|NIAN, SIJI - Luzhou Medical College|
|LOPES, SILVIO - Fundecitrus - Brazil|
|AYRES, ANTONIO - Fundecitrus - Brazil|
|BRLANSKY, RON - University Of Florida|
Submitted to: Plant Pathology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 4/7/2014
Publication Date: 8/1/2014
Publication URL: https://handle.nal.usda.gov/10113/61498
Citation: Hartung, J.S., Nian, S., Lopes, S., Ayres, A.J., Brlansky, R. 2104. Lack of evidence for transmission of Xylella fastidiosa from infected sweet orange seed. Plant Pathology. 96:497-506.
Interpretive Summary: Citrus variegated chlorosis is a very destructive disease of sweet orange that is endemic in South America, but is not present in the United States. It is caused by a bacterium called Xylella fastidiosa which is transmitted to sweet orange trees by a sucking insect and colonizes the internal tissues of the entire tree, from roots to fruits and seed. Because the fruit and seed from diseased trees contain the bacteria the possibility that the pathogen may be transmitted through infected seed to germinating seedling trees was investigated. Fruit was collected from severely diseased trees and seed was removed and planted in a controlled greenhouse. Seed from fruit with symptoms of the disease weighed less than seed from healthy appearing fruit and grew more slowly than seed from healthy appearing fruit. The seedlings were tested by several advanced DNA based assays for the presence of the pathogen at 3 and 17 months after germination. All tests for the pathogen were negative. The experiment was repeated a second year and the seedlings were tested seven months after germination by advanced DNA based methods for the presence of the pathogen. As before, all tests for the pathogen were negative. Altogether, roots and leaves from 757 seedlings were tested with negative results. Thus, no evidence was obtained that Xylella fastidiosa was transmitted through contaminated seeds to seedlings with the establishment of sustained infection. The results will be used by the citrus industry and regulatory agencies. These results do not support results of an earlier study in which Xylella fastidiosa was found in seedlings at earlier stages of growth.
Technical Abstract: 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, and can be transmitted by both leafhopper insects and by bud grafting. Evidence has also been presented that the pathogen is present in sweet orange seed and can be vertically transmitted to seedlings. We have tested seed extracted from fruit heavily infected and symptomatic for Xylella 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 Xylella 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 grown in the greenhouse derived from seed from symptomatic and asymptomatic fruit. Roots and leaf midribs from 260 seedlings were tested for the presence of Xylella fastidiosa by PCR three months after sowing. No positive PCR results were observed. Leaf midribs from 148 seedlings were tested subsequently at seventeen 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 Xylella fastidiosa through contaminated seed was obtained.