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The sensitivity of water stress indicators to changing moisture availability, and their variability, determine the number of measurements that should be taken in order to represent properly plant water status in a certain orchard. In the present study we examined the sensitivity and variability of maximum daily trunk shrinkage, midday stem water potential, and daily transpiration rate in their responses to withholding irrigation from field-grown drip-irrigated `Golden delicious' apple trees in a commercial orchard. Irrigation was withheld from the stressed trees for 17 days starting in mid-July, and the control trees were irrigated daily at 100% of the “Class A” pan evaporation rate. The courses of daily transpiration rate, maximum trunk shrinkage, and midday stem water potential before and 10 days after the drying period were similar in the control and the stressed trees. Highly significant differences between the stressed and the control trees in their midday stem water potentials were apparent from the early stages of the stress period. Daily transpiration rate and maximum daily shrinkage were more variable than midday stem water potential, and differences between treatments became significant only after measurements were expressed relative to the initial values before irrigation was witheld. Differences between treatments (as percentages of the values obtained for the control trees) increased after irrigation stopped where these differences were greatest for maximum daily shrinkage, which reached 90%; moderate for stem water potential (60%); and least for daily transpiration rate, for which the differences remained below 20%. Our data show that the choice of a certain water stress indicator should be based on both the sensitivity to changing moisture availability and the degree of variability. Possible reasons for the different sensitivity to moisture availability and the different variability between the water stress indicators under study are discussed.

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In this study, 18 Petunia ×hybrida Hort. Volm.-Andr. cultivars were mechanically inoculated with the tobamoviruses tobacco mosaic (TMV) or tomato mosaic virus (ToMV) (20 μg·L-1 in 0.05 m sodium phosphate buffer). One and 2 weeks post-inoculation (PI), inoculated and noninoculated upper leaves were harvested and assayed for TMV infection using enzyme-linked immunosorbent assay (ELISA). Local lesions developed on inoculated leaves of 16 cultivars 3-5 days PI. A total of 11 and 16 of the cultivars developed systemic symptoms characteristic of tobamovirus infection 2 weeks after inoculation with TMV and ToMV, respectively. All cultivars were positive in ELISA tests. Large amounts of virus were recovered from the upper, noninoculated leaves of all cultivars, including symptomless plants. Up to 95% infection by TMV occurred when a sterilized knife was passed through an infected shoot of petunia prior to its being used to remove cuttings from healthy petunia plants. Heat sterilization of knives and/or treatment with 2.8 g·L-1 sodium troclosene was very effective in controlling TMV transmission.

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Tomato yellow leaf curl virus (TYLCV), transmitted by the tobacco whitefy (Bemisia tabaci Genn.), can be devastating to tomato (Lycopersicon esculentum L.) crops in tropical and subtropical regions. The development of resistant cultivars is the best option for control of TYLCV. However, all the available resistant commercial cultivars tested at the Volcani Center, when inoculated with TYLCV, developed different levels of disease symptoms. In this study, we report the development of a breeding line, TY172, which is a symptomless carrier of TYLCV. Line TY172, whether infected in the greenhouse with viruliferous whiteflies, or when grown in the field under natural infection, showed no symptoms of the disease. Viral DNA was detected in infected TY172 plants, albeit at much lower levels than a susceptible infected control. In addition, grafting experiments using infected susceptible scions grafted onto TY172 stocks, showed that even when exposed continuously to very high levels of virus, line TY172 did not develop disease symptoms, nor did it accumulate high levels of the virus. When TY172 was crossed with susceptible lines, the hybrids exhibited milder symptoms and lower viral content than the susceptible parent, yet higher than that of TY172, suggesting a partial dominance for the TY172 resistance. Upon inoculation of F2 populations, the amount of symptomless individuals appeared in a ratio of≈7:64. This suggests that at least three genes may account for the resistance.

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Recently, yellows diseases have become more common in Israel, and phytoplasmas have been detected in some of these diseased crops. Commercial fields of two celosia species (Celosia plumosa L. and C. cristata L.) also have exhibited yellows symptoms and total crop failure. Typical mycoplasma-like bodies were observed in infected but not in healthy plants. The same plants were analyzed for the presence of phytoplasma by polymerase chain reaction (PCR), using the universal oligonucleotide pair r16SF2/r16SR2, followed by nested PCR using group-specific primers. Restriction analyses performed with these products indicated that two different types of phytoplasmas are infecting celosia. PCR-RFLP analysis of one type revealed a restriction pattern typical of aster yellows. Similar analysis of the second type indicated possible relatedness, though not identity, to the pattern of phytoplasmas of the Western-X group. This is, to our knowledge, the first report of phytoplasma infection in celosia.

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Abstract

Nucellar and zygotic offspring of ‘Satsuma’ mandarin (Citrus reticulata Blanco) were differentiated by gas chromatographic analysis of gaseous emanations from fragmented leaves of 6-month-old seedlings. Analysis was rapid (< 3 minutes/sample), could be performed on individual leaves, and required virtually no tissue processing. Thus, the method is amenable to the screening of large progeny at the young seedling stage.

Open Access

`Eureka' lemons [Citrus limon (L.) Burro. f.] treated for commercial storage were held for 6 months at 13C. One-half of the fruits were individually sealed in high-density polyethylene (HDPE) plastic film and half not sealed. The HDPE-seaIed lemons showed little change in the water relations characteristics, while unsealed lemons lost weight and decreased in water potential throughout the storage period. The maturity indices in the two treatments were generally similar during the first 3 months of storage, after which maturation of wrapped fruit was slower than that of the control. The overall marketable quality of the fruit was higher in HDPE-sealed lemons than in unsealed. From these results, it appears feasible to introduce seal packaging in packing lines where lemons will be placed in extended storage.

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The effect of Cucurbita and melon rootstocks on the horticultural and pathological performance of grafted Fusarium-susceptible melons was studied in four field experiments conducted in Fusarium-infested and Fusarium-free soils. The melon/melon combinations performed better than the melon/Cucurbita combinations regarding yield and disease control. In the 1999 experiment conducted in infested soil, Fusarium wilt symptoms were observed only in the nongrafted susceptible melons whereas all grafted combinations were symptom-free. In the 2000 experiment, nongrafted susceptible melons were totally wilted, whereas disease incidence in the melon/melon combinations and in one of the melon/Cucurbita combinations was low. The response of grafted plants to Fusarium wilt was also affected by the susceptibility of the scion. Among nongrafted melon cv. Ananas Ein Dor and those grafted onto Brava rootstock, 82% and 20%, were diseased, respectively, compared with only 36% and 0%, of the nongrafted and grafted `Ofir' melons, respectively. Negligible quantities of fruit were harvested from the nongrafted plants grown in infested soil, whereas high and moderate yields were obtained from melons grafted onto melon and Cucurbita rootstocks, respectively. The yield of the nongrafted melons in Fusarium-free soils were similar to those of all the grafted plant combinations. Susceptible melon scions grafted onto resistant melon rootstocks were less colonized by F. oxysporum f. sp. melonis than the same melons grafted onto the Cucurbita rootstocks.

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