STUDIES ON SHARKA DISEASE IN EGYPT

Abstract

During four successive seasons (2013-2016), incidence and spread of Plum pox virus (PPV) on apricot trees were determined in eight governorates. Among 4630 externally examined apricot trees during March and April months, 2710 ones (58.5%) showed typical symptoms of plum pox (Sharka) disease. Double Antibody Sandwich-Enzyme Linked Immuno-Sorbent Assay (DAS-ELISA) ensured that these trees were infected with PPV and proved the efficiency and accuracy of external test by 98%. Visual examination after April is doubtful due to the confusion associated with appearance of powdery mildew symptoms. Natural PPV-infection percentages of apricot trees varied between 16.4% and 87.7% according to the location, tree age, soil type and tested cultivar. The virus was transmitted mechanically (by using some special treatments) and by grafting but not through the infected seeds. Graft transmission gave better results than mechanical transmission. Varietal susceptibility studies revealed that under natural conditions, Amal cultivar was the most susceptible one followed by Canino, Amar and then by Hamawy cultivar. A positive correlation between trees-age and susceptibility to PPV-infection was detected.Infected apricot seedlings showed decrements in totalchlorophyll content as compared with healthy ones.In addition, comparing with the control plants (uninoculated), virus infectionsincreased the activity of phenylalanine ammonia-lyase, peroxidase and polyphenol oxidaseat all stone fruit samples.On the other side, a total phenol was increased in virus-infected apricot and peach trees as compared with uninfected ones.Several diseased samples (isolates) collected from naturally diseased apricot were analyzed by Reverse Transcription-Polymerase Chain Reaction (RT-PCR). The reaction yielded a 243 bp product. Restriction Fragment Length Polymorphism (RFLP) was used to identify PPV isolates obtained from apricot trees. Nested PCR primers were used to identify PPV-M and specific primers were used for PPV-C. In all cases, no mixed infection was found. All virus isolates were found to be belonging to PPV-EA (El-Amar strain). Sequence analysis confirmed the 99% identity between obtained isolate and PPV-EA isolates available in the GenBank. Under Egyptian conditions, obtained data revealed the El-Amar is stable and not variable strain. No other strain was detected in the country.