ‘Dove’ was used at Tifton.
In the Lincoln experiments, damage from chinch bug was assessed by determining the percentage of plant mortality at about 5-day intervals beginning 10 July and ending 23 September in 2003 and beginning 8 July and ending 10 September in 2004. In the 2003 Tifton experiment, the percentage of plant damage was determined based on an overall assessment of total plant damage expressed by stunting, chlorosis, and necrosis, as compared to undamaged plants (Fig. 2). Damage was assessed twice during the growing season. In the 2004 Tifton experiment, percentages of stunting, percent leaf sheath necrosis, and percent dead tillers were scored on individual plants at weekly intervals on 27 July, and 3, 10, and 17 August.
Relative resistance within the experiments at Lincoln and the 2004 experiment
at Tifton was ranked by the area under the insect damage curve (AUIDC),
Expression of Resistance in the Field
Across all experiments, each genotype expressed some damage from chinch bug feeding, therefore differences in resistance were expressed as a relative degree of reaction. Resistance in the 2003 Lincoln experiment was expressed quantitatively by a slower rate of plant mortality of resistant entries compared to susceptible entries. When relative resistance in the 2003 Lincoln experiment was ranked by AUIDC derived from plant mortality ratings, 59464B, 59668M-1, and NM-7R were among the more resistant inbreds, and 16RmR1, 03GH706, and Tift 99B were the most susceptible (Table 1). Differences in plant mortality were often evident by the second or third evaluation date, and mortality in the susceptible inbreds was more than double that of the more resistant inbreds by the 30 July evaluation. Mortality in susceptible inbreds was greater than 90% by 9 August.
In several instances, hybrids were more resistant than parental inbreds. Hybrid 03GH705 × Tift 454 had less mortality than either inbred parent by 19 August. Tift 99A × Tift 454 had lower mortality than Tift 99B from 15 July to 3 September, and had lower mortality than Tift 454 from 19 August to 3 September (not all data are presented in Table 1). Hybrid 03GH706 × Tift 454 had less mortality than inbred 03GH706 by 4 August, and consistently less mortality than Tift 454 by 3 September (not all data are presented in Table 1). Hybrid 03GH707 × Tift 454 had less mortality than the inbred parents only on 19 August and 24 August (not all data are presented in Table 1). When hybrids with Tift 454 as a common parent were compared, minor differences were observed throughout the growing season, but 02F266 × Tift 454 consistently had less mortality than 02F291 × Tift 454 after 29 August.
As in 2003, resistance in the 2004 Lincoln experiment was expressed as a slower rate of plant mortality compared to that of susceptible genotypes (Table 2). When relative resistance in the 2004 Lincoln experiment was ranked by AUIDC calculated by plant mortality, 59668M-1, 59464B, and 1361B were among the most resistant inbreds, whereas inbreds Tift 99B, 16RmR1, and 59052M were among the most susceptible.
More hybrids were evaluated in the experiment at Lincoln in 2004 compared to 2003. 59668A × 58001R, 03GH707 × Tift 454, and 03GH706 × Tift 454 were ranked among the most resistant hybrids, whereas 59052M × 9Rm/4Rm, 59668M × 54026AmRm, and 59668M × 9RM/4Rm were among the more susceptible hybrids. Some hybrids expressed greater levels of resistance than one or both parents. Hybrid 03GH707 x Tift 454 had less mortality than 03GH707 from 28 July, and except for assessments on 7 and 12 August, also had less mortality than did Tift 454 from 28 July (not all data are presented in Table 2). Tift 99A × Tift 454 had consistently less mortality than did Tift 99B from 23 July. In contrast, 03GH706 x Tift 454 and 03GH705 × Tift 454 only rarely expressed less mortality than their parental lines.
59668M-1 often had less mortality compared to most of the hybrids made with the inbred, but these differences were not significant. Among the hybrids made with the 59668 genotype, hybrid 59668A × 58001R had less mortality than hybrid 59668M × 54026AmRm from 2 August, but otherwise, this hybrid did not differ from the other 59668 hybrids. There were some differences in mortality among 59052M and some of its hybrids. This inbred was more susceptible than hybrid 59052M × 54026AmRm for a short duration between 18 July and 2 August (not all data are presented in Table 2).
In the 2003 experiment at Tifton, inbreds NM-7R, 03GH705, and Tift 454 were among the most resistant inbreds, whereas Tift 99B, 03GH706, and 9Pm/4Rm exhibited the most plant damage by 9 September (Table 3). When hybrids were compared to their inbred parents, hybrid Tift 99A × Tift 454 was less damaged than Tift 99A, but did not differ from Tift 454. Hybrid 03GH706 × Tift 454 was less damaged than 03GH706, but did not differ from Tift 454. Hybrid 03GH707 × Tift 454 was less damaged than 03GH707 only on 9 September, whereas hybrid 03GH705 × Tift 454 did not differ from its parental inbreds.
Table 3. Damage to pearl millet from chinch bug feeding at Tifton,
Comparisons in the 2004 Tifton experiment were based on individual ratings of percentages of stunting, leaf sheath necrosis, and mortality. When ranked by AUIDC calculated from the stunting data, NM-7R, 59668M-1, and Tift 454 were among the most resistant inbreds, whereas 03GH706, Tift 99B, and 03GH705 were among the most susceptible (Table 4). When hybrids were compared to parental inbreds, hybrids Tift 99A × Tift 454 and 03GH706 × Tift 454 were less stunted than Tift 99B and 03GH706 at each assessment date. Hybrid 03GH705 × Tift 454 was less stunted than 03GH705 only on the last two assessments. Hybrid 03GH707 × Tift 454 was less stunted than 03GH707 only on 3 August. Hybrids with the 59668 genotype were as resistant as 59668M-1. Hybrid 59052A × NM-72 was less stunted than 59052M only at the 17 August evaluation.
Relative rankings differed when AUIDC was calculated using data of percentage of leaf sheath necrosis. 59464B, 03GH707, and 9Pm/4Rm exhibited the lowest levels of necrosis and were among the most resistant inbreds, whereas 03GH705, 03GH706, and Tift 99B had the most necrosis (Table 4). When hybrids were compared to inbreds, 03GH705 × Tift 454 had less necrosis than 03GH705 at all assessment dates, and less necrosis than Tift 454 on 3 August and 17 August. Tift 99A × Tift 454 exhibited less necrosis than Tift 99B on the last three dates, and less necrosis than Tift 454 on the last two dates. 03GH706 × Tift 454 exhibited less necrosis than both parents on the last three dates. 03GH707 × Tift 454 exhibited less necrosis than 03GH707 at the 3 August and 10 August assessments, and less necrosis than Tift 454 at the last three dates. Hybrid 59052A × NM-72 exhibited less necrosis than 59052M at each of the last three dates. 59668M × 59052B exhibited less necrosis than 59052M on 3 August and 17 August. Hybrid 59668M × 9Rm/4Rm exhibited less necrosis than 59668M-1 on the last three assessments. Hybrids 59668M × 59052B and 59668A × NM-7R1 exhibited less necrosis than 59668M-1 only on one assessment each, on 3 August and 17 August, respectively.
When resistance of genotypes was ranked by AUIDC calculated using tiller mortality data, 59668M-1, 59464B, and 03GH707 were among the most resistant inbreds, while 03GH706, Tift 99B, and 9Pm/4Rm were among the most susceptible (Table 4).
Resistance in hybrids frequently differed from that of the parental inbreds. Tift 99A × Tift 454 had less tiller mortality than Tift 99B on the last three dates. 03GH705 × Tift 454 had less tiller mortality than 03GH705 on the last two dates, but less mortality than Tift 454 only on 10 August. 03GH706 × Tift 454 had less tiller mortality than 03GH706 on the last three dates. 03GH707 × Tift 454 did not differ in resistance from either parental inbred. 59052M did not differ in resistance from hybrids produced with this genotype. Hybrid 59668M × 9Rm/4Rm had more tiller mortality than inbred 59668M-1, but only at the first two dates.
These experiments demonstrate that evaluations at multiple locations are useful to identify differences in reactions to chinch bug in pearl millet. Expression of resistance is a quantitative trait, and can vary across locations and seasons. Although a universally resistant inbred was not identified, 59464B and 59668M-1 were most frequently identified as resistant. 59464B was among the most resistant inbreds in the 2003 and 2004 Lincoln experiments, and in the 2004 Tifton assessments for leaf sheath necrosis and tiller mortality. 59668M-1 was among the most resistant inbreds in the 2003 and 2004 Lincoln experiments, and by the 2004 Tifton assessments for stunting and tiller mortality. Although a universally resistant inbred was not identified, Tift 99B was identified as a susceptible inbred in every experiment and by every assessment criterion (Fig. 3).
Evidence exists for location-specific response to chinch bug feeding, due either to environment or perhaps genetic differences in the insect populations between the two locations. "Resistant" inbreds, notably 03GH707 and Tift 454 developed at Tifton, were classified as resistant only in some of the assessments at Tifton, but not at Lincoln. 16RmR1, developed at Lincoln, was identified as a susceptible inbred in both experiments at Lincoln, but not at Tifton. 03GH706 was susceptible in the 2003 Tifton experiment and the 2004 Tifton stunting assessments, but was not among the most susceptible inbreds at Lincoln.
Inbreds 9Pm/4Rm and 03GH705 were resistant in some assessments and susceptible in others. The reaction of these inbreds to feeding by chinch bugs had a greater environmental sensitivity, suggesting that resistance to chinch bugs may be affected by the temperate and subtropical macroenvironments as well as by unknown microenvironmental conditions.
These evaluations also provided some insights into the expression of resistance in hybrids. Four hybrids made with Tift 454 were evaluated in all four experiments. Resistance in these hybrids tended to be dominant or overdominant in expression. Across the four experiments, a total of 41 assessments was made. Tift 99A × Tift 454 was more resistant than Tift 99B in 81% of the assessments, and more resistant than Tift 454 in 15% of the assessments. 03GH705 × Tift 454 was more resistant than 03GH705 in 39% of the assessments, and more resistant than Tift 454 in 24% of the assessments. Hybrid 03GH706 × Tift 454 was more resistant than 03GH706 in 56% of the assessments, and more resistant than Tift 454 in 22% of the assessments. 03GH707 × Tift 454 was more resistant than 03GH707 in 32% of the assessments, and more resistant to damage than Tift 454 in 27% of the assessments. These evaluations demonstrated that an agronomically desirable but susceptible inbred might be commercially useful if crossed to an inbred that has a dominant expression of resistance. Resistance in both inbreds is desirable, however. ‘Tifgrain 102’ is a hybrid of Tift 99A × Tift 454. Although Tifgrain 102 has equal or greater resistance compared to Tift 454, this hybrid can sustain significant damage if chinch bug pressure is high (J. P. Wilson, personal observation).
Starks et al. (12) indicated that resistance in selected pearl millet breeding lines from Oklahoma was dominant in the F1 hybrids, and segregated for a single dominant gene for resistance in F2 populations; however, no data were presented to support their claim. Some of these breeding lines were eventually released as resistant germplasms (9). Resistance is not always fully dominant; both positive and negative general combining ability for plant damage were observed in diallel crosses of pearl millets from Africa (14).
In prior evaluations, Starks et al. (12) and Ni et al. (10) determined that plant damage was a more effective measure of resistance than assessing chinch bug infestation in the field. Our present study confirms an observation by Starks et al. (12) that plant damage is most evident in field evaluations during certain time intervals. Repeated assessments of plant damage over time are necessary to identify germplasms with resistance. This study demonstrates that pearl millet reaction to chinch bug feeding is affected by both genetic and environmental effects. Experiments at multiple locations are an important approach to more effectively identify resistance to chinch bug feeding in pearl millet.
Mention of trade names or commercial products in this article is solely for the purpose of providing specific information and does not imply recommendation or endorsement by the United States Department of Agriculture. We thank T. Perla, M. Purvis, and J. C. Mullis for their technical assistance.
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