Control of Stem Pitting Disease Caused by Citrus Tristeza Virus Using Protective Mild Strains in Navel Orange

Abstract

Virus-free plants of Morita navel orange obtained by heat treatment were preinoculated with 5 mild strains of citrus tristeza virus (CTV) collected from citrus trees in orchards, two mild strains obtained artificially by heat treatment, and one strain of citrus vein enation virus, respectively. Nursery plants preinoculated with each mild strain were challenge-inoculated by Toxoptera citricida with a severe strain of CTV, and after 1-2 years they were placed outdoor. About 3 years later the trees were replanted in a field close to citrus trees infected with severe strains of CTV. Field surveys were periodically carried out to observe the occurrence of stem pitting, tree growth, fruit production, etc. Based on the results of indexing with Mexican lime seedlings and the occurrence of stem pitting on branches, cross-protection against the severe strain of CTV was recognized in the field for 7-9 years after challenge, thereafter the cross-protection ability was lost. Initial growth of trees inoculated with the mild strains was more vigorous, tree and fruit sizes became larger, and yield was higher compared with the trees inoculated with the severe strain. The M-16A and M-15A mild strains showed better protection than the other strains. Yield of trees preinoculated with both mild strains increased by about 50 percent, and also the size of fruit was larger than that of fruits in trees affected with the severe strain. There were no conspicuous differences in fruit quality among the strains tested.