Peach latent mosaic viroid in naturally infected sweet cherry trees in southern Italy

Peach latent mosaic viroid (PLMVd) was found in naturally infected sweet cherry trees grown in commercial orchards in southern Italy. The viroid was detected in nucleic acid extracts of symptomless leaves by molecular hybridization with a PLMVd cRNA probe. The viroid was transmitted by grafting from sweet cherry to peach seedlings and identified in peach by molecular hybridization.

Comparison of Chilling Requirement in `Ta Tao' Budded Peach Trees by Shoot Forcing

Peach trees exposed to `Ta Tao' vegetative chip bud grafts have been shown to have physiological changes that include bloom delay, delayed maturity, reduced shoot vigor, and early autumn defoliation. `Ta Tao' is known to be infected with the graft transmissible agent Peach Latent Mosaic Viroid (PLMVd). PLMVd has been associated with bloom delay and reduced shoot vigor. `Coronet' peach trees planted in a high-density, Y-trained orchard system were treated with vegetative chip buds from `Ta Tao'. Transmission of PLMVd was confirmed in the treated trees by cRNA probe hybridization. A shoot forcing study was set up to determine if exposure to `Ta Tao' would alter the chilling requirement of `Coronet' peach. Terminal fruiting shoots were collected periodically during the dormant season from 8 Jan. 1999 to 19 Feb. 1999, which represented a range of chill hour accumulation from 574 to 927, respectively. Shoots were forced in a greenhouse, and chilling requirement was considered complete when 50% of the flowers opened. Chilling requirement was not changed by exposure to `Ta Tao' chip buds. The number of days required for shoots to bloom was significantly and consistently longer for the `Ta Tao' treated trees. The number of shoots responding to forcing conditions was significantly less in the treated trees. The data suggest that the graft transmissible effects from `Ta Tao' buds increased the growing degree hours required for `Coronet' leaf and flower bud emergence after rest completion under greenhouse forcing conditions.

Pear Blister Canker Viroid: Host Range and Improved Bioassay with Two New Pear Indicators, Fieud 37 and Fieud 110

An investigation was conducted to improve the biological detection of pear blister canker viroid (PBCVd), which over a period of 2 to 3 years induces pear blister canker disease on the perry pear (Pyrus communis) cv. A20. PBCVd was not detected by dot blot hybridization or bioassay in any of the tested species of Amelanchier, Aronia, Cotoneaster, Crataegus, and Pyracantha. However, some species of Chaenomeles, Cydonia, and Sorbus, five out of 13 species of Malus, 15 Pyrus species, and 16 commercial pear cultivars were susceptible to PBCVd, although none developed symptoms. Only in perry pear seedlings did approximately 5% of the population react to infection with pure PBCVd strains by developing petiole, leaf, or bark necrosis 2 to 3 years (cv. A20) or 3 to 5 months (cv. Fieudière) after inoculation. The selected Fieud 37 and Fieud 110 clones are proposed as PBCVd indicators to replace A20. Results from bioassays on the indicators and from detection by a PBCVd-cRNA probe were essentially in agreement except for some Malus species.

A cRNA Probe Detects Prunus Necrotic Ringspot Virus in Three Peach Cultivars after Micrografting and in Peach Shoots following Long-term Culture at 4 °C

As part of a program to develop transgenic peach (Prunus persica L. Batsch) cultivars with resistance to Prunus necrotic ringspot virus (PNRSV), we are testing a system for measuring virus in peach shoot cultures. Micrografting in vitro is used for inoculation and slot-blot hybridization, with a digoxigenin (DIG)-labeled cRNA probe complementary to the 5′ open reading frame (ORF) of PNRSV RNA 3, for detection. In this study, we investigated whether infected shoots maintain virus infection over long periods of culture at 4 °C and if PNRSV-infected `Suncrest' shoot cultures can serve as graft bases to transmit virus equally well into cultivars Nemaguard, Springcrest, and Suncrest. The results of RNA hybridization analysis showed that virus was present in extracts of leaf samples from 2-year-old PNRSV-infected `Suncrest' shoots that had been subjected to varying lengths of incubation at 4 °C in the dark, suggesting that infected shoots can be maintained for repeated use. Rates of graft success were higher in heterografts between `Suncrest' bases and tips of `Springcrest' or `Nemaguard' than in autografts between `Suncrest' and `Suncrest', and there was equal efficacy of graft inoculation from `Suncrest' into these three cultivars.

Improved Detection of Potato Leafroll Luteovirus in Leaves and Tubers with a Digoxigenin-Labeled cRNA Probe

A digoxigenin-labeled cRNA probe of approximately 2,100 bp was more than 2,000 times more sensitive in detecting potato leafroll virus (PLRV) in leaf extracts of Datura stramonium, Physalis floridana, and potatoes than enzyme-linked immunosorbent assay (ELISA). The limit of detecting PLRV with the probe was 1 pg/ml compared with 2 ng/ml by ELISA. The probe detected PLRV easily in dormant tuber tissues at dilutions of up to 1:100. There was no background reaction with healthy extracts. No reactions were observed between the probe and potato X potexvirus or potato Y potyvirus.