Effect of Citrus Viroids and the Influence of Rootstocks on Field Performance of Navel Orange

Studies continue on testing the field performance of Washinton navel orange trees on sour orange and Troqer citrange rootstocks inoculated with citrus exocortis viroid (CEV) as well as several citrus viroids and mixtures characterized by mild to moderate reactions in citron. Tree size, fruit production, color development and fruit quality were evaluated during 10 fruiting seasons at the Lindcove Research Field Station in the Central Valley of California. The presence of CEV and combinations of citrus viroid-Ia, -1Ia and -1IIb reduced tree size and yield. Navel orange on Troyer citrange rootstock continued to outyield trees on sour orange rootstock. The retardation of color development noted in 1985 was not observed in the 1988 or 1989 seasons. Fruit quality was not affected by presence of viroids. However, fruit from trees on Troyer citrange rootstock continued to show earlier higher sugar-acid ratios. In 1977, an experiment was designed to test the effect of CEV and a number of "mild" isolates of CEV on the performance of navel orange trees grafted on two rootstocks: sour orange and Troyer citrange. At the inception of this experiment certain isolates of CEV were judged as "mild" based on intensity of bark cracking symptoms on trifoliate rootstocks and also on mild symptoms induced on greenhouse-indexed citron plants. The range of symptoms induced on citron by these "mild" isolates were all less severe than those caused by CEV. Several years after the initiation of this experiment, these milder reactin "exocortis isolates" were shown to be a series of distinct citrus viroids (CV) in pure form or in various mixtures containing CV-Ia, -1Ia or -1IIb (3, 4, 5, 10, 13). Nauer et al. (8) reported that after six fruiting seasons (1980-1985), presence of CEV significantly reduced fruit yield and tree size. Also, in the fruit color evaluation of 1985, presence of CEV induced a slower color development of navel fruit on trees on both rootstocks when compared to fruit on viroid-free trees. The presence of certain mixtures of citrus viroids were also shown to have an effect on retarding fruit coloring during this one season. Although some trends were noted on the effects of CV on reduced tree size and fruit maturity, they were not significant statistically. There were no effects on fruit shape, rind thickness or juice percentage. A major finding of this first report was the superiority in performance of navel orange trees on Troyer citrange rootstock in fruit yield, size and color development when compared to trees on sour orange rootstock (8). This rootstock difference was independent of viroid content. This paper reports the results of continuing studies over four additional fruiting seasons on the effects of CEV and CV on tree size, fruit yield, fruit size, fruit coloring and sugar-acid ratios for navel oranges on sour orange and Troyer citrange rootstocks. MATERIALS AND METHODS Old-line Parent and Atwood navel orange buds containing various citrus viroids and free of all other known viruses were grafted on sour orange and Troyer citrange seedlings in 1977. The citrus viroids present and the citron reaction induced are given in Table 1. Details of the background and history of the inoculum sources were presented in the previous report (8). All inoculum sources had been previously indexed to appropriate indicator plants and were found free of all other known graft transmissible Viroids and Viroid Induced Diseases