Biological and molecular study, incidence, and spatial distribution of viruses in pitaya (Selenicereus spp.)

Abstract

Raising pitaya, which belongs to the genus Selenicereus spp., has increased uninterruptedly in recent years, due to its nutraceutical and organoleptic properties. Furthermore, it has proven to be quite profitable for small producers in Brazil. However, due to its recent introduction in the country, studies involving the occurrence and control of diseases and pests are rare, with most of them focusing on their biochemical and bioactive compounds. Among the diseases that could affect pitaya, viruses stand out for the lack of curative control methods and for being underreported, as symptoms can be confused with nutritional disorders. Vegetative propagation and the need for several manual operations in pitaya crops have a high potential to increase the spread of viral diseases. Therefore, plants with symptoms of viruses in orchards is frequently reported by producers. In this work, 212 samples of cladodes with symptoms suspected of viruses were collected in different regions of Brazil, with the purpose of identifying and carrying out the biological characterization of the viruses found, as well as studying their molecular variability based on the sequence of the coat protein gene (CP). Furthermore, a survey was carried out on orchards in five municipalities located in the

southern of the states of Minas Gerais to determine the incidence and spatial pattern of virus- infected plants, using the Run, Doublet and dispersion index models. The Chenopodium

quinoa and C. amaranticolor plants that were mechanically inoculated reacted with symptoms of chlorotic and necrotic lesions. Among the 212 analysed samples, 181 were infected with one or more of the following species of the genus Potexvirus: cactus virus X (CVX); pitaya virus X (PiVX); zygocactus virus X (ZyVX) and schlumbergera virus X (SchVX). The mixed infections were found with two to four viruses in the same sample. The analysis of the virus CP gene showed the CVX isolates present in two distinct groups, one of which presented a deletion of nine nucleotides in the initial region of the CP gene, in position 42 to 50, in the 5'→3' direction. All the orchards inspected showed high incidences of potexviruses, varying between 80 and 100%. Most orchards showed an aggregated pattern when analysed by Run and Doublet models, while the dispersion index prevailed with a regular pattern, suggesting a spread through cultural practices from infected seedlings. This is the first work involving the detection and identification, as well as the incidence and spatial pattern of viruses in commercial orchards of pitaya. The results revealed that Brazilian pitaya producers do not have the necessary know-how to adopt preventive measures to control viruses in pitaya orchards. It was also evident that the commercialized seedlings must be responsible for the introduction of viruses into orchards, which shows the urging need for a system to control the production and commercialization of seedlings in the country.