Transmission of Xylella Fastidiosa Through Pecan Rootstock

PLANT PATHOLOGY

HORTSCIENCE 41(6):1455–1456. 2006. and Oct. 2005. The data from each year were combined, and statistical significance evalu- Transmission of Xylella fastidiosa ated by the chi-square test. through Pecan Rootstock Results and Discussion 1 The overall graft success rates were low R. S. Sanderlin and R. A. Melanson (54.5% for grafts on uninfected rootstocks Louisiana State University Agricultural Center, Pecan Research-Extension and 40% on infected rootstocks), probably Station, PO Box 5519, Shreveport, LA 71135 because the trees had been confined to small pots for several years and were in poor Additional index words. Carya illinoinensis, graft transmission, pecan bacterial leaf scorch nutritional condition. Abstract. Xylella fastidiosa Wells et al., the pathogen that causes pecan [Carya illinoinensis In 2004, infected rootstocks came from (Wangenh.) C. Koch] bacterial leaf scorch disease, was demonstrated to be highly six seed sources and uninfected rootstocks transmissible through graft unions from infected rootstock into new growth developing came from five of the same six seed sources from scions. Infected rootstocks were obtained by inoculation of pecan seedlings in pots (Table 1). Eleven of the 12 successful grafts with in vitro cultures of the pathogen. If rootstock infection occurs in nature, trans- on infected rootstocks developed PBLS mission of the pathogen into tissue growing from scions could serve as a significant source symptoms in either 2004 or 2005, and 10 of of introduction of the disease into pecan orchards. Because symptom development in these tested positive for X. fastidiosa infec- infected trees typically begins in midsummer and grafting takes place in the early spring, tion in at least one of the serological assays it would be difficult to identify infected rootstock before grafting. Commercial pecan during 2004 and 2005 (Table 1). Six of the growers sometimes attempt to eliminate bacterial leaf scorch from trees by regrafting to seven trees developing from the healthy other cultivars. The high rate of transmission from infected rootstocks observed in this rootstock grafts did not exhibit scorch symp- test and the lack of knowledge of cultivars with resistance to the disease makes this toms and all seven tested negative for practice ineffective. infection. Scorchlike symptoms were recorded on one of the trees developing from a graft on an Pecan [Carya illinoinensis (Wangenh.) Materials and Methods uninfected rootstock and one from a graft on C. Koch] is one of the few economically impor- an infected rootstock that tested negative for tant crops native to North America. It is To test for rootstock transmission of the X. fastidiosa in several serological assays. It grown commercially in its native range of pathogen, infected and uninfected trees in is probable that these two developing trees the United States, and production has ex- pots were grafted to healthy scions. Infected were not infected because there are other panded into nonnative areas of the southeast- rootstocks were obtained from a group of problems that can mimic the symptoms of ern United States and into some of the pecan seedlings that had been inoculated with bacterial leaf scorch, including drought and western states including Arizona, New Mex- in vitro isolates of X. fastidiosa in 2001 nutritional imbalances. ico, and California. Pecan cultivars are clon- (Sanderlin, 2005). Uninfected seedlings in- Eight infected rootstocks from four seed ally propagated with scion wood or buds oculated with distilled water in 2001 served as sources were grafted in 2005; seven of these grafted onto rootstocks that are grown from uninfected rootstocks. The seedlings had been tested positive for infection before the end of seed. grown from seed of several cultivars (Table the year (Table 2). The five grafts onto Pecan bacterial leaf scorch (PBLS) dis- 1). The potted seedlings were maintained in uninfected rootstocks all tested negative for ease, incited by Xylella fastidiosa Wells et al., a greenhouse. Prior assays using an enzyme- infection. Scions from all sources used (ÔCape has a chronic course and is able to cause linked immunosorbent assay (ELISA) kit FearÕ, ÔPawneeÕ, and ungrafted ÔRiversideÕ defoliation and economically significant crop (Agdia, Inc., Elkhart, Ind.) were used to seedling) became infected when grafted onto loss in severely infected trees (Sanderlin and estimate the presence or absence of X. fastid- diseased rootstocks. Heyderich-Alger, 2000, 2003). The modes of iosa. Uninfected scions were collected in Feb. During the 2-year period, 85% of the trees introduction of the pathogen into pecan 2004 from a ÔCape FearÕ tree and an ungrafted that developed after grafting onto infected orchards has not been completely estab- tree grown from a ÔRiversideÕ nut, and were rootstock became infected with the bacterium. lished; however, the pathogen can be trans- stored wrapped in moist paper towels near None of the trees grafted onto uninfected mitted into new growth from infected scions 4 °C until used for grafting in May. The scion rootstock were infected. The chi-square test (Sanderlin, 2005). Thus, use of scion wood sticks were 0.5 to 1.0 cm in diameter and was highly significant (P < 0.0001) for X. from infected trees could be a significant about 15 cm long. Grafting was carried out fastidiosa transmission from infected root- source of pathogen introduction into or- using the four-flap technique (Vanerwegen, stock after grafting. This clearly demon- chards. If pecan propagators try to avoid 1975). In 2004, 25 infected rootstocks and 12 strates the ability of the pathogen to pass collecting scions from infected sources, it uninfected rootstocks were used. The growth into newly developing grafts at a high becomes equally important to know whether that developed from the scions was visually rate through the rootstock–scion union. the pathogen can be transmitted from in- evaluated for PBLS symptoms through the Because of the high rate of pathogen trans- fected rootstock into trees developing from summer and assayed by ELISA for the mission through infected rootstock, regraft- uninfected scions. In addition, pecan growers bacterium two or three times during Sept. ing pecan trees as a means of reducing the sometimes try to eliminate trees with PBLS and Oct. 2004 and again in Sept. 2005. incidence of the disease is not an effective in their orchards by regrafting diseased trees The test was repeated in 2005 using the control measure, particularly because to other cultivars. The purpose of this study same procedures with other seedlings that cultivars resistant to PBLS have not been was to verify that the pathogen can be passed had been inoculated with either X. fastidiosa identified. from infected seedling rootstock into newly or distilled water in 2001. Scions used in The ability of the pathogen to pass easily developing grafts. 2005 were collected in February. and came through pecan graft unions from infected from either an uninfected ÔPawneeÕ tree or an rootstocks and from infected scions into uninfected, ungrafted tree grown from a newly developing growth may represent one ÔRiversideÕ nut. Twenty-five infected root- of the primary ways that the pathogen is Received for publication 17 Mar. 2006. Accepted stocks and 10 uninfected rootstocks were introduced into new orchard plantings. The for publication 21 Apr. 2006. grafted in 2005. Developing growth from rootstocks used in this test were infected 1To whom reprint requests should be addressed; the scions was monitored for symptoms and through artificial inoculation using in vitro e-mail [email protected]. assayed for infection by ELISA in September isolates of the pathogen. Pecan nurseries will

HORTSCIENCE VOL. 41(6) OCTOBER 2006 1455 Table 1. Results of 2004 rootstock transmission of Xylella fastidiosa test. Symptoms and serologyy Rootstock seed source and 8 Sept. 2004 14 Sept. 2004 24 Sept. 2004 22 Oct. 2004 6 Sept. 2005 test tree numberz Sym Ser Sym Ser Sym Ser Sym Ser Sym Ser Infected Barton I – + – – + + Barton II – – + + + + Cape Fear I + + + – + + Cheyenne I – – + + + + – – Cheyenne II + + – + + + Desirable I – + + + + + Desirable II – – – – + – – – Kiowa I – + + + + + Kiowa II – – – – + + Kiowa III – + + + + + Kiowa IV + + + + + + Schley I – – – – – – Uninfected Barton I – – – – – – Barton II – – – – – – Cape Fear I – – – – – – Cheyenne I – – – – + – Kiowa I – – – – – – Schley I – – – – – – Schley IIx –––– zRootstocks were grown from seed of listed cultivars, inoculated with Xylella fastidiosa or distilled water in 2001, and grafted to uninfected scions of ÔCape FearÕ or a ÔRiversideÕ seedling tree in 2004. yPresence (+) or absence (–) of symptoms of pecan bacterial leaf scorch were recorded at the time tissue was taken for serological assay, date shown. Ser (+) or (–) indicates a positive or negative enzyme-linked immunosorbent assay for X. fastidiosa. xThis tree died during 2005.

Table 2. Results of 2005 rootstock transmission of It should be possible to reduce the degree stocks that have at least 1 year’s growth, there Xylella fastidiosa test. of transmission through infected scions, would be ample opportunity for insect trans- Symptoms and serologyy about 14% in one test (Sanderlin, 2005), by mission to occur. Although seedlings used in Rootstock seed visually identifying infected trees throughout this study were not from the typical maternal source and test 6 Sept. 2005 5 Oct. 2005 tree numberz Sym Ser Sym Ser the growing season and avoiding collection cultivars generally used by pecan propaga- Infected of scion wood from trees that develop symp- tors (Grauke and Thompson, 1995), results Barton III + + toms at any time during the summer and fall. indicate that rootstocks from multiple seed Barton IV – + Results reported here suggest that the rate of sources can be infected with the pathogen. Barton V – + rootstock transmission of the pathogen is Cheyenne III + + much higher than the rate of scion trans- Desirable III – – mission. However, it would be more difficult Desirable IV – + to identify infected rootstocks and avoid their Literature Cited Kiowa V + + use. Pecan grafting is typically conducted Gould, A.B. and J.H. Lashomb. 2005. Bacterial Kiowa VI – + during the spring either before leaf growth or Uninfected leaf scorch of shade trees. 7 Nov. 2005. http:// Barton III – – shortly after the initiation of leaf develop- www.apsnet.org/online/feature/bls. Cheyenne II – – ment, and symptoms of PBLS usually do not Grauke, L.J. and T.E. Thompson. 1995. Rootstock Desirable I – – become visible until midsummer. In addition, development, p. 167–175. In: M.W. Smith, W. Kiowa II – – results from this test (Table 1) and previous Reid, and B.W. Wood (eds.). Sustaining pecan st Kiowa III – – tests (Sanderlin, 2005) illustrate that it is not productivity into the 21 century. Second zRootstocks were grown from seed of listed always possible to detect the presence of National Pecan Workshop Proceedings. U.S. cultivars, inoculated with Xylella fastidiosa or infection solely by occasional observation Department of Agriculture, Agricultural Re- search Service, Ser. Pub. ARS-1995-3. distilled water in 2001, and grafted to uninfected for symptoms. Symptoms may be somewhat scions of ÔPawneeÕ or a ÔRiversideÕ seedling tree in Sanderlin, R.S. 2005. Cultivar and seedling sus- transient, because defoliation occurs and ceptibility to pecan bacterial leaf scorch caused 2005. leaves with scorch may not be present at yPresence (+) or absence (–) of symptoms of pecan by Xylella fastidiosa and graft transmission of bacterial leaf scorch were recorded at the time a specific point in time. Because it can take the pathogen. Plant Dis. 89:446–449. tissue was taken for serological assay, date shown. from two months to more than a year for Sanderlin, R.S. and K.I. Heyderich-Alger. 2000. Ser (+) or (–) indicates a positive or negative symptoms to develop after infection (R. S. Evidence that Xylella fastidiosa can cause leaf enzyme-linked immunosorbent assay for Sanderlin, unpublished data), it would be scorch disease of pecan. Plant Dis. 84:1282– X. fastidiosa. difficult to verify that rootstocks are not 1286. infected. It can be speculated that if root- Sanderlin, R.S. and K.I. Heyderich-Alger. 2003. Effects of pecan bacterial leaf scorch on growth stocks are naturally infected, it would be and yield components of cultivar Cape Fear. through transmission of the pathogen by Plant Dis. 87:259–262. be evaluated to determine whether natural insect vectors (Gould and Lashomb, 2005). Vanerwegan, J. 1975. A new grafting procedure. infection of rootstocks occurs. Because grafting is usually done on root- Pecan South 2:70–71.

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