LACK OF EVIDENCE FOR TRANSMISSION OF XYLELLA FASTIDIOSA FROM INFECTED SWEET ORANGE SEED
Citrus variegated chlorosis is among the principle diseases that affect sweet orange in Brazil and Argentina, and is viewed as an emerging threat by the U.S. sweet orange industry. The disease is caused by the fastidious bacterium Xylella fastidiosa. We have tested seed extracted from fruit heavily infected and symptomatic for X. fastidiosa infection as well as seed obtained from normal asymptomatic fruit from the same trees in Brazil. Seeds from symptomatic fruit were tested by PCR and found to be contaminated with X. fastidosa, and to weigh less than seeds from normal-sized fruit. Seed from both symptomatic and asymptomatic fruit were sown in the greenhouse and studied following germination. The growth of shoots, but not of roots, was reduced in seedlings produced from seed collected from symptomatic as compared to asymptomatic fruits. Symptoms of citrus variegated chlorosis were not observed in any of the seedlings derived from seed from symptomatic and asymptomatic fruit. Roots and leaf midribs from 260 seedlings were tested for the presence of X. fastidiosa by PCR three months after sowing. No positive PCR results were observed. Leaf midribs from 148 seedlings were tested subsequently at 17 months after sowing using both standard format and qPCR assays, also without any positive results. Leaf midribs from 349 seedlings from a second seed harvest were tested seven months after sowing by qPCR also without any positive tests. Thus in this study no evidence for the vertical transmission of X. fastidiosa through contaminated seed was obtained.