DISTRIBUTION OF XYLELLA FASTIDIOSA IN CITRUS ROOTSTOCKS AND TRANSMISSION OF CITRUS VARIEGATED CHLOROSIS BETWEEN SWEET ORANGE PLANTS THROUGH NATURAL ROOT-GRAFTS

Authors: HE, C - UNIV. SAO PAULO, BRAZIL; LI, WEN - FUNDECITRUS, SP, BRAZIL; AYRES, ANTONIO - FUNDECITRUS, SP, BRAZIL; Hartung, John; MIRANDA, V - FUNDECITRUS, SP, BRAZIL; TEIXEIRA, DIVA - FUNDECITRUS, SP, BRAZIL

Submitted to: Plant Disease
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 1/28/2000
Publication Date: 6/1/2000
Citation: He, C.X., Li, W.B., Ayres, A.J., Hartung, J.S., Miranda, V.S., Teixeira, D.C. 2000. Distribution of xylella fastidiosa in citrus rootstocks and transmission of citrus variegated chlorosis between sweet orange plants through natural root-grafts. Plant Disease.

Interpretive Summary: Citrus variegated chlorosis (CVC) affects up to 40% of the sweet orange trees in Brazil and can cause substantial loss due to reduced fruit size. The disease is not yet present in the United States. It is caused by the bacterium Xylella fastidiosa and is transmitted by leafhopper insects that feed in the xylem tissue where the bacterium lives in infected plants. The insect vectors for the disease are abundant in the US. CVC is also spread through contaminated budwood used in propagation. We show here that although the insects feed in the foliage and branches of the tree, the bacterium is also found in the root systems of 11 of 15 rootstocks tested. The mechanism used by the bacterium for systemic movement in infected plants is not known. We have also shown using potted plants in a screened greenhouse that the pathogen can be transmitted between plants through naturally occurring root grafts. The rate of natural root graft formation in citrus is not known but should be more frequent at higher planting densities as are used in nurseries. Other strains of the same bacterium cause important diseases of coffee in Brazil and of grapevines, oleander, peach, plum, and several shade trees in the United States. This is the first report of root graft transmission of this pathogen and thus has implications for the management of this pathogen in several of these hosts in addition to citrus, particularly those grown in hedgerows where extensive natural root grafting is likely. These crops include coffee in Brazil and oleander in the western United States.

Technical Abstract: To study translocation of Xylella fastidiosa to citrus rootstocks, budsticks from Citrus Variegated Chlorosis (CVC) affected 'Pera' sweet orange (C. sinenesis (L.) Osb.) were top-grafted on fifteen citrus rootstocks. Disease symptoms were conspicuous three months later on all fifteen rootstocks tested. The presence of X. fastidiosa was confirmed by light microscopy, double antibody sandwich enzyme-linked immunosorbent assays, and by polymerase chain reaction in rootlets and main roots of CVC symptomatic 'Pera' sweet orange in eleven of the fifteen rootstocks tested. These results suggest that bacterial translocation from the aerial plant parts to the root system is not essential for X. fastidiosa to induce symptoms in the aerial parts. Bacterial translocation to the roots was not correlated with CVC leaf symptom severity in the 'Pera' scion. To determine if CVC disease could be transmitted by natural root grafts, two matched seedlings of each of four sweet oranges, 'Pera', 'Natal', 'Valencia' and 'Caipira', were transplanted into single pots. One seedling rootstock of each pair was inoculated by top-grafting with a CVC-contaminated budstick while the other seedling rootstock was cut but not graft-inoculated. Transmission of X. fastidiosa from an inoculated plant to a non-inoculated plant sharing the same pot was observed in all four sweet orange varieties tested. Transmission was confirmed by observation of natural roots grafts between the two plants, presence of X. fastidiosa in the root grafts and disease development in the uninoculated plants. This is the first report of transmission of CVC disease through natural root grafts.