Tomato yellow leaf curl virus (TYLCV) is one of the most important tomato (Lycopersicon esculentum Mill.) pathogens in the Middle East. Reports from the region (Pilowsky and Cohen, 1974; El-Hammady et al., 1976; Abu Gharbiah et al., 1978; Hassan et al., 1882, 1985; Kasrawi et al., 1988) and India (Nariani and Vasudeva, 1963) indicated severe losses from the disease and a lack of resistance to the virus in commercial tomato cultivars. Resistance to the virus has, however, been reported by the same authors in some accessions of L. cheesmanii ssp. minor, L. chilense, L. hirsutum, and L. peruvianum . Also resistance (slight susceptibility) was reported in tomato line EC 104395 in India (Varma, 1980) and Sudan (Fadl and Burgstaller, 1986). L. peruvianum CMV INRA was found to be susceptible to graft inoculation, but resistant to the natural mean of infection by the tobacco whitefly Bemisia tabaci Gennadius (Hassan et al., 1982; Mazyad et al., 1982). It's susceptibility, as being virus-carrier, was later confirmed by Kasrawi et al. (1988) who indicated that all symptomless graft-inoculated genotypes in their investigation (5 other accessions of L. peruvianum, 1 of L. peruvianum f. humifusum, and 1 of L. hirsutum) were, likewise, virus-carriers. Also, tolerance to TYLCV was reported in L. pimpinellifolium LA 121 which is susceptible to virus infection by the insect vector, supports virus multiplication, but shows very slight symptoms or none (Pilowsky and Cohen, 1974; Makkouk, 1978; Hassan et al., 1982; Mazyad et al., 1982; Kasrawi et al., 1988). Meanwhile, several other accessions of L. hirsutum, L. pimpinellifolium, and L. peruvianum were found to be susceptible to TYLCV (Nariani and Vasudeva, 1963; El-Hammady et al., 1976; Hassan et al., 1982; Kasrawy et al., 1988).
Though tolerance of different L. pimpinellifolium accessions (LA 121, LA 1582, and selections of LA 1478 and Hirsute-INRA) was reported to be controlled by an incompletely dominant (Pilosky and Cohen, 1974) or dominant (Geneif, 1984; Yassin, 1987, Kasrawi, 1989) gene, others reported a quantitative nature of tolerance in L. pimpinellifolium LA 121 and LA 373 (Hassan et al., 1984a) and quantitative resistance in both L. cheesmanii ssp. minor LA 1401 and L. hirsutum LA 386 (Hassan et al., 1984b). Additionally, difficulties were encountered in selecting resistant segregates in the early generations of the cross 'Mortelglan' x L. peruvianum CMV INRA (Hassan et al., 1987). Currently, no resistant tomato cultivar has been released; with the exception of the Israeli cultivar 'TY 20' which was described as 'tolerant', with its tolerance derived from L. peruvianum.
The objective of these trials was to search for a reliable source of resistance in the wild and domestic Lycopersicon germplasm, particularly in L. esculentum, and to study the response of some of the previously-reported TYLCV-resistant or tolerant germplasm to the virus under U.A.E. conditions.
One thousand seven hundreds ninety-six wild and domestic Lycopersicon accessions were evaluated for TYLCV resistance under field conditions at the Sulymat Agricultural Experiment Stations, Al-Ain, United Arab Emirates during the 1988/1989 and 1989/1990 autumn plantings. The first year trial included the following: (a) 1716 tomato plant introductions (PIs), viz., all L. esculentum accessions listed in the U.S.D.A. inventory of PIs from PI 303751 to PI 499370 except the following PIs: 303753, 303757, 303771, 321057, 325153, 330337, 358501, 358816, 358819, 367963, 379410, 390405, 390409, 390492, 390494, 390497, 390499-390504, 390510, 391605, 391614, 401764, 401767, 406756, 406764, 406781, 406826, 406903, 406972, 410948, 438882, 438892-438894, 451941, 476385, 479204, and 499370; (b) tomato line EC 104395; and (c) 76 accessions of 11 wild Lycopersicon species, subspecies, and hybrids, including 1 unspecified Lycopersicon hybrid, 1 of L. cheesmanii f.sp minor, 20 of L. esculentum x L. pimpinellifolium, 1 of L. esculentum var. cerasiforme, 10 of L. hirsutum, 1 of L. hirsutum f. glabratum, 1 of L. pennellii, 12 of L. peruvianum, 1 of L. peruvanum x L. esculentum, 27 of L. pimpinellifolium, and 2 of L. pimpinellifolium hirsute. The second year trial included the following: (a) re-evaluation of 39 L. esculentum accessions and 31 wild accessions of Lycopersicon species and hybrids and (b) tomato cultivars TY 20, 'Midi', Petropride No. 2, and 'Rossol VFN' as control. Thus, a total of 1720 L. esculentum accessions and 76 accessions of wild Lycopersicon species and hybrids were evaluated. Seeds were kindly provided by the U.S.D.A. through Dr. Charles Block (Plant Introduction Station, Ames, Iowa) for the PI accessions which accounted for 99.3% of the accessions evaluated; Dr. C.M. Rick (University of California, Davis) for 6 LA accessions; and Dr. H. Laterrot (INRA, France) for tomato line EC 104395, F2 ('Floradade' x L. pimpinellifolium hirsute), L. pimpinellifolium hirsute, and L. peruvanum CMV INRA. Seeds of commercial cultivars were obtained from seed dealers.
Screening depended on natural infection which was enhanced by placing heavily TYLCV-infected, whitefly-infested tomato plants nearby seedlings in the nursery. Each accession was assigned to one field row in the first year and four rows in a randomized complete block design in the second year of the study. In the first trial, each accession was given one of 19 reaction categories based on combinations of symptom severity (slight, moderate, and severe), disease incidence (0.0, >0.0-50, and >50-100% of the plants showing symptoms), and the earliest date of full symptom development (1.5, 2.0-2.5, and 3 - 3.5 months after transplanting for early, late, and very late categories, respectively). Also, the potentially high yielding tomato cultivars were visually selected in the first year trial. In the second year, each plant was given a disease score according to the following scale: 1, no symptoms: 2, slight; 3, moderate; and 4, severe symptoms. Individual plant ratings for each accession were added and divided by the number of evaluated plants to obtain the corresponding mean disease score.
In the first trial, 90.09%, 9.27%, 0.47%, and 0.17% of the L. esculentum accessions were, respectively, highly susceptible, moderately susceptible (to different degrees), slightly susceptible, and symptomless. Respective percentages of the wild accessions were 42.1% (mostly of the L. esculentum x L. pimpinellifolium hybrids), 15.8%, 1.3% and 1.3% (mostly of L. hirsutum and L. peruvianum). In the second year trial, only 2 L. peruvianum PIs remained symptomless (viz. PI 390670 and PI 390687), while all other re-evaluated accessions showed various degrees of susceptibility. Based on performance in both years of the study, the following accessions were selected from the germplasm evaluated as the best sources of 'tolerance' to infection with TYLCV: L. esculentum PI 365923, PI 365925 and PI 390648 (PI 475838 was the least susceptible, with a 1.3 mean disease score in the second year, but its identity as L. esculentum is questionable); L. hirsustum PI 390662; L. peruvianum PI 390669, PI 390670, PI 390681, and PI 390687; and L. pimpinellifolium PI 407543 and PI 407546. Additionally, seeds were saved from 4 symptomless plants of L. peruvianum x L. esculentum PI 306812. These first-reported-tolerant accessions and selections showed lesser symptom severity than those screened of the previously-reported-resistant (L. peruvianum CMV INRA) or -tolerant (L. pimpinellifolium LA 121, LA 373, and hirsute INRA and tomato line EC 104395) accessions which were found in this study to be either slightly or moderately susceptible. Mean disease scores were 3.7 in cv. 'TY20' and 4.0 in each of the commercial cultivars 'Rossol VFN', 'Midi;, and 'Petopride No.2' which were used as control. All plants evaluated of cv. 'TY20' exhibited TYLCV-symptoms, but their vegetative growth was relatively vigorous as compared with growth of the other accessions. Also 17 improved tomato cultivars (viz, PIs 406868, 432946, 432947, 433116, 433145, 433171, 433191, 435339, 451963, 451970, 451983, 451985, 452015, 452020, 452025, 466915, and 466917) were found by visual selection in the 1988/1989 trial to be potentially good yielders in spite of their infection with TYLCV. Their yield tolerance is reported here for the first time and merits further investigation.
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