EVALUATION OF SWEET POTATO (IPOMEA BATATAS, L.) CULTIVARS AND ADVANCED BREEDING LINES FOR RESISTANCE TO TWO ROOT-KNOT .NEMATODE SPECIES (M. INCOGNITA & M. JAVANICA)

ABSTRACT

by Conrad K. Bonsi Several sweet potato cultivars and advanced breeding lines were screened in both the greenhouse and a heavily nematode infested field for resistance to root-knot nematode species (Meloidogyne incognita and Meloidogvne javanica). Two greenhouse methods, "The Bed Method" and "The Improved Method" were used for indoor investigations. A heavily nematode infested field was used for outdoor investigations. The infestation in the field had been built up over a period of three years. Plants in all investigations were evaluated on a scale of 1-5 based on the percentage number of roots infested using both gall and egg mass indices. Several breeding lines, namely, FH-7-72, TI-1&92, L3-186, TI-15S-76, TI-152-76 and TI-I8 4 -7 6 were resistant to M. incognita; however only FH-7-72, TI-1892 and L3-186 were comparable to ’Jewel’ in their resistance. ’Carver’ and ’Rojo Blanco’, two newly released cultivars from Tuskegee Institute were intermediate in resistance and susceptibility respectively when inoculated with M. incognita. When plants were screened for resistance to M. .jayanica, TI-1892, ’Carver’, FH-7-72, TI-1895, TI-164-76, TI-152-76, viii TI-1S6-76, TI-6 2-7 6 , TI-174-76 and TI-184-76 were found to be resistant, with TI-1392, TI-152-76 and TI-174-76 showing the highest resistance levels. TI-1892 and TI-152-76 were higher in their resistance levels than other breeding lines when plants were inoculated with a mixed population of M. incognita and M. javanica. Although FH-7-72, TI-I8 4 -7 6 and TI-174-76 were resistant to both of these species in pure cultures, they were intermediate in resistance when tested in a mixed population. This indicated that plants which are resistant to either of the species in pure cultures may become susceptible in a mixed species population. Plants evaluated in the field study indicated that FH-7-72 and TI-1892 were comparable to 'Jewel’ in resistance, while •Carver* was intermediate in resistance and ’Rojo Blanco’ was susceptible. TI-263-76 and TI-1895 showed high tolerance levels to nematode population densities at levels below 1,000 larvae/pint of soil. Although cracking and rotting of storage roots were observed in all cultivars, it is believed that this might have been enhanced by root-knot nematode infestations. The Improved Method allowed plants to be evaluated within the shortest period of time when all the three tests were compared. It also allowed periodic root observations -without physically damaging the plants. All three methods produced comparable results. ix INTRODUCTIO