Host Specificity of Pecan Strains of Xylella fastidiosa subsp. multiplex.

Pecan (Carya illinoinensis) bacterial leaf scorch disease, caused by Xylella fastidiosa subsp. multiplex, causes defoliation and reduces terminal growth and nut yield. The pathogen is transmitted to pecan by xylem-feeding spittlebugs and leafhoppers and through graft transmission in the clonal propagation of cultivars. Xylella fastidiosa subsp. multiplex has a broad host range, infecting numerous hardwood tree species and some herbaceous species. There is evidence of additional host specialization within subsp. multiplex. Data presented here support the existence of host specialization with X. fastidiosa that infect pecan. In this study, mechanical inoculation was used to inoculate several plant species that are naturally infected by subsp. multiplex, including sycamore, red maple, purple-leafed plum, and blueberry with strains of X. fastidiosa from pecan. Hosts of three other subspecies were also inoculated with the pecan strains: grapevine (subsp. fastidiosa); oleander (subsp. sandyi); and mulberry (subsp. morus). Pecan was also inoculated with a strain of the pathogen from sycamore (subsp. multiplex) and a strain from grapevine (subsp. fastidiosa). In greenhouse tests, inoculum prepared from X. fastidiosa obtained from naturally infected pecan almost exclusively infected pecan. In addition, the subsp. multiplex strain from sycamore generally did not infect pecan, and the subsp. fastidiosa strain from grapevine did not infect pecan. The inability of the pecan strain to readily infect other hosts commonly located in the vicinity of pecan orchards affects the management recommendations for the disease in commercial pecan production by allowing management practices to focus on pecan orchards and insect vectors.

A Systematic Study Reveals that Xylella fastidiosa Strains from Pecan Are Part of X. fastidiosa subsp. multiplex.

Xylella fastidiosa causes disease in a number of economically important crops, ornamental plants, and shade trees, including grapevine, citrus, oleander, and sycamore. In pecan, X. fastidiosa causes pecan bacterial leaf scorch (PBLS), which leads to defoliation and reduces nut yield. No economically effective treatments are available for PBLS. In order to improve PBLS management practices, it is necessary to determine the subspecies of X. fastidiosa strains that infect pecan so that potential sources of inoculum may be identified. Multiprimer polymerase chain reaction (PCR) and phylogenetic analyses using nucleotide sequence data from the 16S-23S rRNA intergenic transcribed spacer (ITS) region and pglA consistently identified strains of X. fastidiosa isolated from pecan as X. fastidiosa subsp. multiplex. Enterobacterial repetitive intergenic consensus PCR and repetitive extragenic palindromic (REP)-PCR analyses were congruent with phylogenetic analyses. REP-PCR analyses indicated genetic variation within strains of X. fastidiosa from pecan. From these same analyses, X. fastidiosa strains from sycamore, grapevine, and oleander from Louisiana were identified as subsp. multiplex, subsp. fastidiosa, and subsp. sandyi, respectively. This study provides additional information about the host ranges of X. fastidiosa subspecies.

Insect Transmission of Xylella fastidiosa to Pecan.

Pecan bacterial leaf scorch (PBLS), caused by the bacterium Xylella fastidiosa, can cause economically significant crop loss to some pecan (Carya illinoinensis) cultivars in the southeastern United States. X. fastidiosa is typically vectored by spittlebugs (Cercopidae) and leafhoppers (Cicadellidae). Because no vector species had been reported for pecan, an attempt was made to identify potential vectors that are capable of acquiring the bacterium from infected pecan trees and transmitting to pecan. Several spittlebug and leafhopper species collected from various sources, including sorghum and pecan, were tested as potential vectors of the pathogen from pecan to pecan. When tested in groups, the pecan spittlebug, Clastoptera achatina; the Johnson-grass sharpshooter, Homalodisca insolita; and the glassy-winged sharpshooter (GWSS), Homalodisca vitripennis, had transmission rates to pecan of 11.4, 19.3, and 4%, respectively, following a pathogen acquisition period on infected pecan terminals. The pecan spittlebug is common in pecan orchards in the southeastern United States, and the GWSS was observed on young vigorous pecan shoots. Limited testing with the diamond-backed spittlebug, Lepyronia quadrangularis, and the lateral-lined sharpshooter, Cuerna costalis, suggested that these could be occasional vectors of X. fastidiosa to pecan. There is a need for studies on the identification and population dynamics of Cicadellidae that inhabit pecan orchards to determine if management of vectors is needed in commercial pecan production to reduce the spread of PBLS.

Reduction of Xylella fastidiosa Transmission Through Pecan Scion Wood by Hot-Water Treatment.

Pecan (Carya illinoinensis) bacterial leaf scorch disease, caused by the bacterium Xylella fastidiosa, causes leaf loss and reduction in yield of pecans. One of the ways that the pathogen infects newly developing trees is by graft-transmission through infected scion wood. Submersion of pecan scion wood in 46°C water for 30 min greatly reduced transmission of X. fastidiosa following grafting. During a 2-year study with potted rootstock grafted to either hot-water-treated or nontreated scion wood collected from limbs of 'Cape Fear' pecan infected with X. fastidiosa, the pathogen was detected in 21% of the trees that developed from the nontreated scion wood and 0.7% of the trees from the hot-water-treated scions. The hot-water treatment of 46°C for 30 min did not affect graft success. Likewise, scion diameter had no effect on success of grafting or on the efficacy of hot-water treatment. Similar hot-water treatments have been efficacious in elimination of X. fastidiosa and some other pathogens from grapevine cuttings. The use of the hot-water treatment demonstrated in this report could be helpful to individual pecan growers and nurseries that use scion wood that may be infected with X. fastidiosa to reduce the occurrence of pecan bacterial leaf scorch disease in new trees.

Cultivar and Seedling Susceptibility to Pecan Bacterial Leaf Scorch Caused by Xylella fastidiosa and Graft Transmission of the Pathogen.

The xylem-limited bacterium Xylella fastidiosa was detected in 20 cultivars and 4 seedling trees with pecan bacterial leaf scorch (PBLS) symptoms during a 2-year survey of commercial pecan (Carya illinoinensis) cultivars. The orchard survey and inoculation trials indicated that several of the currently recommended cultivars, as well as older, popular cultivars, are susceptible to infection by the bacterium and develop PBLS. The severity of disease development was variable between cultivars and within cultivars. Eight of 9 cultivars growing in pots that were needle-puncture inoculated with X. fastidiosa became infected and developed the disease. Currently, no cultivars have been identified that are not susceptible to infection and symptom development. All 6 seedling types that were inoculated became infected and developed PBLS. The frequency of infection of the seedlings was greater than the cultivars; 88% of the inoculated seedlings and 44% of the inoculated cultivar trees were infected. It was demonstrated that the pathogen could be transmitted through scion wood. Because pecan cultivars are clonally propagated, graft-transmission may represent a major source of pecan infection.

Effects of Pecan Bacterial Leaf Scorch on Growth and Yield Components of Cultivar Cape Fear.

Pecan bacterial leaf scorch (PBLS) recently was recognized to be caused by the bacterium Xylella fastidiosa. The objective of this work was to compare certain tree growth and yield components of trees with and without PBLS. The evaluations were made for 3 years (1999 to 2001) on the disease-sensitive cv. Cape Fear. At nut maturity (October) each year, the number of leaves and leaflets and the leaflet weight were significantly smaller on terminals from trees with leaf scorch than on terminals from trees not infected with X. fastidiosa. Similar amounts of defoliation occurred on bearing and nonbearing terminals. The symptomatic terminals averaged 58% fewer leaflets at the end of the growing season. The weight of terminals from diseased trees was significantly lower in 2 of the 3 years for nonbearing terminals compared with similar terminals from uninfected trees. Terminals of infected trees generally were not significantly shorter than terminals of uninfected trees. The weight of the nuts from terminals with leaf scorch was lower than the weight from uninfected trees each year. The primary effect was on kernel development, which averaged 16% lower weight. Phosphorus concentration was slightly but significantly lower in infected leaflets during October in three of six comparisons. There were no other consistent differences in the concentration of nine other elements in leaflets between diseased and uninfected terminals. PBLS is capable of causing economically significant yield reductions. Because the disease is chronic, the potential yield reduction over the life of a tree is large on disease-sensitive trees.

Evidence that Xylella fastidiosa Can Cause Leaf Scorch Disease of Pecan.

The disease known as pecan fungal leaf scorch has been reported to be either caused by or associated with several fungi since it was first recognized in 1972. Data are presented that indicate the disease is initiated by the fastidious xylem-limited bacterium Xylella fastidiosa. X. fastidiosa was found consistently associated with fungal leaf scorch disease of pecan (Carya illinoinensis) in commercial orchards in Louisiana. It was generally not detected in symptomless trees. The disease was reproduced by inoculation of greenhouse seedlings and grafted trees with cultures of the bacterium obtained from leaves with fungal leaf scorch. The bacterium was reisolated from symptomatic tissue of inoculated pecan seedlings, but not from symptomless plants inoculated with water to complete Koch's postulates. It is proposed that the name of the disease be changed to pecan bacterial leaf scorch because fungi do not appear to be necessary for symptom development.

Evidence That Xylella fastidiosa Is Associated with Pecan Fungal Leaf Scorch.

Several different leaf scorch symptomatologies occur on the foliage of pecan (Carya illinoinensis). The causes of these different scorch symptoms have been associated with environmental stresses, nutritional imbalances, mites, and pathogens. One type of leaf scorch is characterized by necrosis beginning at the tips or margins of the leaflets and progressing toward the midrib and base of the leaflets. The most distinguishing feature of this type of leaf scorch is a dark brown, black, or purplish band of tissue at the interface of the necrotic and green leaflet tissue. This band does not occur on all affected leaflets, but is a consistent feature with this symptomatology. Leaflets with this leaf scorch usually abscise before the entire leaflet becomes necrotic. Affected leaflets will abscise from a compound leaf, while leaflets without symptoms remain on the rachis. When this scorch is severe, the entire leaf including the rachis may abscise. The symptoms are often confined to one area of the tree. Symptoms of this disease appear as early as June but often begin in July. Incidence and severity increase through the remainder of summer and into fall. Over the past 25 years, several different genera of fungi have been reported as being associated with this leaf scorch (2). The phenomenon was named fungal leaf scorch (FLS) because of the association with fungi and the observation that some fungicides reduced the severity of leaf scorch. Genera of fungi implicated in the development of FLS were Pestalotia, Epicoccum, Curvularia, and Fusarium. Recent work has indicated that the disease could be caused by a Phomopsis sp. or Glomerella cingulata or both (1). The symptoms and epidemiology of the FLS are similar to other leaf scorch diseases of hardwood caused by the fastidious xylem-limited bacterium Xylella fastidiosa. A commercial enzyme-linked immunosorbent assay (Agdia, Elkhart, IN) was used to screen for this organism in association with FLS. Assays were conducted by extracting from a composite sample of 9 to 12 sections (approximately 3 cm long) taken from 3 to 4 rachises of each tree tested. Positive reaction for the presence of X. fastidiosa in rachises was recorded from 10 of 10 trees with symptoms of FLS on the Cape Fear cultivar. One of two trees was positive from Cape Fear leaves without symptoms. Symptomless foliage from two less susceptible cultivars, Stuart (two trees) and Sumner (one tree), assayed negative for the bacterium. The association of X. fastidiosa with symptoms of FLS provides evidence that this organism is involved in the etiology of pecan leaf scorch; however, further work is needed to determine the exact etiology. References: (1). A. J. Latham et al. Plant Dis. 79:182, 1995. (2) R. H. Littrell and R. E. Worley. Phytopathology 62:805, 1972.