Communicated by Edîte Kaufmane To evaluate the occurrence of nine viruses infecting
Apple chlorotic leaf spot virus (ACLSV), Apple stem pitting virus (ASPV), Apple stem grooving virus (ASGV) and Apple mosaic virus are economically important viruses infecting fruit tree species worldwide. To evaluate the occurrence of these pome fruit viruses in Latvia, a large-scale survey was carried out in 2007. Collected samples were tested for infection by DAS ELISA and multiplex RT-PCR. The accuracy of the detection of the viruses in multiplex RT-PCR was confirmed by sequencing amplified PCR fragments. The results showed a wide occurrence of viruses in apple and pear commercial orchards established from nontested planting material. More than 89% of the tested apple trees and more than 60% of pear trees were infected with one or more pome fruit viruses. Analyses showed that the high occurrence of viruses in several apple cultivars is due to the propagation of infected clonal rootstocks and scions from infected mother trees. Sequence analyses targeting the 3¢-terminal region of the tested viruses showed various degrees of genetic diversity within respective virus isolates. This is the first report of the occurrence of ACLSV, ASGV and ASPV in apple and pear trees in Latvia and demonstrates their genetic diversity in different host genotypes.
An internal blackening disorder may cause substantial losses in the value of tomatoes grown for processing. The disorder resembles an internal form of blossom-end rot and appears to be more common in dry seasons. In an experiment to test whether the internal blackening is caused by water deficit and whether it is indeed blossom-end rot, plots of cv. Cannery Row were irrigated to keep the soil moisture deficit c50 mm and others were sheltered from rain and not irrigated from early flowering onwards. Shoot growth (total and fruit dry mass) was measured destructively at intervals, and root growth and death nondestructively using minirhizotrons. There was a greater incidence of internal blackening and blossom-end rot, and lower Ca concentrations, in the fruit of non-irrigated plants. Root growth and root death were accelerated in these plants around the time that internally-blackened fruit were set. Although the internal blackening syndrome shared some features with blossom-end rot some differences were apparent in this experiment. It is suggested that internal blackening could have resulted from increased root competition for photosynthate, leading to abnormal seed development. Root turnover was appreciable (3040% of the roots survived < 28 days). This suggests there may be substantial errors in contemporary models of dry matter partitioning in tomato crops.
Apples and pears are among the most important commercial fruit species grown in Latvia. Because of suitability to local climatic conditions, mainly domestic cultivars and cultivars originating in neighbouring countries are grown. The planting material of pome fruits produced and used for establishment of new orchards in Latvia corresponds to the Conformitas Agraria Communitatis standard due to the unavailability of nuclear stock. To establish virus-tested, experimental nuclear stock for apple and pear, one to two years old candidate plants were exposed to thermotherapy at +38 °C for 40 to 70 days. The mother trees and candidate plants before treatment were tested for the presence of the four most widespread pome fruit viruses by RT-PCR. The shoot tips of the heat-treated plants were grafted onto seedling rootstocks and were re-tested for the four viruses by RT-PCR during the next three to five vegetation seasons. Several plants of apple cultivars ‘Dace’, ‘Zarja Alatau’, ‘Rubin’, and ‘Ausma’ remained infected either with Apple chlorotic leaf spot virus, Apple stem growing virus or Apple stem pitting virus after the thermotherapy. Tests on woody indicators were carried out to determine possible presence of graft-transmittable organisms according to EPPO guidelines for the establishment of nuclear stock material for pome fruits.
Two Golden Queen peach blocks, one propagated from virus-free scions (Block A), the other from non-indexed sources (Block B), were ELISA-tested 5-7 years after planting for prune dwarf virus (PDV) and prunus necrotic ringspot virus (PNRSV). Trees in Block A were 18% infected by PNRSV at year 5 and 60% by year 7, at which time 1% also had PDV. Block B was 100% infected by PNRSV and 9% with PDV by year 5, increasing to 15% by year 7. Between years 5 and 7, salable yield reduced by 18% as a result of PNRSV-infection. Dual infection was likely to have reduced average salable yield by 32% (9.6 t/ha) and gross returns by $4,805/ha.
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