The tomato gene Mi-1.2 confers resistance against root-knot nematodes and some isolates of potato aphid. Resistance to the whitefly Bemisia tabaci previously has been observed in Mi-bearing commercial tomato cultivars, suggesting that Mi, or a closely linked gene, is responsible for the resistance. The response of two biotypes of B. tabaci to tomato carrying the cloned Mi was compared with that of the isogenic untransformed tomato line Moneymaker. Our results indicate that Mi-1.2 is responsible for the resistance in tomato plants to both B- and Q- biotypes. Mi-1.2 is unique among characterized resistance genes in its activity against three very different organisms (root-knot nematodes, aphids, and whiteflies). These pests are among the most important on tomato crops worldwide, making Mi a valuable resource in integrated pest management programs.

Developmental rates of the B- and Q-biotypes of Bemisia tabaci (Gennadius, 1889) were studied at seven constant temperatures (17, 20, 23, 26, 30, 33,

Two commercial cultivars of tomato, Alta and Peto 95, the accession line number LA716 ofLycopersicon pennelliiand lines 94GH-006 and 94GH-033 (backcrosses between Peto 95 and LA716), with different leaf acyl sugar contents were screened for resistance toBemisia argentifoliiBellows & Perring (corresponding to the Spanish B-biotype ofBemisia tabaci(Gennadius)), in greenhouse- and field-no-choice experiments. There was no oviposition on LA716 (with the highest acyl sugar content) while the greatest fecundity and fertility values were observed on the cultivar Alta (no acyl sugar content). However, no clear relationship was found between the low acyl sugar content in the other tomato cultivars tested and whitefly reproduction. Thus, resistance toB. tabacidid not appear to correlate with acyl sugar content below a threshold level of 37.8 μg cm^-2leaf. In a greenhouse choice-assay,B. tabaciexhibited reduced host preference and reproduction on the commercial tomato cultivars Motelle, VFN8 and Ronita all of which carry theMigene resistance toMeloidogynenematodes and the aphidMacrosiphum euphorbiae(Thomas), than on theMi-lacking cultivars Moneymaker, Rio Fuego and Roma. When data ofMi-bearing plants were pooled, the mean values for daily infestation and pupal production ofB. tabaciwere significantly lower than those ofMi-lacking plants. This reflected a level of antixenosis- and antibiosis-based resistance in commercial tomato and indicated thatMi, or another closely linked gene, might be implicated in a partial resistance which was not associated either with the presence of glandular trichomes or their exudates. These findings support the general hypothesis for the existence of similarities among the resistance mechanisms to whiteflies, aphids and nematodes in commercial tomato plants.