Ābeļu hlorotiskās lapu plankumainības vīrusa epidemioloģija un ģenētiskā daudzveidība Latvijā un augļkoku atveseļošanas iespējas: promocijas darba kopsavilkums zinātnes doktora grāda (Ph.D.) iegūšanai lauksaimniecības, meža un veterinārās zinātnēs | Epidemiology and genetic diversity of apple chlorotic leaf spot virus in Latvia and its elimination possibilities from fruit trees: Summary of the doctoral thesis for the doctoral degree of science (Ph.D.) in agriculture, forestry and veterinary sciences

Virus diseases of fruit trees have been known for centuries before it was discovered that many diseases are caused by viruses. In the modern commercial fruit-growing industry, viruses are one of the most dangerous plant diseases. In the case of viral infections, the vegetative growth and yield of fruit trees decrease, the susceptibility of trees to other pathogens increases, but infected producing trees are not possible to recover. The intensive development of fruit growing, and the introduction and distribution of untested planting material have contributed the spread of latent viruses in orchards and increased the genetic diversity of viruses. Currently, around 160 fruit tree diseases caused by viruses and virus-like organisms have been described in the world, but only a few of their causal agents have been identified. Apple chlorotic leaf spot virus (ACLSV) is one of the most widespread fruit viruses and has become one of the dominant pathogens in fruit orchards. The goals of this work were: 1) to determine the occurrence of apple chlorotic leaf spot virus in orchards; 2) to find out whether there are present different genetic types of the virus in Latvia; 3) to evaluate the effect of ACLSV on the yield and vegetative growth of apple genotypes grown in Latvia; 4) to evaluate the possibilities of ACLSV elimination with thermotherapy. Fifty apple, 36 pear, 28 plum orchards and 13 wild localities were surveyed, and 2034 leaf samples were collected to determine ACLSV occurrence in Latvia. Two methods were used to detect the virus in plants – Enzyme-Linked Immunosorbent Assay (ELISA) and Reverse Transcription Polymerase Chain Reaction (RT-PCR). The results of the study showed that in Latvia, 65.4% of apple trees, 32.3% of pear trees, and 2.0% of plum trees are infected with ACLSV, and this virus is also found in wild-growing apple trees. In addition, several apple, pear, and plum genotypes are significantly more infected with ACLSV, which indicates on the propagation of infected mother plants and the further use of infected planting material in orchard establishment. When analysing the nucleotide sequences of the ACLSV coat protein gene, the high genetic diversity of the virus was also observed, where nucleotide sequence identity varied from 79.55% to 98.45% and amino acid sequence identity from 84.54 to 100%. The detected recombination events in virus genome by the computer program RDP5 indicated that genetic variation is an ongoing process in the virus population, both in cultivated and in the wild-growing plants. In Latvia, two genetic types of the virus were found in pomme fruits, whereas three different genetic types were identified in stone fruits, one of which is related to plum bark split disease. Comparing growth and yield parameters of ACLSV-infected and virus negative trees, the study confirmed that ACLSV does not have a statistically significant (p value >0.05) negative effect on the yield and vegetative growth of apple trees 'Antonovka' and 'Gita’ but showed ACLSV infection statistically significantly contributes the increasing of tree crown volume, which does not significantly increase the amount of cumulative yield in relation to the crown volume. The elimination of ACLSV from fruit trees by thermotherapy, showed that the effectiveness of the virus elimination depends not only on whether the plant is infected with one or more viruses, but also on the sensitivity of the plant genotype to increased temperatures and the ability to recover after the thermotherapy. The results of the work can be used in the planning of a virus-free planting material production system, in the evaluation of existing fruit tree genetic resources and new varieties, and as the basis for further research.