Resistance to root-knot nematodes (Meloidogyne spp.) in tomato (Lycopersicon esculentum Mill.) plants has been reported to break down at soil temperatures >28C. We evaluated in vitro root explants of tomato heterozygous (Mimi), homozygous (MiMi) at the Mi locus, or lacking the Mi-1 gene for resistance to Meloidogyne incognita (Kofoid & White) Chitwood and Meloidogyne arenaria (Neal) Chitwood at 28, 31, 34, and 37C. Genotypes Ace-55 UF and Rutgers, lacking the dominant allele, were susceptible to M. incognita and M. arenaria at all temperatures. Genotypes possessing the dominant allele (heterozygous or homozygous) were equally resistant to both nematode species. The resistance level in these genotypes was maintained fully at 31C, partially maintained at 34C, and lost at 37C. Resistance in the heat-tolerant Mi-heterozygous accession CLN 475-BC1F2-265-4-19 was not different from that of the heat-sensitive genotypes. As temperature increased, the genotypes differed in their sensitivity to resistance conferred by the Mi-1 locus.
The Mi gene, which is the only source of resistance to the root-knot nematodes M. incognita and M. javanica in tomatoes, is effective only at soil temperatures below 28C. This single dominant gene exists in a homozygous form in certain tomato cultivars, in a heterozygous form in others, and is lacking in others. It has also been introduced into heat-tolerant and heat-sensitive cultivars. The availability of such genotypes allows determining whether a) the homozygous form provides more resistance than the heterozygous form and b) heat tolerance protects the Mi gene at high-temperature stress. The results of in vitro tests using excised roots show that the resistance offered by the Mi gene in the homozygous or the heterozygous form to M. incognita and M. arenaria was the same. The presence of heat tolerance gene did not protect the Mi gene from losing its effectiveness above 28C.