Distribution of Xylella fastidiosa in Citrus Rootstocks and Transmission of Citrus Variegated Chlorosis Between Sweet Orange Plants Through Natural Root Grafts

Abstract

To study translocation of Xylella fastidiosa to citrus rootstocks, budsticks from citrus variegated chlorosis (CVC)-affected cv. Pera sweet orange (Citrus sinenesis (L.) Osb.) were top grafted on 15 citrus rootstocks. Disease symptoms were conspicuous 3 months later on all 15 rootstocks tested. The presence of X. fastidiosa was confirmed by light microscopy, double-antibody sandwich enzyme-linked immunosorbent assays, and polymerase chain reaction in rootlets and main roots of CVC-symptomatic Pera sweet orange in 11 of the 15 rootstocks tested. These results suggest that bacterial translocation from the aerial plant parts to the root system occurs but 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 orange cultivars (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 noninoculated plant sharing the same pot was observed in all four sweet orange cultivars 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.

Evaluation of the Genetic Structure of Xylella fastidiosa Populations from Different Citrus sinensis Varieties

“…CVC was first described in Brazil in 1987 during an outbreak of the disease in the northwest region of the state of São Paulo (3,35). The pathogen is naturally transmitted by sharpshooter (Cicadellinae) leafhoppers (33) and by natural root grafts and top grafting with infected budsticks (12). The rapid spread of CVC through Brazilian orchards and the high level of damage caused by this disease have resulted in heavy economic losses for citrus growers and the orange juice industry.…”

Scientific Opinion on the risks to plant health posed byXylella fastidiosain the EU territory, with the identification and evaluation of risk reduction options

“…Rootroot transmission of pathogens between neighbouring plants can occur when the roots make intimate associations called root grafts (Epstein, 1978). A report shows transmission of X. fastidiosa via citrus root grafts in 31 % of experimental plants tested (He et al, 2000). Another study with grapevines did not observe root grafts between plants and, consequently, no transmission (Krell et al, 2007).…”

“…Several studies have addressed the plant varietal resistance and/or tolerance to X. fastidiosa infection on different plant host species (He et al, 2000;Krivanek et al, 2005;Ledbetter and Rogers, 2009;Ledbetter et al, 2009;Cao et al, 2011;Wilhem et al, 2011;Sisterson et al, 2012). It is clear that varietal differences within plant species and genera are relevant to the development of X. fastidiosa infections and disease symptoms.…”

Bacterial Leaf Scorch of Shade Trees.

“…Their role in the transmission of BLS has yet to be confirmed. In some hosts, such as alfalfa, citrus, grape, and peach, X. fastidiosa is known to spread through root grafts and by budding (12,14,38). In citrus, there is some evidence that the pathogen can colonize fruit tissues and pass from seed to seedling (22).…”