Relationships between Root-Knot Nematode Resistance and Plant Growth in Upland Cotton: Galling Index as a Criterion Jinfa Zhang,* C. Waddell, C. Sengupta-Gopalan, C. Potenza, and R. G. Cantrell ABSTRACT The southern root-knot nematode (RKN) [Meloidogyne incognita (Kofoid and White) Chitwood] is one of the most destructive pests in the Cotton Belt of the USA. The lack of an economical evaluation method for RKN resistance has hindered the development of resistant cultivars. This investigation was conducted to develop an improved RKN evaluation technique. Twelve genotypes including seven suscep- tible (S) lines, one moderately resistant (MR) line, three highly resis- tant (R) lines derived from ‘Auburn 623 RNR,’ and one F 1 between DP 33B (S) and ‘Auburn 634 RNR’ (R) were evaluated in the green- house for plant growth, RKN egg reproduction, and root galling in Experiment 1. RKN egg reproduction was highly and positively corre- lated with galling index and both were highly and negatively corre- lated with plant growth characteristics including plant height, number of leaves, plant and root weight. Galling index was confirmed to be highly significantly and negatively correlated with plant height and fresh weight in Experiment 2 with 9 parents and their 36 F 1 hybrids. Galling index had highest genotypic F value and comparable coef- ficient of variance (CV) to the plant characteristics, while CV for egg counts was very high. Correlation between the two greenhouse tests in the 9 parental lines as measured by galling index was highly signif- icant. Comparison between F 1 and their parents in egg reproduction and galling revealed that the RKN resistance is partially dominant. Using a common check and double inoculation in each pot, galling index is an easy, quick and reliable method for screening large numbers of cotton plants. T HE southern root-knot nematode Race 3 has be- come one of the major threats to US cotton pro- duction with an estimated annual yield loss of 2.2% (Blasingame and Patel, 2005). Yield loss in the western cotton-growing region is even higher (for example, 5% in Arizona and New Mexico). Management of RKN in- cludes crop rotation, application of pesticides, and use of resistant cultivars. Among these options, development and use of resistant cultivars should be the most cost effective method of controlling RKN. The history of breeding for RKN resistance in cotton can be traced back to the early 1900s, but it was not until the 1950s when the first cultivar that showed moderate RKN resistance, Auburn 56, was developed. However, a major breakthrough in RKN resistance breeding was the release of the highly resistant line, Auburn 623 RNR (Shepherd, 1974c). Its resistance was transferred by backcrossing into many commercial cotton backgrounds, resulting in the development of Auburn 634 RNR, other Auburn germplasm, and many M series of lines (e.g., M 240 and M 315) in the 1980s (Shepherd, 1982; Shepherd et al., 1996). The resistant plants permit penetration of second-stage juveniles (J2) into the roots and initiation of feeding sites, but most J2 do not ultimately develop into egg-bearing adults. RKN reproduction is reduced by more than 98% at the end of the normal reproductive cycle (Jenkins et al., 1995; Creech et al., 1995). Even though these lines had improved agronomic traits, the high level of RKN resistance has never been transferred to any commercial cotton cultivars (Robinson et al., 1999). Only a moderate level of RKN resistance was re- ported and confirmed in a few commercial cultivars, i.e., ST LA 887, PM 1560, and Acala Nem-X (Robinson et al., 1999; Ogallo et al., 1999). The proportion of RKN resistance that is due to total genetic or additive genetic variation, i.e., broad-sense heritability or narrow-sense heritability, has not been re- liably estimated due to high experimental errors in RKN resistance evaluation. The lack of a reliable and efficient RKN resistance evaluation method has been one of the major limiting factors in analyzing the genetics of RKN resistance, and breeding this trait in cotton. Because of this, few consistent conclusions have ever been drawn, even though the inheritance of RKN resistance for Auburn 623 RNR and its derived resistant lines has been investigated since the 1970s using traditional Mendelian and quantitative genetics. Both RKN egg counting and galling index have been extensively used by cotton geneticists, agronomists, and nematologists (Shep- herd, 1974a, 1974b, 1974c; McPherson, 1993; McPherson et al., 1995, 2004; Colyer et al., 2000; Robinson et al., 1999, 2004; Ogallo et al., 1999; Bezawada et al., 2003; Zhou, 1999; Zhou and Starr, 2003; Zhang et al., 2004; Ynturi et al., 2004). In comparison with counting root eggs or egg masses per plant, galling index would be preferred in breeding because it can be more easily assayed on large numbers of plants as needed in breeding programs. In addition, galling index is a direct measure- ment of cotton plant response to RKN infection. Zhou and Starr (2003) reported that the initial RKN population density is significantly correlated with galling index and seedcotton yield reduction in susceptible and resistant cultivars. However, the relationship between RKN repro- duction and RKN damage to cotton in terms of galling and cotton growth characteristics has not been exten- sively investigated. The objectives of the present study were to develop a reliable, easy, and fast RKN screening technique, and demonstrate the validity of the technique Jinfa Zhang, C. Waddell, C. Sengupta-Gopalan, C. Potenza, Dep. of Plant and Environ. Sci., Box 30003, New Mexico State University, Las Cruces, NM 88003; R.G. Cantrell, Cotton Incorporated, Cary, NC 27513 USA. Received 24 Sept. 2005. *Corresponding author (jinzhang@nmsu.edu). Published in Crop Sci. 46:1581–1586 (2006). Crop Breeding & Genetics doi:10.2135/cropsci2005.09-0333 ª Crop Science Society of America 677 S. Segoe Rd., Madison, WI 53711 USA Abbreviations: ANOVA, analysis of variance; BG, Bollgard; CV, coefficient of variation; LSD, least significant difference; RKN, root knot nematode. Reproduced from Crop Science. Published by Crop Science Society of America. All copyrights reserved. 1581 Published online May 18, 2006