Abstract
The relationship between yield and disease symptoms of two major fungal diseases of citrus in Trinidad was investigated to determine the effect of the diseases on infield yield variability. Surveys of incidence of greasy spot (Mycosphaerella citri Whiteside) and of footrot (Phytophthora spp.) were conducted from 1996 to 1998 on ‘Valencia’ orange (Citrus sinensis [L.] Osbeck), ‘Portugal’ mandarin (C. reticulata Blanco) and ‘Ortanique’ tangor (C. sinensis x C. reticulata). These were measured in terms of foliar coverage by lesions and trunk and branch girdling, respectively. A greasy spot index was developed and used for analysis. Yields as yield level or fruit count were recorded for the subsequent crop years of 1998-2000.
A small but significant relationship of greasy spot with mean fruit count per tree was observed for ‘Valencia’ R2 = 13%, P = 0.014, P= 0.028 and for ‘Ortanique’ R2 = 12.2%, P = 0.012. Greasy spot scores on ‘Portugal’ mandarin were relatively low. There was no significant relationship of footrot symptoms with yield level. However, the incidence of tree death after symptom expression in ‘Portugal’ mandarin was very high (50%). It is concluded that the effect of these two diseases on tree yield was generally minor and contributed little to tree to tree yield variation.
