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a USDA-ARS, Crop Genetics and Breeding Res. Unit, P.O. Box 748, Coastal Plain Exp. Stn., Tifton, GA 31793
b USDA-ARS, Crop Protection and Management Res. Unit, P.O. Box 748, Coastal Plain Exp. Stn., Tifton, GA 31793
c Plant Path. Dep., Univ. of Georgia, P.O. Box 748, Coastal Plain Exp. Stn., Tifton, GA 31793
d National Environmentally Sound Production Ag. Lab., Univ. of Georgia, P.O. Box 748, Coastal Plain Exp. Stn., Tifton, GA 31793
* Corresponding author (Corley.Holbrook{at}ars.usda.gov).
ABSTRACT
‘Tifguard’ (Reg. No. CV-101, PI 651853) is a runner-type peanut (Arachis hypogaea L. subsp. hypogaea var. hypogaea) cultivar released by the USDA-ARS and the Georgia Agricultural Experiment Stations in 2007. Tifguard was developed at the University of Georgia Coastal Plain Experiment Station, Tifton, GA. Peanut cultivars are available that have high resistance to the peanut root-knot nematode [Meloidogyne arenaria (Neal) Chitwood race 1] or spotted wilt caused by tomato spotted wilt tospovirus (TSWV). However, no cultivars exist that have resistance to both pathogens. Our research objective was to combine resistance to both pathogens in a single cultivar. Breeding populations were developed by hybridizing the TSWV-resistant ‘C-99R’ with the nematode-resistant ‘COAN’. Selection for nematode resistance was conducted using standard greenhouse screening techniques. Selection for TSWV resistance was conducted in the field with natural virus infection. A breeding line (C724-19-15) was selected that had high resistance to both pathogens. Tifguard exhibited higher resistance to TSWV and higher yield than standard check cultivars when grown in fields with little or no nematode pressure. Because of its high level of resistance to both TSWV and M. arenaria, Tifguard had significantly higher yield than all others entries when grown in two locations with high pressure from both pathogens. This cultivar should be valuable for peanut growers who have to deal with both pathogens.
Abbreviations: TSWV, tomato spotted wilt tospovirus
‘Tifguard’ (Reg. No. CV-101, PI 651853) is a runner-type peanut (Arachis hypogaea L. subsp. hypogaea var. hypogaea) cultivar that was released by the USDA-ARS and the Georgia Agricultural Experiment Stations in 2007. Tifguard was developed at the University of Georgia Coastal Plain Experiment Station, Tifton, GA, and was tested under the experimental designation C724-19-15.
Peanut cultivars are available that have high resistance to the peanut root-knot nematode [Meloidogyne arenaria (Neal) Chitwood race 1] or tomato spotted wilt tospovirus (TSWV), the cause of tomato spotted wilt. However, no cultivars exist that have resistance to both pathogens. The objective of this research was to combine resistance to both pathogens in a single cultivar.
Methods
The original population was developed by crossing ‘C-99R’ (Gorbet and Shokes, 2002), a cultivar with good field resistance to TSWV (Wells et al., 2002) with ‘COAN’ (Simpson and Starr, 2001), a cultivar with near immunity to the peanut root-knot nematode. The population was advance to the F4 using single seed descent. One-hundred twenty-three individual F4 plants were harvested.
A few seeds from each plant were used to evaluate the population for resistance to M. arenaria using the greenhouse screening technique described by Holbrook et al. (1983) with three replications. Plants were grown in steam-pasteurized loamy sand (850 g kg–1 sand, 110 g kg–1 silt, 40 g kg–1 clay). Each pot was inoculated with 3500 eggs of M. arenaria race 1 (Gibbs isolate) that had been cultured alternatively on tomato (Lycopersicon esculentum Mill. cv. ‘Rutgers’) and peanut to reduce potential contamination from M. incognita (a parasite of tomato but not peanut). Nematode inoculum was prepared using the NaOCl method (Hussey and Barker, 1973) and applied at 10 d after planting.
Approximately 70 d after inoculation, plants were uprooted and washed clean of soil. Roots were placed in 1000-mL beakers containing 300 mL of 0.05% (v/v) phloxine B solution for 3 to 5 min (Daykin and Hussey, 1985). Each plant was indexed for root galls and egg masses using the following scale: 0 = no galls or no egg masses, 1 = 1 to 2, 2 = 3 to 10, 3 = 11 to 30, 4 = 31 to 100, and 5 = more than 100 galls or egg masses root system–1 (Taylor and Sasser, 1978).
The remaining F4:5 seeds were planted the following year (2002) in single replicate plots at the Gibbs farm (Tifton loamy sand [fine-loamy, Kaolinitic, thermic Plinthic Kandiudult]) in Tift County, GA. Plots consisted of two rows 80 cm apart and 3 m long. Spotted wilt intensity was evaluated in each plot using a disease intensity rating that represents a combination of incidence and severity as described by Culbreath et al. (1997). The number of 0.3-m portions of row containing severely stunted, chlorotic, wilted, or dead plants was counted and converted to a percentage of row length for comparison of genotypes. Plot C724-19 was selected on the basis of resistance to TSWV and visual observation of yield after digging. We harvested 30 individual plants from this plot because the nematode screening data indicated that this family may still be segregating for nematode resistance. These 30 lines were evaluated in additional greenhouse and field screens, and a nematode resistant (C724-19-15) line was selected for more intensive study. A near isogenic line (C724-19-25) without resistance to the peanut root-knot nematode was also developed (Holbrook et al., 2008).
Tifguard, along with nematode resistant and susceptible check cultivars was tested for resistance to M. arenaria using the greenhouse technique described above with six replications. After the plants were indexed for root galls and egg masses, roots were blotted dry and weighed, and nematode eggs were collected with 1.0% (v/v) NaOCl and counted.
The same genotypes were also planted on 14 May 2004 and 28 Apr. 2005 in fields with little or no M. arenaria at the Gibbs Farm in Tift County, GA. Each test was planted in a randomized complete block design with two replications in 2004 and three replications in 2005. Plots consisted of two rows 80 cm apart and 4.6 m long. Entries were planted at 15 seeds m–1. Plots were managed throughout the growing season by standard production practices and were irrigated as needed. Spotted wilt intensity was evaluated in each plot using the disease intensity rating as previously described. Plots were dug 25 Sept. 2004 and 9 Sept. 2005. The crop was picked the following day using a self-propelled small plot combine. Pods were dried with forced air (35°C) until kernel moisture reached about 8%.
Tifguard, along with check genotypes, was also tested in two fields that were heavily infested with M. arenaria. One field was at the Bowen Farm (Ocilla loamy coarse sand [loamy, siliceous, semiactive, thermic Aquic Arenic Paleudults]) in Tift County. This test was planted on 11 May 2006, dug on 19 September, and picked on 20 September. The other field was at the Gibbs Farm. This test was planted on 24 May 2006, dug and picked on 10 October. Immediately after digging the roots from 10 randomly selected plants per plot were clipped and bagged. These roots were taken to the laboratory where they were visually rated for root galling using a 0 (no galling) to 10 (severe galling) scale. With the exception of no nematicide usage, plots were managed throughout the growing season by standard production practices and were irrigated as needed.
All data were subjected to analysis of variance, and genotypic means were compared by Fisher's protected least significant difference. Unless otherwise stated, all differences referred to in the text were significant at P 
0.05.
Characteristics
Root-gall index, egg-mass index, and eggs per gram of fresh root all clearly indicated that Tifguard is resistant to M. arenaria (Table 1 ). Results for Tifguard were very similar to COAN and ‘NemaTAM’ (Simpson et al., 2003), the two nematode-resistant check cultivars. The values for all three criteria for Tifguard were significantly lower than the nematode-susceptible cultivars Georgia Green (Branch, 1996) and C-99R.
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Availability
The Georgia Seed Development Commission (2420 South Milledge Ave., Athens, GA 30605) will produce and distribute Foundation seed for this cultivar. The USDA-ARS will be responsible for maintenance of Breeder seed. For breeding and research use, small quantities of Tifguard seed can be obtained from the corresponding author for at least 5 yr from the date of this publication. Seed of Tifguard was deposited in the National Plant Germplasm System. Protection for Tifguard under the U.S. Plant Variety Protection Act Title V will be sought.
Acknowledgments
The contributions and technical support of Brad Buchanan, Weibo Dong, Jason Golden, Vickie Hogan, Dannie Mauldin, and Betty Tyler are gratefully acknowledged. This work was supported in part by funds from the Georgia Peanut Commission.
Footnotes
All rights reserved. No part of this periodical may be reproduced or transmitted in any form or by any means, electronic or mechanical, including photocopying, recording, or any information storage and retrieval system, without permission in writing from the publisher. Permission for printing and for reprinting the material contained herein has been obtained by the publisher.
Received for publication December 4, 2007.
References
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