Reaction severity and infection symptoms in citrus cultivars susceptible to citrus yellow vein clearing virus in the greenhouse conditions

Citrus yellow vein clearing virus (CYVCV), is the causal agent of a significant and devastating disease that has spread in Iran and a number of countries in the region in recent years. In order to identify susceptible and tolerant cultivars for disease management, the reaction of a number of commercial citrus cultivars from the groups of orange, mandarin, grapefruit, citron, tangelo, pomelo, kumquat, lime and lemons was investigated after inoculation in the greenhouse conditions. The relative susceptibility of the cultivars was then determined according to the severity of the symptoms.

The buds of 19 commercial citrus cultivars of the country including Thomson navel orange, Valencia sweet orange, Moro blood orange, Siavaraz local orange, Clementine mandarin, Ponkan mandarin, Page mandarin, Satsuma mandarin, Sour orange, Sweet lime, Eureka lemon, Persian lime, Mexican lime, Flame grapefruit, Redblush grapefruit, Mineola tangelo, Etrog citron, Pummelo and Kumquat were propagated on sour orange rootstocks and the plants were simultaneously graft inoculated with a CYVCV isolate (Gene Bank number: KX902488). In order to confirm the infection, the total RNA of the symptomatic plants was extracted by SDS-Potassium acetate method. A two-step Reverse Transcription Polymerase chain reaction (RT-PCR) was carried out using a specific primer pair of the virus coat protein (CP) gene. The reaction of the inoculated plants was monitored under greenhouse conditions with controlled temperature.

14 cultivars susceptible to citrus yellow vein clearing were detected based on the appearance of four types of symptoms in the inoculated plants including yellowing of the lateral veins on the upper surface of young leaves (symptoms type A), corresponding water soaked areas under the young leaves (symptoms type B), wrinkling of the old leaves (symptoms type C) and leaf spots on the old leaves (symptoms type D). These cultivars were classified into three groups with high, medium and low susceptibility according to the symptom severity. No symptoms were found in tolerant cultivars of Sweet lime, Mexican lime, Page mandarin and two grapefruit cultivars up to one year after inoculation. Descriptive grouping of frequency, severity and stability of symptoms was consistent with cluster analysis based on Euclidean Ward distance coefficient.

The management of citrus yellow vein clearing disease is based on preventive methods and the cultivation of resistant or tolerant cultivars instead of susceptible varieties. The results of the present study showed that Citrus yellow vein clearing virus has a wide host range and can infect different citrus species or groups. Sour orange, Eureka Lemon and Persian lime are the most susceptible hosts of the virus. Monitoring of four typical symptoms of the disease is recommended for disease management in citrus nurseries and orchards.