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  • Author or Editor: David J. Chitwood x
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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.

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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.

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Clove oil derived from the clove plant [Syzygium aromaticum (=Eugenia caryophyllata)] is active against various soil-borne plant pathogens and therefore has potential for use as a bio-based pesticide. A clove oil formulation previously found to be toxic to the southern root-knot nematode (Meloidogyne incognita) in laboratory assays was investigated in greenhouse studies for nematode suppression and phytotoxicity on vegetable crops. Phytotoxicity studies were conducted with 0.1%, 0.2%, and 0.3% clove oil applied to soil 0, 2, 5, and 7 days before transplant of cucumber (Cucumis sativus), muskmelon (Cucumis melo), pepper (Capsicum annuum), and tomato (Solanum lycopersicum) seedlings. Tomato seedlings were the most sensitive to clove oil. The 0.2% and 0.3% clove oil concentrations applied as drenches at transplant (0 day) were the most phytotoxic to seedlings of all the tested vegetable species, with only 0% to 50% seedling survival. Most of the clove oil concentrations applied as drenches at transplant decreased shoot heights and fresh shoot weights of all seedlings. Some applications of clove oil at 0.2% and 0.3%, applied 2, 5, or 7 days before transplant also significantly reduced shoot growth, especially of pepper and tomato. Greenhouse experiments evaluating suppression of nematode populations on cucumber were conducted with 0.10%, 0.15%, and 0.20% clove oil applied 7 days before transplant. Overall, plants inoculated with nematodes tended to have smaller shoots and heavier roots than plants without nematodes. Effects of clove oil treatments on nematode population densities were inconsistent between the two trials. In Trial 1, 0.10% and 0.15% clove oil decreased population densities compared with the carrier control. In Trial 2, nematode population densities were lowest in the water and carrier control treatments. The results indicate that, with the tested clove oil formulation and application times, southern root-knot nematode populations would not be consistently reduced with clove oil concentrations that were not phytotoxic to one or more of the tested vegetable crops.

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