Shlomo Cohen

Comparison of Resistance Level to Tomato Yellow Leaf Curl Virus Among Commercial Cultivars and Breeding Lines

The effect of tomato yellow leaf curl virus (TYLCV) on total yield and yield components of various resistant F1 tomato cultivars and new breeding lines was evaluated in the field. Seeds of the F1 hybrids 8484, 3761, Fiona, and Tyking and the new breeding lines TY172 and TY197 were sown in an insect-proof greenhouse. When the seedlings reached the first-leaf stage, they were inoculated with TYLCV by means of the whitefly vector Bemisia tabaci. Noninoculated plants of the same cultivar or line which were exposed to nonviruliferous whiteflies served as controls. After a short recovery period, the plants were transplanted to the field. The inoculated plants of each cultivar or line were compared with their respective control, noninoculated plants, in terms of total yield, average fruit weight and number, and plant fresh weight. Disease symptom development and virus accumulation in the inoculated plants were monitored. There were substantial differences among the different entries tested in the extent of yield loss relative to the corresponding noninoculated control plants as well as viral DNA accumulation levels. Plants of TY172 and TY197 suffered the least relative yield loss and contained the lowest level of viral DNA. Therefore, these two lines exhibited the highest level of resistance. These results clearly demonstrate the ability of the various resistant tomato cultivars and lines to inhibit the effects of the virus, even following inoculation at a very early stage of plant development.

Effect of host plant resistance to Tomato yellow leaf curl virus (TYLCV) on virus acquisition and transmission by its whitefly vector

ABSTRACT The effect that Tomato yellow leaf curl virus (TYLCV)-infected resistant tomato plants may have on virus epidemiology was studied. Four tomato genotypes that exhibit different levels of viral resistance, ranging from fully susceptible to highly resistant, served as TYLCV-infected source plants. Viral acquisition and transmission rates by white-flies following feeding on the different source plants were evaluated. TYLCV transmission rate by whiteflies that had fed on infected source plants 21 days postinoculation (DPI), shortly after the appearance of TYLCV symptoms, was negatively correlated with the level of resistance displayed by the source plant. Therefore, the higher the resistance, the lower the transmission rate. In addition, TYLCV DNA accumulation was shown to be lower in the resistant source plants compared with the susceptible plants. Whitefly survival rate, following feeding on source plants 21 DPI, was similar for all the cultivars tested. Significant differences in whitefly survival were found, however, following feeding on the infected source plants at 35 DPI; here, whitefly survival rate increased with higher levels of resistance displayed by the source plant. At 35 DPI, the susceptible plants had developed severe TYLCV disease symptoms, and transmission rates from these plants were the lowest, presumably due to the poor condition of these plants. Transmission rates from source plants displaying a medium level of resistance level were highest, with rates declining following feeding on source plants displaying higher levels of TYLCV resistance. TYLCV DNA accumulation in whiteflies following feeding on infected source plants at both 21 and 35 DPI was directly correlated with viral DNA accumulation in source plants. Results show that, in essence, the higher the resistance expressed, the less suitable the plant was as a viral source. Consequently, following acquisition from a highly resistant plant, TYLCV transmission by whiteflies will be less efficient.

Ultraviolet-deficient greenhouse environment affects whitefly : Attraction and flight-behavior

Abstract The effect of a UV-deficient environment on the attraction and dispersal behavior of whiteflies, Bemisia argentifolii (Bellows & Perring), and on the transmission efficiency of the whitefly-borne tomato yellow leaf curl geminivirus, was tested under field conditions and through controlled experiments. We found that the rate of tomato yellow leaf curl virus-disease spread to tomato plants grown under walk-in tunnels covered with regular greenhouse plastic sheets increases sharply with time, whereas the virus infection-rate under UV-absorbing sheets proceeds at a very slow pace. Average number of whiteflies trapped under regular plastic sheet tunnels was significantly higher than numbers trapped in UV-absorbing plastic sheet tunnels. Similarly, the average number of whiteflies trapped on yellow-sticky traps placed on the outside walls of tunnels covered with regular plastic was higher than the number trapped on the outside walls of tunnels covered with UV-absorbing plastic sheets. No differences were found in the whitefly’s ability to transmit tomato yellow leaf curl virus under the two types of plastic covers. Whitefly dispersal pattern under the two types of plastic covers was examined using a release-recapture experiment. In each type of walk-in tunnel we established a grid of yellow-sticky traps forming two concentric circles: an inner and an external. Under UV-absorbing tunnels, significantly higher numbers of whiteflies were captured on the internal circle of traps than the external circle. The fraction of whiteflies that were captured on the external circle was much higher under regular covers, when compared with UV-absorbing covers, suggesting that filtration of UV light hindered the ability of whiteflies to disperse inside these plastic tunnels. Our results indicate that the mechanisms by which UV-deficiency protects covered crops from insect infestation and spread of viruses are that the lack of UV interferes with insect flight orientation; and that the lack of UV radiation alters the normal behavior of the invading insects, resulting in reduced dispersal activity.

Accumulation of tomato yellow leaf curl virus DNA in tolerant and susceptible tomato lines

Four tomato lines (M-60, M-54, 1970, and 2950), an F 1 hybrid (TY-20) tolerant to tomato yellow leaf curl virus (TYLCV), and the susceptible cultivars 1651, 1630, 614-1, and 199-2 were compared for accumulation of viral DNA in TYLCV-infected plants. DNA was analyzed by either alkaline transfer or dot spot hybridization using a cloned viral DNA as a probe. Results obtained with either procedure indicated that in all tested lines viral DNA accumulation peaked at 12-15 days after inoculation. At this time, accumulation of viral DNA in the tolerant genotypes was 10-50% lower than in the susceptible ones. Viral DNA then decreased up to 50% in both the tolerant and the susceptible lines, and it remained at that level (.)

Evaluation of Oreochromis niloticus×O. aureus hybrid progeny of different geographical isolates, reared under varying management regimes

Abstract Ten Oreochromis niloticus × O. aureus hybrids, progeny of different geographic isolates, were evaluated in polyculture with common carp, silver carp and grass carp. Traits tested included growth rate, survival, yield and proportion of males. The hybrid combinations O. niloticus ‘Nile’ (Egypt)× O. aureus ‘Mehadrin’ (Israel), and O. niloticus ‘Ghana-74’× O. aureus ‘Nile’ showed similar or better performance compared to the ‘Ghana-74’בMehadrin’ hybrid developed earlier at Dor. This suggests that the ‘Nile’ strains of both O. niloticus and O. aureus are potential candidates for commercial hybridization. The low proportion of males (30%) obtained with O. niloticus ‘Ghana-88’× O. aureus ‘Mehadrin’ crosses suggests that the later introduction from the Volta River differs from the 1974 introduction.

Tolerance to cucumber mosaic virus in pepper : Development of advanced breeding lines and evaluation of virus level

Tolerance to cucumber mosaic virus (CMV) was introduced from an Indian small-fruited hot pepper accession, Perennial, into several bell-type sweet pepper lines by means of pedigree and backcrossing breeding procedures. Tolerance was determined to be incompletely dominant and quantitatively inherited. Breeding lines with variable degrees of tolerance were developed based on inspection of visual symptoms after mechanical inoculation. The breeding lines were subsequently tested for their agronomic performance in the field after mechanical inoculation. Their levels of tolerance in the field closely resembled their previous performances in the greenhouse. There was no association between virus accumulation levels in the upper leaves, as determined by enzyme-linked immunosorbent assay (ELISA), and the degree of tolerance to the virus, as determined by either visual symptoms or field performance. We concluded that the basis for developing tolerant breeding lines from Perennial is primarily their ability to recover from high virus titer and not their restriction of virus multiplication.

Appearance and Expansion of TYLCV: a Historical Point of View

In 1959, the Israeli Ministry of Agriculture urged farmers in the Jordan Valley to replace the tasty but soft tomato “Marmande” with the long‐shelf life variety “Money Maker,” which was more suitable for export. A month after transplanting (August), most of the tomato plants in the region were affected by a disease of unknown etiology. Symptoms included severe stunting of plant growth, erect shoots, and markedly smaller and misshaped leaflets. The leaflets that appeared immediately after infection were cupped down and inward, and subsequently developing leaves were strikingly chlorotic and showed an upward curling of the leaflet margins. When young plants were infected, they barely produced any marketable fruits (Cohen & Nitzany, 1960). The growers’ first reaction was to blame the change in tomato variety and they demanded compensation from the Ministry of Agriculture. Dr. F. E. Nitzany, head of the Virology Laboratory at the Volcani Center, Agricultural Research Organization (ARO), Israel, was asked to determine the causal agent of the disease and find solutions to the problem. A field survey revealed that most of the tomato plots in the area had been completely destroyed, and that the disease was accompanied by large populations of whiteflies. The whitefly population had built up in the nearby cotton fields, a crop which was being grown on a commercial scale for the first time in Israel. Soon enough, the suspicion that the whiteflies were the vector of the disease was confirmed, following controlled transmission experiments in the laboratory. Moreover, the “Marmande” tomato was found to be as susceptible as “Money Maker” to the disease, which was found to be viral in nature (Cohen & Nitzany, 1960). The virus was named Tomato yellow leaf curl virus (TYLCV) by the late Professor I. Harpaz of the Hebrew University (Cohen & Harpaz, 1964). Interestingly, similar disease symptoms had first been

Social control of growth in Macrobrachium rosenbergii. II. The “leapfrog” growth pattern

Abstract Bi-weekly recordings of size and morphotype of individually marked Macrobrachium rosenbergii raised in large communal cages support the “leapfrog” hypothesis, i.e. the orange-clawed male (OC) metamorphoses into the blue-clawed male (BC) only after becoming larger than the largest BC male in its vicinity. This results in a series of differently sized BC males, whose size is positively correlated with the time of their metamorphosis. This growth pattern is achieved mainly through a delay in the transition from the fast growing OC morphotype into the slow growing BC one. This delay may be due to social interaction among males, as males isolated in small cages did not follow this pattern. The function of the “leapfrog” growth pattern is discussed.

Screening additional wild tomatoes for resistance to the whitefly-borne tomato yellow leaf curl virus.

Plants of 25 wild Lycopersicon accessions were screened in the greenhouse for resistance to the whitefly-borne tomato yellow leaf curl virus (TYLCV). High levels of resistance were detected in 7 of 9 accessions of L. peruvianum and in all 5 accessions of L. chilense tested. In contrast, plants of 7 accessions of L. hirsutum and 3 of 4 accessions of L. pimpinellifolium were highly susceptible. Plants of accession CIAS 27 (L. pimpinellifolium) showed moderate resistance to TYLCV.

Interference with Ultraviolet Vision of Insects to Impede Insect Pests and Insect-Borne Plant Viruses

Publisher Summary During the last decade, peoples awareness of the drawbacks of chemical control has increased. Integrated pest management (IPM) is a better alternative for controlling various pests, including insect-borne plant viruses. It is considered to be a system that, in the socioeconomic context of farming systems and the associated environmental and populational dynamics of pest species, utilizes all suitable techniques in a compatible manner to maintain pest populations at levels below those causing economic loss. This chapter describes some aspects of ultraviolet (UV) vision in insects and the use of this knowledge to create an innovative approach to control. This new approach, already implemented in Israeli agriculture, alters UV vision to prevent insects from invading greenhouses. Moreover, by altering the normal behavioral paradigm of insect vectors by changing their UV vision, one can reduce plant virus transmission.