|Yokomi, Raymond - Ray|
|RATTNER, RACHEL - Cooperative Agricultural Support Services|
|HAJERI, SUBHAS - Central California Tristeza Eradication Agency|
Submitted to: Citrograph
Publication Type: Other
Publication Acceptance Date: 3/10/2021
Publication Date: N/A
Technical Abstract: Eradication of trees with citrus tristeza virus (CTV) that react with MCA13 continues in three of five pest control districts in central California, despite prevalent use of CTV-tolerant or resistant rootstocks in California’s citrus industry. This study was conducted to examine the genetic diversity, virulence and spread of local MCA13-reactive CTV strains. MCA13 reacted to four genotypes (T36, RB, S1, and VT) of CTV collected from central California. These strains were graft inoculated to six commercial cultivars on Carrizo rootstock and grown for four-years in a screenhouse and examined for virulence. Based on metabolic changes and smaller trunk diameters, VT strains were considered more virulent; whereas, non-VT strains were asymptomatic and considered mild. Virus titer of VT strains remained consistently high, while titer of non-VT strains fluctuated seasonally and, in many cases, became non-detectable during hot summer weather. The annual spread of CTV was 0.9 percent in orchards with mandatory aphid vector control and eradication of trees infected with MCA13-reactive strains, whereas spread rate was 1.4 percent in non-eradication plots in the Navel and Valencia Export to Korea (NAVEK) program. This small 0.5% difference of spread rate suggested that pest control in the NAVEK program may be slowing CTV spread by reducing aphid vector populations. Spatial and temporal spread of MCA13 CTV in non-eradication plots showed spread was not occurring from outside of the orchard, but from MCA13 sources within the field. These studies highlight the differences between mild and severe strains that react to MCA13 and eradication efforts are limiting MCA13 CTV spread to adjacent non-eradication orchards.