DISEASE AND PEST MANAGEMENT HORTSCIENCE 50(8):1183–1186. 2015. transmission from infected scions through the application of a hot-water treatment to dormant scions before grafting (Sanderlin Susceptibility of Some Common Pecan and Melanson, 2008). The application of the hot-water treatment to pecan rootstock has Rootstocks to Infection by Xylella not been tested and may be challenging because of the size of the rootstocks, and fastidiosa the need to treat the rootstock or grafted tree while dormant before transplanting into an Randy S. Sanderlin1 orchard. Louisiana State University Agricultural Center Pecan Research-Extension Pecan rootstocks are produced from Station, 10300 Harts Island Road, Shreveport, LA 71115 open-pollinated seed of grafted cultivars. Certain cultivars are preferred as sources of Additional index words. Carya illinoinensis, pecan bacterial leaf scorch, pathogen graft seed for rootstocks based on size of the nuts, transmission, mechanical inoculation vigor of growth, and the effects on early tree growth and nut production following graft- Abstract Carya illinoinensis . Pecan [ (Wangenh.) K. Koch], a hardwood tree native to ing to scions (Grauke and Thompson, 1995). North America, is grown for commercial production of nuts in southeastern, central, and The use of pecan rootstocks resistant to western regions of the United States. Pecan is also grown commercially in Mexico, South infection by X. fastidiosa combined with Africa, Australia, and some countries in South America. Pecan trees can be infected by hot-water treatment of scions may provide Xylella fastidiosa the broad host range xylem-limited bacterial pathogen Wells et al. a way for commercial pecan nurseries and Infection incites a leaf scorch disease that can cause significant defoliation, reduced tree pecan growers to establish new trees free of growth, and lower nut yield. Pecan cultivars are clonally propagated onto rootstocks the pathogen. Rootstock resistance to X. X. fastidiosa grown from open-pollinated seed of selected cultivars. is transmitted at fastidiosa infection has been identified in a high frequency from infected rootstocks into newly developing grafted trees. some other agricultural commodity hosts of Rootstocks resistant to infection would be beneficial to pecan nurseries and pecan the pathogen including, grapevine, almond, producers to prevent infection of young trees through grafting, especially when and peach (Gould et al., 1991; Gubler et al., combined with hot-water treatment of scions to eliminate the pathogen. Some common 2008; Ledbetter and Rogers, 2009). Pres- rootstocks were tested for variation in susceptibility to infection using mechanical ently, there is no information available on the inoculation with the pathogen. No outstanding level of resistance to infection was susceptibility of rootstocks used by the pecan detected among the seven rootstocks tested. The rootstocks from ‘Curtis’, ‘Elliott’, and nursery industry to infection by X. fastidiosa. ‘Riverside’ were less susceptible than one standard rootstock in the test (‘VC1-68’) and The objective of this research was to test less susceptible than highly susceptible ‘Cape Fear’ rootstock. Conversely, the rootstocks some of the pecan rootstocks commonly from ‘Apache’, ‘Moore’, ‘Stuart’, and ‘VC1-68’ seed had a level of susceptibility to used by commercial nurseries for their de- infection comparable to ‘Cape Fear’ and perhaps are not the best choice for rootstocks in gree of susceptibility to infection by the X. fastidiosa geographic areas where is prevalent. The results of this research suggest that PBLS pathogen. This report provides the X. fastidiosa there is variation in rootstock susceptibility to infection by . The use of first data on pecan rootstock susceptibility mechanical inoculation may facilitate identification of susceptibility categories of pecan to X. fastidiosa. rootstocks to the pecan bacterial leaf scorch pathogen. Materials and Methods Pecan [C. illinoinensis (Wangenh.) K. In pecan, the disease causes defoliation, Seven rootstock types grown from open- Koch], a tree species native to the Mississippi yield loss, and reduced growth of trees pollinated seed of the following cultivars River floodplain of North America, is culti- (Sanderlin and Heyderich-Alger, 2003). As were used in the study: ‘Apache’, ‘Curtis’, vated for nut production across the south- with other hosts of this bacterium, the path- ‘Elliott’, ‘Moore’, ‘Riverside’, ‘Stuart’, and eastern United States, and other states ogen is transmitted to pecan through feeding ‘VC1-68’. Trees grown from seed of ‘Cape including Texas, Oklahoma, Kanas, New by certain members of the Cicadellidae (leaf- Fear’, which is not typically used as a root- Mexico, Arizona, and California (Graham hoppers) and Cercopidae (spittlebugs) insect stock, were included as a standard for com- and Gibson, 2005). Commercial pecan pro- families (Redak et al., 2004; Sanderlin and parison because of its known high degree of duction also occurs in Mexico, South Africa, Melanson, 2010). In addition to insect trans- susceptibility as a cultivar and as nongrafted several South American countries, and Aus- mission, the pathogen can be transmitted in trees grown from ‘Cape Fear’ nuts (Littrell tralia (Geisler, 2011). Pecan is susceptible to pecan through infected rootstocks or scions and Worley, 1975; Sanderlin and Heyderich- infection by X. fastidiosa ssp. multiplex (Sanderlin and Melanson, 2006). Once in- Alger, 2000). Seeds for production of root- resulting in the development of pecan bacte- fection is established, the bacterium tends to stocks from cultivars of ‘Apache’, ‘Elliott’, rial leaf scorch (PBLS) disease (Melanson become systemic in trees with chronic dis- ‘Moore’, ‘Riverside’, and ‘VC1-68’ were et al., 2012). The multiplex subspecies has ease development. Because there is no effec- supplied by the U.S. Department of Agricul- a large host range, including numerous hard- tive treatment to eliminate the pathogen from ture Pecan Breeding and Genetics Station at wood species, and causes disease in a variety infected trees, pecan trees infected through Somerville, TX. Seeds of cultivars ‘Cape of agriculturally important species including grafting will be affected by the disease Fear’, ‘Curtis’, and ‘Stuart’ were collected fruit and nut trees in the United States, and throughout their life span, which can be from trees at the LSU AgCenter Pecan was recently reported to have infected olive several decades. In new orchards, graft trans- Research-Extension Station, Shreveport, trees in Italy (European Food Safety Author- mission can provide the primary inoculum of LA. The seeds were stored near 4 °C through ity, 2013; Hopkins and Purcell, 2002). X. fastidiosa, which may be subsequently the winter in moist vermiculite for stratifica- spread by insect vectors. Without the estab- tion, and a single seed was planted into 25 · lishment of the pathogen in an orchard 25 cm (9.5-L volume) plastic pots contain- through graft transmission, an orchard may ing MetroMix 902 (Sungro, Agawam, MA) Received for publication 4 Mar. 2015. Accepted for publication 29 June 2015. escape several years before the pathogen is plant growth medium in Mar. 2013. The trees I thank Dr. L.J. Grauke for providing seed for introduced by insect vectoring (Sanderlin, were grown for 1 year in a greenhouse and rootstock production. unpublished data). inoculated in 2014. At the time of inocula- 1Corresponding author. E-mail: rsanderlin@agcenter. It should be possible to reduce the tion, most trees were between 30 and 40 cm lsu.edu. incidence of PBLS by reduction of graft in height. HORTSCIENCE VOL. 50(8) AUGUST 2015 1183 Trees were observed for PBLS symptoms PBLS symptoms from their inoculation date types including ‘Cape Fear’. The rootstocks and assayed for the pathogen by enzyme- until the trees went dormant in the green- from ‘Apache’, ‘Moore’, and ‘Stuart’ were linked immunosorbent assay (ELISA) before houses in late December. also not significantly different from ‘Cape inoculation to confirm that X. fastidiosa was Tissue for ELISA of the inoculated trees Fear’. Even though rootstocks from seed of not detectable in the plants by serological was collected initially at 9- to 12-weeks post ‘Apache’ and ‘Moore’ had a lower inci- assay. For ELISA, the two or three oldest inoculation. Additional assays were done as dence than ‘Cape Fear’ type rootstocks, leaves were removed from each tree and the symptoms developed. Trees in each of the they still each had higher than a 50% in- rachises used for assay. All ELISAs conduct- three inoculation periods that had not pre- fection level. The rootstock grown from ed throughout the study were performed with viously assayed positive for infection were ‘Curtis’, ‘Elliott’, and ‘Riverside’ had a sig- a commercial kit for X. fastidiosa (Agdia given a final ELISA at 17 to 22 weeks nificantly lower incidence of infection than Inc., Elkhart, IN). The preinoculation assays postinoculation. Five noninoculated control ‘Cape Fear’ and ‘VC1-68’; but were not were done within a week before inoculation. trees of each rootstock type were assayed 18 lower than any of the other rootstocks tested For each inoculation date, the inoculum weeks after the first X. fastidiosa inoculation (Table 1). was started from cultures stored at –80 °Cin date. The majority (86%) of the trees of that 30% glycerol. Cultures were regenerated on For statistical analysis, inoculation dates were positive for infection by ELISA either a single petri plate of periwinkle