Combination of Cold Therapy and Chemotherapy for Eradication of Citrus exocortis Viroid

Abstract

Citrus Excortis Viroid (CEVd) is California's first viroid disease described on citrus trees in 1948. It is the largest viroid of citrus with 371 nucleotides. The disease causes distinguished symptoms of bark scaling on rootstocks of citrus trees grafted onto trifoliate rootstocks causing severe stunting of trees grafted on these rootstocks. The use of indicator host plants for CEVd detection had been successful using clean matured fresh cuttings of specific indicators (Etrog lemon, Volkameriana lemon), which were grafted with the infected plants and cultured in plastic bags after a short dipping in IBA solution for 10 sec. Ideal clear symptoms were observed 18 days after grafted inoculation on new growing leaves of unique indicators. The obtained results of the biological indexing trail were confirmed using RT-PCR and transmission assays using some hosts such as Solanum melongena, Piper nigrum , Gynura auranisa and Solanum esculentum (Castel rock) with infectious sap. Results showed that cold treatment at 4 °C for 6 months companies with chemotherapy with 10, 20 and 30 mg/L Virazol and thiouracil gave plants free-viroid. viroid infection was reduced by using the highest concentrations of Virazol and thiouracil at 30 mg/L reaching 80 and 55%, respectively. The recovered plants were examined by RT-PCR. Cold therapy treatment and chemotherapy are effective and attractive techniques for eradicating CEVd from citrus trees in Egypt. Virazol has the greatest effect on the elimination of viroid because the viroid-free percentages by Virazol were higher than those obtained using thiouracil.