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The southern root-knot nematode, Meloidogyne incognita is a noxious and widespread plant-parasitic nematode, that causes serious damage to sweet potato, Ipomoea batatas storage root both in quality and yields (Clark and Moyer 1988). In this crop, therefore, many resistant cultivars have been developed to deal with this destructive pest. One such cultivar, ‘Minamiyutaka’ was developed using a wild hexaploid accession (K123-11) can be crossed with sweet potato and is highly resistant to nematode (Ono et al. 1977). Tokui et al. (1992) examined the mode of inheritance of the resistance to an M. incognita population in diploid I. trifida. However, in M. incognita, several races with different pathogenicity to sweet potato have recently been identified (Sano et al. 2002). It is thus necessary to evaluate the resistance of I. trifida to these pathogenic races and clarify the mechanism of the resistance. In the present study, the resistance of diploid I. trifida to two major races of M. incognita was evaluated and the histological responses occurring in resistant and susceptible plants were examined. Two major races of M. incognita, SP1 and SP2, both widely prevalent in Kyushu, Japan, were used in this study. The latter race reproduces on a differential sweet potato host, ‘Norin No. 2’, unlike the former (Sano et al. 2002). These two races were raised from an egg-mass and maintained on tomato (cv. Pritz) at a temperature of 25°C. Egg-masses produced on the root systems of the tomato plant were extracted and second stage juvenile (J2) worms, freshly hatched in tap water at 25°C, were used for inoculation. The F1 plants were produced from two original (Mexico, Tehuantepec) and resistant clonal lines (4FR15-3 × 4FR18-1), and these populations were crossed again to a sensitive clonal line (G5-6) (unpublished). Seeds from seven cross combinations were provided from Dr. I. Shiotani. The nineteen plants (Fig. 1) from 7 cross combinations of I. trifida (diploid) were evaluated for the above two races. Sweet potato cultivars ‘Kokei No.14’ and ‘Minamiyutaka’ were used as susceptible and resistant controls, respectively. Each of the plants was rooted in vitro using a culture medium of MS (Murashige and Skoog 1962) for all plants and the cultivars were transplanted into a pot with 200 g of sterilized soil. After five days in a growth chamber at 25°C, each pot was inoculated with 500 J2 worms and plants were grown in the same growth chamber. Forty days after inoculation, all root systems were washed and stained with Phloxine B and the eggmasses produced were counted. The experiments were repeated four times (two plants each time). Based on the results of the resistance evaluation, a highly susceptible plant ‘C-3’ and a highly resistant plant ‘C-1’, both derived from the same parents, were selected and subjected to histological examination. Plant materials were inoculated with J2 worms of M. incognita SP1 in the same manner as described above. Three, five and seven days after inoculation, the five replicated root systems were harvested and stained with aniline blue by a modified cold stain method (De Guiran 1967) to count the number of penetrated J2 worms and to observe the histological reactions. In addition, the root tips with J2 worms penetrating the Note

Resistance to Two Races of Meloidogyne incognita and Resistance Mechanism in Diploid Ipomoea trifida