Peanut (Arachis hypogaea L.) is an important cash crop in the southeastern U.S. Peanut growers are demanding cultivars and management practices that reduce production costs so they can grow peanuts profitably to compete with imports. Peanut breeders have provided high yielding cultivars with good quality profiles for industry and consumer use. However, all of these cultivars are susceptible to one or more commonly occurring diseases. Few cultivars with multiple disease resistances are available, and none that has a sufficient level of leaf spot resistance to be grown without fungicide applications. Further, adverse weather conditions may delay timely sprays leading to heavy disease infestation that reduces yield and quality of peanuts. Thus, there is a need for high yielding, novel disease resistant peanut cultivars and progress in breeding for resistance has been slow due to the limited number of resistant genotypes available within the A. hypogaea germplasm. However, it is noteworthy that extensive work has been reported in developing leaf spot resistant runner cultivars using resistant plant introductions available within A. hypogaea gene pool (Branch, 2002; Gorbet and Shokes, 2002a and 2002b).

A potential source of disease resistance genes for peanut improvement is the wild species. The genus contains about 80 diploid (2n  =  2x  =  20), aneuploid (2n  =  2x  =  18) and tetraploid (2n  =  4x  =  40) species that have been grouped into nine botanical sections (Krapovickas and Gregory, 1994; Valls and Simpson, 2005) and A. hypogaea will only hybridize with species in section Arachis. Disease evaluations of section Arachis species revealed several with high levels of resistance to many important pathogens and pests (Lynch and Mack, 1995; Stalker and Moss, 1987; Stalker and Simpson, 1995). Although wild species of section Arachis have been suggested as sources of resistance genes since the 1960s (Smartt and Gregory, 1967), only two disease resistant, runner commercial cultivars have been released to-date from interspecific hybridization (Simpson and Starr, 2001; Simpson et al., 2003). A major reason for this slow and limited use of Arachis species for peanut improvement has been their ploidy level differences and genomic incompatibilities with A. hypogaea (Tallury et al., 2005). Nonetheless, several tetraploid interspecific hybrid lines derived from A. cardenasii Krapov. & W.C. Gregory, A. stenosperma Krapov. & W.C. Gregory, A. diogoi Hoehne, A. batizocoi Krapov. & W.C. Gregory and A. correntina (Burkart) Krapov. & W.C. Gregory, have been developed in the NCSU peanut genetics program and 16 germplasm lines were released (Stalker and Beute, 1993; Stalker and Lynch, 2002; Stalker et al., 2002a; Stalker et al., 2002b; Isleib et al., 2006a). These germplasm lines are publicly available and have high levels of resistances to early and late leaf spots (Cercospora arachidicola Hori and Cercosporidium personatum (Berk. & Curt.) Deighton), root-knot nematode (Meloidogyne arenaria [Neal] Chitwood), tomato spotted wilt virus (TSWV), Sclerotinia blight (S. minor Jagger), Cylindrocladium black rot (CBR, C. parasiticum Crous, Wingfield & Alfenas), web blotch (Phoma arachidicola) and several insects, such as potato leafhopper (Empoasca fabae [Harris]), corn earworm (Helicoverpa zea [Boddie]), and lesser cornstalk borer (Elasmopalpus lignosellus [Zeller]). Several of these germplasm lines and other disease resistant selections have been hybridized with susceptible commercial cultivars and additional selections from the resulting progenies were evaluated in the present study (Table 1) for possible cultivar releases.

The overall goal of our research program is to develop peanut cultivars with resistance to the most common diseases in the V-C production area, which include early leaf spot (ELS), TSWV, Sclerotinia blight (SB) and CBR. The specific goals of the research reported in this paper are 1) to compare tetraploid, interspecific hybrid derived breeding lines with commercial peanut cultivars for their agronomic performance and leaf spot resistance, and 2) to evaluate the potential of tetraploid interspecific hybrid derived breeding lines for commercial cultivation in the V-C peanut production area.

Twenty-six tetraploid, interspecific hybrid derived breeding lines, six A. hypogaea lines resistant to early- and late leaf spots (resistant checks), and 11 commercial cultivars (susceptible checks) (Table 1) [NC 6 (Campbell et al., 1977); NC 12C (Isleib et al., 1997); Gregory (Isleib et al., 1999); Perry (Isleib et al., 2003); Brantley ( Isleib et al., 2006b); Phillips ( Isleib et al., 2006c); VA-C 92R (Mozingo et al., 1994); VA 98R (Mozingo et al., 2000); Wilson (Mozingo et al., 2004); NC 7 (Wynne et al., 1979) and NC-V11 (Wynne et al., 1991)] were evaluated in field tests at the Peanut Belt Research Station in Lewiston, NC from 2004 to 2006. Experimental plots consisted of 2-rows each, 7.3 m long on 91 cm wide beds and 28 seeds/row were planted with 25 cm spacing. The plots were planted on 11 May in each of the 3 years. In each year the experimental design was a lattice with two replications, with a 10×10 simple square lattice in 2004 and 10×9 double rectangular lattices in 2005. The additional lines evaluated in these experiments (57 in 2004, 47 in 2005 and 2006) were experimental A. hypogaea lines from the N.C. State Univ. peanut-breeding project. Data on these experimental lines were not reported in this study (T.G. Isleib, unpublished data). Recommended management practices were followed during the growing of the crop but no fungicidal sprays were used to control leaf spots.

Defoliation ratings were recorded on plots on 2, 14, and 21 Sept. in 2004, on 21 and 27 Sept. in 2005, and on 9 and 21 Sept. in 2006, respectively. The severity of the disease was rated on a proportional scale from 1  =  no defoliation (resistant) to 9  =  complete defoliation (susceptible). The 3 yrs of data were combined to generate mean disease scores. Pod yield and grade were measured on all plots and combined to generate means. Analysis of variance was performed on defoliation scores, yields and grades. Means were adjusted for block effects in the incomplete block design if block effects were found to be significant by F-test. The means for the 43 entries from each year were used to perform an analysis of variance as if for a randomized complete block design using years as the blocking variable. Means were separated by t-tests. All analyses were performed using the general linear models procedure (PROC GLM) of the SAS statistical software package (SAS Institute, Inc. 2001).

Currently grown cultivars in the V-C area produced less yield with more disease than the interspecific hybrid derived breeding lines. Many interspecific hybrid derived breeding lines yielded 3000 kg/ha without chemical control of leaf spots. This indicates that the interspecific hybrid derived breeding lines evaluated in this study have superior leaf spot resistance combined with high yield. Because the diploid Arachis species used in the pedigrees of the interspecific hybrid breeding lines have a broad range of disease resistances to several common pathogens and pests of peanut (Stalker and Moss, 1987), they may also be resistant to other diseases of peanut. These lines need to be evaluated in multiple environments in the V-C area before a recommendation for commercial use can be made.