Relationship of citrus yield and virus infection in Trinidad

Abstract: Low yield is a serious problem of citrus in Trinidad but it is not known to what extent virus/viroid diseases contribute to yield reduction. This study is an attempt to quantify both the extent of infection of major virus/viroid diseases known to exist in citrus locally and the relationship of infection level with yield. The virus and vims-like diseases assessed in surveys were citrus tristeza virus (CTV), citrus exocortis viroid (CEV) and psorosis. The study began in 1996 and was conducted on Valencia orange, Ortanique tangor and Portugal mandarin established on sour orange rootstock. Techniques used in the survey included visual assessment of symptoms and both biological and serological indexing. In 1997, Ortanique had the highest level of CTV infection of 48.8 % of trees, while in the other cultivars < 10 % tested positive. There were significantly fewer high yielding Ortanique CTV positive trees compared to CTV negative trees (P = 0.042). Fruit count of CTV positive trees was significantly lower than CTV negative trees in Valencia 2000 (P = 0.004) and Ortanique for a cumulative period of 1998 - 2000 (P = 0.001). All Ortanique trees and few trees of the other cultivars appeared infected with CEV. The yield pattern of infected trees over time did not suggest a reduction in yield associated with CEV infection. Valencia field 12069 had the most (51 %) trees with psorosis-like bark symptoms. Presence of bark-scaling symptoms showed no relationship with yield. Of the three diseases studied, only CTV was responsible for yield reduction.