ARTICLES

Variability in Field Response of Peanut Genotypes from the U.S. and China to Tomato Spotted Wilt Virus and Leaf Spots

Authors: Yan Li , Albert K. Culbreath , Charles Y. Chen , Steve J. Knapp , C. Corley Holbrook , Baozhu Guo

  • Variability in Field Response of Peanut Genotypes from the U.S. and China to Tomato Spotted Wilt Virus and Leaf Spots

    ARTICLES

    Variability in Field Response of Peanut Genotypes from the U.S. and China to Tomato Spotted Wilt Virus and Leaf Spots

    Authors: , , , , ,

Abstract

Tomato spotted wilt, caused by Tomato spotted wilt virus (TSWV) and transmitted by thrips, and early leaf spot and late leaf spot are among the most important diseases of peanut in the southeastern United States. The objective of this study was to compare field susceptibility of diverse peanut lines to TSWV and leaf spot pathogens for selection of lines for mapping population development. In field trials in 2007 and 2008, 22 genotypes were evaluated for reactions to TSWV and leaf spots. Early leaf spot was the predominate pathogen in both years. There was a near-continuous range of spotted wilt from 18% to 79% for the total incidence rating with any symptoms caused by TSWV. In general, NC94022, ‘Georganic’, C689-6-2, ‘Georgia-01R’, C724-19-25, TifGP-1, C11-154-61, C12-3-114-58, and ‘Tifguard’ were among the most resistant genotypes to TSWV, whereas GT-C20, GT-C9 and PE-2 were the most susceptible. Final percentage of defoliation by leaf spots ranged from 10% to 97% for both years. Genotypes C689-2, Georgia-01R, C12-3-114-58, C11-154-61, Tifguard and Georganic showed resistance to leaf spots, whereas ‘NC-6’, ‘Spancross’, GT-C9, GT-C20 and PE-2 were susceptible to leaf spots. There were 3 cultivars and 3 breeding lines classified as resistant to both TSWV and leaf spots; and there were 3 genotypes from China susceptible to both TSWV and leaf spots. These phenotypic disease reaction data can be used in conjunction with genetic characterization of these genotypes for development of recombinant inbred line populations in efforts to develop markers for resistance to TSWV and leaf spots.

Keywords: Cercospora arachidicola, Cercosporidium personatum, Arachis hypogaea, Disease resistance

How to Cite:

Li, Y. & Culbreath, A. & Chen, C. & Knapp, S. & Holbrook, C. & Guo, B., (2012) “Variability in Field Response of Peanut Genotypes from the U.S. and China to Tomato Spotted Wilt Virus and Leaf Spots”, Peanut Science 39(1), p.30-37. doi: https://doi.org/10.3146/PS11-6.1

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Published on
01 Jan 2012
Peer Reviewed

Introduction

Tomato spotted wilt, caused by Tomato spotted wilt virus (genus Tospovirus; family Bunyaviridae) (TSWV) and transmitted by thrips (Thysanoptera: Thripidae), is a serious problem to southeastern U.S. peanut production. Tomato spotted wilt occurs every year with fluctuations in severity within years and locations (Culbreath et al., 2003). Losses to TSWV in peanut increased steadily from the late 1980s, with annual losses as high as $40 million in Georgia alone (Bertrand, 1998). TSWV is transmitted by several species of thrips (Ullman et al., 1997), but the control of thrips has not shown much promise for management of spotted wilt in peanut. The management of spotted wilt is dependent upon the use of cultivars with moderate levels of field resistance (Culbreath et al., 1996), and cultural practices which reduce the incidence and severity of spotted wilt (Brown et al., 1996; Culbreath et al., 1999). New cultivars such as AP-3 (Gorbet, 2007), Georgia-02C (Branch, 2003) , and Tifguard (Holbrook et al., 2008a) have greater levels of field resistance than Georgia Green, the moderately resistant cultivar used as a standard for genotype comparisons (Culbreath et al. 2008), but still can be severely affected. Higher levels of field resistance have been reported in the cultivar Georganic (Culbreath et al., 1997; Culbreath et al., 1999; Holbrook and Culbreath, 2008) and breeding line F NC94022-1-2-1-1-b3-B (Culbreath et al., 2005; Li et al., 2009), henceforth referred to as NC 94022.

Two of the most prevalent and severe foliar diseases of peanut worldwide are early leaf spot, caused by the fungus Cercospora arachidicola S. Hori and late leaf spot, caused by the fungus Cercosporidium personatum (Berk. & M. A. Curtis) Deighton (Smith and Littrell, 1980). In the U. S., most of the peanuts are grown in the southern states where environmental conditions are often favorable for leaf spot epidemics. One or both of the leaf spot diseases occur in all peanut producing states, and multiple applications of fungicides are necessary for control of the diseases.

The development and use of resistant cultivars is one of the most desirable ways to manage both leaf spot diseases and TSWV (Culbreath et al., 2003). Currently, peanut breeding programs focus on extensive field screening to look for sources of resistance and making selections of resistant lines once crosses have been made and breeding lines are available. Field selection for resistance to leaf spot pathogens and TSWV requires considerable space and a relatively large number of plants. Development of molecular markers for assisting in selection for resistance to TSWV and the leaf spot pathogens could increase the efficiency of breeding programs (Li et al., 2010; Guo et al., 2012). Marker assisted selection has been used in development of nematode resistant peanut cultivars COAN (Simpson and Starr, 2001) and NemaTAM (Simpson et al., 2003), but markers are not currently available for resistance to either of the leaf spot pathogens or TSWV. The objective of this study was to compare field susceptibility of diverse peanut genotypes, including cultivars, germplasm accessions, and advanced breeding lines to TSWV and leaf spot pathogens in order to relate to genotypic characterization of these lines in an effort to develop markers for resistance to TSWV and leaf spot pathogens (Li et al., 2010) through genetic mapping (Qin et al., 2012).

Materials and Methods

Twenty-two genotypes were evaluated in this study, including commercial cultivars, advanced breeding lines, and germplasm accessions (Table 1). Two separate field trials were conducted at the University of Georgia, Coastal Plain Experiment Station Belflower Farm, Tifton, GA in 2007 and 2008. Soil type is Tifton loamy sand (Fine-loamy, siliceous, thermic Plinthic Kandiudult). Peanut was in an annual rotation sequence following corn (Zea mays L.). In each year, one experiment was planted in April to maximize potential for development of spotted wilt epidemics (Culbreath et al., 2003), and one was planted in May to reduce potential for spotted wilt epidemics and increase the likelihood of heavy leaf spot epidemics.

Table 1.
Table 1.

Twenty-two genotypes used for the field evaluations for reaction to Tomato spotted wilt and early leaf spot.

The field trials were conducted using randomized complete block designs with 4 replications. Experiment plots were 6.0 m long, separated by 2.4-m alleys. Peanut seed was planted to 91-cm-spaced twin-row plots. Planting dates were 23 April and 23 May in 2007, and 18 April and 20 May in 2008. Seeding rate was 3.3 seed/m of row. Sparse seeding rate was used to maximize pressure of Tomato spotted wilt (Culbreath et al., 2003) and to allow Tomato spotted wilt severity evaluation on individual plants. Only April planted trials were used for the spotted wilt evaluations, and both April and May planting trials were used for leaf spot evaluations. Fungicide applications of chlorothalonil (Bravo WeatherStik, Syngenta Crop Protection, Greensboro, N.C.) (1.26 kg a.i./ha) were made at ca. 30 days after planting with subsequent applications every 14 days until ca. 80 days after planting.

Tomato spotted wilt severity was evaluated for each plant on 25 July, 2007 and 21 July, 2008. Severity of Tomato spotted wilt was assessed using a 0 to 5 severity scale adapted from Baldessari (2008) based on visual determination of presence of symptoms and estimation of the degree of stunting (reduction in plant height, width, or both) for symptomatic plants (Table 2). Genotype comparisons were made for total incidence of Tomato spotted wilt (severity ratings of 1 or greater), as well as incidence of plants with severity ratings in classifications (2 or greater, 3 or greater, 4 or greater and 5).

Table 2.
Table 2.

The rating scale of severity of Tomato spotted wilt caused by TSWV.

Leaf spot severity was evaluated on 25 September 2007, and 26 September 2008 for each plot using the Florida 1 to 10 scale (Chiteka et al., 1988) where 1  =  no leaf spot; 2 = very few lesions on the leaves and none on upper canopy; 3  =  very few lesions on upper canopy; 4  =  some lesions with more on upper canopy, and 5% defoliation; 5  =  noticeable lesions on upper canopy with 20% defoliation; 6  =  numerous lesions on upper canopy with 50% significant defoliation; 7  =  numerous lesions on upper canopy with 75% defoliation; 8  =  Upper canopy covered with lesions with 90% defoliation; 9  =  very few leaves covered with lesions remain and some plants completely defoliated; 10  =  plants dead (Chiteka et al., 1988). Final percent defoliation (% defoliation) was estimated from the Florida scale rating taken just before peanut plants were inverted (Chiteka et al., 1988).

Final leaf spot ratings were converted to estimates of % defoliation according to the function:

This equation was obtained by non-linear regression of defoliation level on Florida scale rating (FSR) values as reported by Chiteka et al. (1988).

Data were analyzed using Proc Mixed with ddfm  =  satterth option that performs a general Satterthwaite approximation for the denominator degrees of freedom and produces an accurate F-approximation (SAS v.8.3, SAS Institute, Cary, NC), unless otherwise stated. Incidence of Tomato spotted wilt and % defoliation by leaf spot estimated from Florida scale ratings were used as response variables. Genotype was considered a fixed variable, and replication and year were treated as random effects. Main effects and interactions as well as specific treatment effects were considered significant if P ≤ 0.05. Fisher's least significant difference (LSD) was computed using standard error and t values of adjusted degrees of freedom from ddfm  =  satterth option. If the interaction effect between year and genotype was not significantly different (P > 0.05), the data were pooled and presented averaged across years; if the interaction was significant (P ≤ 0.05); the data were analyzed separately and are presented by year.

Genotypes were classified as susceptible, moderately susceptible, moderately resistant, and resistant, based on TSWV incidence and leaf spot intensity compared to that in Georgia Green. As an interesting note for 2007 and 2008 growing seasons, early leaf spot was the predominate disease in the field and we rated the total defoliation including both early and late leaf spots.

Results and Discussion

There was no significant interaction between year and genotype for the total incidence of Tomato spotted wilt (severity ≥ 1), but there were significant interactions for the incidence of severity categories, ≥ 2, ≥ 3, ≥ 4 and 5 (data not shown). Therefore, data were pooled across years for the analysis of incidence of severity ≥ 1, but incidence of plants in all other severity ratings categories are presented separately for each year.

For the incidence of severity ≥ 1, NC 94022 had the lowest numerical ranking for total incidence of spotted wilt, but did not differ from that of thirteen genotypes (Table 3). Only NC 94022 had total incidence of spotted wilt (severity rating ≥ 1) lower than that of Georgia Green (Table 3). For other categories of incidence of severity, NC 94022 had lower incidence of Tomato spotted wilt than Georgia Green in both years (Table 3). Several additional genotypes in addition to NC 94022 had incidence of Tomato spotted wilt that ranked lower than Georgia Green in all severity classes (Table 3). Among them, only U.S. breeding line C689-6-2 showed a lower incidence of Tomato spotted wilt than Georgia Green in all severity classes for at least one year (Table 3). Others including U.S. released cultivars Georganic, Georgia-01R, Tifguard, Tifrunner, AP-3 and breeding lines C724-19-25, TifGP-1, C 12-3-114-58, C11-154-61 showed significantly lower incidences of Tomato spotted wilt than Georgia Green in one or more severity classes in each year (Table 3).

Table 3.
Table 3.

Reactions of different peanut genotypes to TSWV in five severity classes based on the data collected in Tifton, Georgia from 2007 and 2008.

GT-C20, which originated from China, had the highest rating in total incidence (Table 3). There were no significant differences among GT-C20, GT-C9 and PE-2, and they were all greater than Georgia Green in all severity classes in both years (Table 3). Incidence of Tomato spotted wilt in SunOleic 97R, Tifton 8, and PE-2 typically ranked higher than that of Georgia Green and was significantly higher in one or more severity classes in each year (Table 3).

Leaf spot epidemics were severe in both trials in both years and caused noticeable defoliation in all genotypes evaluated (Table 4). In 2008, leaf spot evaluations were not made in GT-C20 in the early planted trial because of severe stunting and mortality from Tomato spotted wilt by the time of leaf spot evaluation. A significant year by genotype interaction for percent defoliation for both early and late planted trials was observed; therefore, means of defoliation are presented by trial and year.

Table 4.
Table 4.

Effect of peanut genotype on percent defoliation caused by leaf spots based on the data collected in Tifton, GA from 2007 and 2008.

Defoliation for the cultivars Georganic, Georgia-01R and Tifguard, and breeding lines C12-3-114-58, C11-154-61 and C689-6-2 was significantly lower than for Georgia Green in both early and late planted trials for both 2007 and 2008; whereas, defoliation for the cultivars Tifrunner and Georgia-02C and breeding lines C724-19-25 and TifGP-1 was significantly lower than Georgia Green in late planted trials for both years and for the early planted trial in 2008 (Table 4). U.S. breeding line gen 04-14 had significantly less defoliation than Georgia Green for both trials in 2008, but not in 2007. Several additional genotypes including AP-3 and PE-1 had lower defoliation than Georgia Green for the late planted trial in 2008 (Table 4).

Defoliation for GT-C20 and SunOleic 97R was greater than for Georgia Green in both trials in 2007, and defoliation for Spancross and GT-C9 was greater than for Georgia Green for both trials in 2007 and the early planted trial in 2008. Except for GT-C20 and SunOleic 97R, there were no other genotypes that showed higher defoliation than Georgia Green for the late planted trial in 2008. In general, the late planted peanuts had lower levels of defoliation by leaf spot than early planted peanuts. However, the leaf spot evaluations for both tests were made on the same day. Therefore, the different levels of leaf spot in these two tests likely were due to longer epidemic duration in the early-planted trails than the late planted trials.

A summary of the disease responses of the genotypes in the current study, compared to previous reports of resistance to each pathogen, is listed in Table 5. The results from this study generally agreed with previous reports and comparisons of current results to previous reports indicated that the field resistances to TSWV in NC94022 (Culbreath et al., 2005), Georganic (Culbreath et al., 1999; Culbreath et al., 2005), Tifguard (Holbrook et al., 2008b), C724-19-25 (Holbrook et al., 2008c), Tifrunner (Holbrook and Culbreath, 2007), and Georgia-01R (Culbreath et al., 2008) were better than that in Georgia Green (Table 5). SunOleic 97R was reported as susceptible to TSWV (Culbreath et al., 1999; Culbreath et al., 2005). In the current study there were genotypes from China that showed higher severity of Tomato spotted wilt disease compared to that of SunOleic 97R as susceptible to TSWV. Three other U.S. cultivars, NC-6, Spancross, and Tifton8, and one Chinese breeding line PE-1 showed similar incidence of Tomato spotted wilt in the fields as genotypes with moderate susceptibility (Table 5). Field reaction to Tomato spotted wilt had not been reported previously for most of the breeding lines including, C689-6-2, TifGP-1, C12-3-114-58, and C11-154-61, which showed promising levels of field resistance to TSWV. Genotypes PE-2, GT-C9, and GT-C20 were highly susceptible to TSWV (Table 5).

Table 5.
Table 5.

The field response of twenty-two peanut genotypes to Tomato spotted wilt virus and leaf spots, and the comparison to previous reports.

In general, there were three cultivars (Georgia-01R, Tifguard, and Georganic) and three breeding lines (C 11-154-6, C 12-3-114-58 and C 689-6-2) classified as resistant to both TSWV and leaf spots. Whereas, some Chinese lines, GT-C20, GT-C9 and PE-2 were highly susceptible to both TSWV and leaf spots. PE-1 was moderately susceptible to both TSWV and leaf spots (Table 5). There has been no report of Tomato spotted wilt in peanut in China; therefore, resistance to TSWV would not have been selected by Chinese breeders. For those genotypes from China, leaf spot evaluations in the current study were hindered by high incidence and severe symptoms of Tomato spotted wilt. However, susceptibility to leaf spots was obvious.

The results in this study were also in agreement with previous reports of field resistance to leaf spots in Georgia-01R (Culbreath et al., 2008), Georganic (Culbreath et al., 1999; Culbreath et al., 2005) and Tifrunner (Holbrook and Culbreath, 2007) (Table 5). Tifguard and its sister line C725-19-25 showed resistance to moderate resistance to leaf spots in this study. A preliminary report from another study indicated Tifguard also had a moderate level of resistance to late leaf spot (Culbreath et al., 2008). Although field resistance to TSWV in NC 94022 has been reported (Culbreath et al., 2005), this study evaluated the reaction of NC94022 to both leaf spot pathogens and TSWV. NC 94022 proved to be moderately susceptible to early leaf spot in this study but has been observed to be resistant to late leaf spot which was the predominant leaf spot in 2010 (Culbreath and Guo, personal communication). Among the tested breeding lines, three lines, C11-154-61, C12-3-114-58 and C689-6-2, were identified as resistant genotypes showing lower percent defoliation due to leaf spot, whereas C724-19-25 and TifGP-1 were moderately resistant.

Summary

The results in this study indicated a wide range of field reactions to TSWV and leaf spots among the tested lines. This information should be useful for formulating disease management strategies by using different peanut cultivars or breeding lines. In addition, these trials included fourteen of a panel of sixteen genotypes for which genetic diversity was characterized using simple sequence repeat (SSR) markers (Li et al., 2010) and four of these genotypes, NC94022, SunOleic 97R , Tifrunner, and GT-C20, have been used to develop two recombinant inbred line (RIL) populations (Qin et al., 2012). Based on the results of these field trials, the parents, NC 94022 and SunOleic 97R differ markedly in their field resistance to TSWV, while the other pair of parents Tifrunner and GT-C20 differs greatly in field resistance to both TSWV and leaf spots. Information from the combination of the field and genetic characterization of these genotypes could be useful in development of mapping populations and construction of genetic linkage map for quantitative trait loci (QTLs) studies (Qin et al., 2012).

Acknowledgements

We thank Billy Wilson and Jake Fountain for technical assistance in the field. This research was partially supported by USDA Specific Cooperative Agreement 58-6602-6-121 with the University of Georgia, and the graduate student assistantship was partially supported by funds provided by Georgia Peanut Commodity Commission and the National Peanut Board. Mention of trade names or commercial products in this publication is solely for the purpose of providing specific information and does not imply recommendation or endorsement by the U.S. Department of Agriculture. USDA is an equal opportunity provider and employer.

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Notes

    Author Affiliations

  1. University of Georgia, Department of Plant Pathology, Athens, GA 30602
  2. USDA-Agricultural Research Service, National Peanut Research Laboratory, Dawson, GA 39842
  3. University of Georgia, Department of Crop and Soil Science, Athens, GA 30602
  4. USDA-Agricultural Research Service, Crop Genetics and Breeding Research Unit, Tifton, GA 31793
  5. USDA-Agricultural Research Service, Crop Protection and Management Research Unit, Tifton, GA 31793
  6. *Corresponding author (e-mail: Baozhu.Guo@ars.usda.gov)