The silverleaf whitefly (Bemisia argentifolii Bellows & Perring) is an important pest of tomatoes in Florida and elsewhere. Associated with populations of the whitefly is an irregular ripening disorder of fruit characterized by inhibited or incomplete ripening of longitudinal sections of fruit and by an increase in the amount of interior white tissue. Experiments were conducted during the spring and fall tomato production seasons of 1995 and 1996 to elucidate the relationship of nymphal and pupal density with severity of the disorder. Insecticides or insecticide combinations were applied at predetermined densities of whitefly nymphs and pupae and the subsequent severity of the disorder was rated separately for external and internal symptoms on red ripe fruit harvested weekly. Expression of irregular ripening symptoms, especially external symptoms, were correlated positively to the density of whitefly nymphs and pupae (number·10-1 terminal leaflets on the seventh to eighth leaf from the top of a main or lateral stem) increased. Expression of external symptoms tended to be better correlated with whitefly density when symptom severity was rated 1 and 3 weeks after estimating whitefly density for the spring and fall seasons, respectively. Expression of internal symptoms tended to be more consistently correlated with whitefly density when symptom severity was rated 2 and 3 weeks after estimating whitefly density for the spring and fall seasons, respectively.
The whitefly-transmitted Tomato yellow leaf curl virus (TYLCV) is a major pathogen of tomatoes grown in tropical and subtropical regions of the world. Several genes of different origins conferring resistance to TYLCV have been introgressed to the cultivated tomato (Solanum lycopersicum), including the single dominant gene, Ty-2, that originated from S. habrochiates and was previously mapped to a 19-cM region on the long arm of chromosome 11 delimited by restriction fragment length polymorphism markers TG36 and TG393. In the present study, we confirmed the dominant inheritance of the Ty-2 gene from TYLCV evaluation and molecular marker analysis of an F2 segregating population derived from a commercial hybrid that carries the Ty-2 gene. Evaluating recombinants recovered from the F2 progeny for TYLCV resistance localized the Ty-2 gene to a marker interval of 5.5 cM between C2_At1g07960 (82.5 cM) and C2_At4g32930 (88 cM). Additional recombinants were identified for the target region carrying the Ty-2 gene. TYLCV evaluation of the progeny from these recombinants further delimited the Ty-2 gene to a 4.5-cM interval between C2_At1g07960 (82.5 cM) and cLEN-11-F24 (87 cM). The smaller introgressions no longer include the fusarium wilt race 2 resistance locus (I-2), which should facilitate combining the two resistance genes in cis configuration. The polymerase chain reaction-based markers developed from the present study can be used to precisely monitor the introgression of the Ty-2 gene, thus offering the opportunity to pyramid TYLCV resistance genes from different sources as well as resistance genes for other pathogens into elite tomato cultivars.
Cultivated tomato (Solanum lycopersicum L.) accessions have been susceptible to the whitefly-transmitted begomoviruses Tomato yellow leaf curl virus (TYLCV) and Tomato mottle virus (ToMoV) that can cause serious crop damage. S. habrochaites accession LA1777 has been reported to be resistant to TYLCV. To locate putative virus resistance genes, 89 recombinant inbred lines (RILs) previously developed from LA1777 in a tomato background, LA1777 and the susceptible RIL parent E6203, were screened against the begomoviruses TYLCV and ToMoV. An initial study showed 18 RILs had less disease severity to TYLCV or ToMoV. Eight RILs had S. habrochaites alleles at TG27 (restriction fragment length polymorphism marker) on chromosome 1, three RILs had S. habrochaites alleles at TG202 on chromosome 7, and one RIL had S. habrochaites alleles at both marker loci. The RILs with these regions were intercrossed in 10 different cross combinations and F2 seeds were then obtained. The F2 progenies were inoculated separately with both viruses and then evaluated in the field. The F2 plants with less disease severity were selected, but most did not have the markers from the hypothetical resistance regions. The F3 progenies were then inoculated and rated for disease severity to both viruses. None of the F3s demonstrated any increased level of resistance, even if derived from F2s homozygous for the target regions from both chromosomes. All plants from every cross combination were susceptible for both TYLCV and ToMoV, suggesting that there is no begomovirus resistance in the LA1777 RIL population. Some limitations of capturing all genes in an RIL population derived from an outcrossing accession are discussed.
The sweetpotato whitefly, Bemisia tabaci (Gennadius), was associated with symptoms of a silverleaf disorder of acorn squash (Cucurbita pepo L. cvs. Table King Bush and Table Ace) in cage studies in the greenhouse. Symptoms appeared on uninfested leaves that developed after plants were infested with the whitefly. When the infested lower leaves were removed and the young leaves protected from infestation with insecticides, new growth was asymptomatic or nearly so and symptomatic leaves remained symptomatic. Symptom expression was related more to nymphal density than to adult density since the relationship between log nymph density and symptoms was linear when adult densities were equal.
Tomato yellow leaf curl virus (TYLCV), a Begomovirus in the family Geminiviridae, is an important disease of cultivated tomato (Solanum lycopersicum L.) in many parts of the world. Disease is managed primarily by chemical control of the vector, the sweetpotato whitefly, Bemisia tabaci (Genn.), and by growing resistant varieties. Resistance derived from the cultivar Tyking is being used in many breeding programs, but the location of resistance factors has not been reported. The breeding lines Fla. 8753 and Fla. 344 both have high levels of resistance to TYLCV derived from ‘Tyking’ and from S. chilense accession LA 1938, but none of their parent lines contain any of the known genes Ty-1 to Ty-4. An additional resistance locus, Ty-5, was recently identified, and to determine if this locus controls TYLCV resistance in Fla. 8753 and Fla. 344, appropriate segregating populations were analyzed using the Ty-5 marker, SlNAC1. Results show that SlNAC1 cosegregates with a recessive allele derived from ‘Tyking’. We suggest the gene symbol ty-5 be used to describe this gene. Mean disease severity of progeny homozygous for either the resistant or susceptible alleles did not equal parental levels of resistance and susceptibility, respectively, suggesting the involvement of an additional gene that is likely derived from LA1938.