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Diallel Analysis of Resistance in White Clover to Southern Root-Knot Nematode

USDA-ARS, Crop Science Res. Lab., Forage Res. Unit, P.O. Box 5367, Mississippi State, MS 39762

^* Corresponding author.

Persistence of white clover, Trifolium repens L., is reduced in areas infested with the southern root-knot nematode, Meloidogyne incognita (Kofoid & White) Chitwood, in the southern USA. Little information is available on the resistance of white clover to M. incognita. This study was conducted to determine the relative importance of general (GCA) and specific (SCA) combining ability, and maternal and maternal reciprocal effects on resistance to M. incognita in selected white clover plants. Three M. incognita-resistant and three susceptible white clover plants were used as parents in a diallel cross. Progeny from 30 F₁ crosses, including reciprocals, and ‘Regal’ white clover (a susceptible cultivar) were evaluated under greenhouse conditions for reaction to inoculation with 1500 M. incognita (Race 4) eggs per plant. Nine weeks after inoculation, the roots were washed and scored on a 0 to 5 scale for egg mass index (EI; 0 no egg masses, 5 = > 100 egg masses) and for percent of the root system galled (PRSG; 0 = galls, 5 = 91 to 100% galled). Six crosses had EI scores lower than Regal, and nine crosses had PRSG scores lower than Regal. A cross of resistant parents, ND 2 x ND 3, had the lowest EI (2.3) and PRSG (1.1) scores. Both GCA and SCA were significant sources of variation for both EI and PRSG. The GCA effects were more important than SCA effects for both traits. No maternal or nonmaternal reciprocal effects were observed for EI and PRSG. Resistant parents ND 1 and ND 3 produced progeny with the least M. incognita reproduction and galling. No crosses were observed that performed significantly better than expected from the GCA effects of the parents, though two crosses performed worse than expected. Resistance to M. incognita. reproduction and root galling in these and other lines are being used to develop nematode resistant white clover.

Received for publication July 22, 1991.

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