Progress 09/30/02 to 09/30/05
Outputs
The major goals of this project have been to develop information on the method(s)of transmission of the bacterial pathogen that causes pecan bacterial leaf scorch (Xylella fastidiosa) to pecan trees and to acquire knowledge about the epidemiology of the disease. It was established that during the tree dormancy period the pathogen can inhabit terminals of the size and age that are used for scion wood preparation. The pathogen was able to survive scion wood storage and infect newly developing tissue following grafting. In an orchard test the pathogen was detected in about 14% of trees grafted with scion wood that had been collected from infected trees. In 2005, it was demonstrated that transmission could also occur from infected seedling rootstocks into new growth developing from uninfected scions following grafting. The graft-transmission rate from infected rootstocks was 85% in a test conducted with potted trees. The natural infection frequency of rootstocks used by
pecan propagators has not been determined. Because pecan cultivars are clonally propagated, the ease of transmission of the pathogen from either infected scion wood or rootstock may play a significant role in the introduction of the pathogen into orchards. It is probable that insect transmission is also a major factor in spread of the disease. Observations of orchards support this probability. A complete-block randomized orchard with three cultivars, Cape Fear, Candy, and Stuart that was planted in 1987 has been monitored for increase in incidence of bacterial leaf scorch since 1998. In 2004 and 2005, the number of infected trees in this orchard almost doubled each year. The pattern of disease spread suggests that insect transmission is involved in moving the pathogen from pecan tree to pecan tree. Disease increase has been primarily in Cape Fear, which suggests that this cultivar is more susceptible to infection than the other two cultivars in the orchard. Pecan bacterial leaf scorch
disease has been identified in over twenty cultivars in commercial orchards in Louisiana. In 2004 and 2005, the pecan spittlebug (Clastopera achatina) was collected from an orchard and used in transmission tests. A sorghum trap crop was used in 2004 and 2005 to attract other potential vectors (leafhoppers) for testing. The insect transmission tests have not been completed. In a separate test, for a three-year period during the spring and summer, a set of 10 different potted pecan seedlings were placed weekly in an orchard with a high intensity of leaf scorch. These seedlings were examined for two years after exposure, but infection was not detected in any of them.
Impacts
Pecan bacterial leaf scorch can cause leaf loss and yield reduction of nuts. The pathogen was found to be transmitted to newly developing trees from infected rootstocks and scions following grafting. Because pecan trees are produced by grafting, graft-transmission of the pathogen could be an important source of introduction of the disease into orchards. Observations and data suggest that the pathogen is also spread to pecan trees by insects. The disease has been detected in over 20 cultivars, including most of the commonly grown cultivars in the southeastern United States. This basic knowledge about pathogen transmission in pecan and disease epidemiology is needed to eventually develop management approaches that limit the damage of leaf scorch disease to commercial pecan production.
Publications
- Sanderlin, R. S. 2005. Cultivar and seedling susceptibility to pecan bacterial leaf scorch caused by Xylella fastidiosa and graft transmission of the pathogen. Plant Disease 89:446-449.
- Sanderlin, R. S. and Melanson, R. A. 2005. Status report on pecan bacterial leaf scorch research. Proceedings of the Southeastern Pecan Growers Association98:87-91.
- Sanderlin, R. S. and Melanson, R. A. 2005. Insect transmission trials of Xylella fastidiosa to pecan. Citrus, Fruit, and Nut Louisiana State University Agricultural Center Exchange Groups Research Summary 164:37.
- Sanderlin, R. S. and Melanson, R. A. 2005. Spread of pecan bacterial leaf scorch disease within orchards. Citrus, Fruit, and Nut Louisiana State University Agricultural Center Exchange Groups Research Summary 164:38-39.
- Sanderlin, R. S. and Melanson, R. A. 2005. Rootstock transmission of Xylella fastidiosa in pecan. Citrus, Fruit, and Nut Louisiana State University Agricultural Center Exchange Groups Research Summary 164:40-41.
- Sanderlin, R. S. and Melanson, R. A. 2005. Evaluation of graft-transmission of Xylella fastidiosa in pecan. Citrus, Fruit, and Nut Louisiana State University Agricultural Center Exchange Groups Research Summary 164:42-44.
- Sanderlin, R. S. and Melanson, R. A. 2005. Efficacy of reduced rates of fungicides used for pecan scab control in combination products. Citrus, Fruit, and Nut Louisiana State University Agricultural Center Exchange Groups Research Summary 164:45-46.
- Graham, C. J., Sanderlin, R. S., and Gibson, A. 2005. Inglewood orchard and Pointe Coupee parish nut evaluations for 2004. Citrus, Fruit, and Nut Louisiana State University Agricultural Center Exchange Groups Research Summary 164:33-36.
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Progress 01/01/04 to 12/31/04
Outputs
The survey for possible insect vectors of Xylella fastidiosa to pecan continued in 2004. The pecan spittlebug, Clastoptera achatina, was present in abundant numbers in an orchard with a high intensity of pecan bacterial leaf scorch (PBLS) caused by X. fastidiosa. From June through August, spittlebug nymphs and adults were collected from symptomatic terminals and tested for transmission of the bacterium. Spittlebugs were transferred directly from infected trees to uninfected Cape Fear seedlings in a greenhouse. Ten to 30 nymphs were placed on each of 91 seedling trees and 100 nymphs were placed on each of 3 grafted uninfected trees. Over 2900 nymphs collected from infected trees were tested in this manner. By the end of the growing season none of the test plants had exhibited scorch symptoms. Likewise, over 200 adults in groups of 10 to 20 were tested on 15 seedlings for transmission and no symptoms were observed. Over 2600 nymphs in groups of 10, 20, 40, or 125 and
150 adults in groups of 10 were tested for the bacterium by ELISA. No definite positives for the presence of X. fastidiosa were detected. The seedlings will be assayed for possible infection during the next growing season. A common planthopper (Ormenoides venusta) has been observed on pecan trees. Approximately 150 of these were captured in 2003 and placed onto symptomatic pecan tissue for 1 to 2 days then transferred to uninfected seedlings to test for transmission of X. fastidiosa. After 2 years none of the 28 seedlings used developed scorch and all tested negative by ELISA. Last year X. fastidiosa graft-transmission in pecan was demonstrated in a test plot. This year PBLS developed in some additional trees in this plot that were grafted with scions from infected trees. The disease has not been confirmed in trees grafted with scions from an uninfected source or in ungrafted trees within this plot. Another experimental block has been monitored by symptoms observation for spread of
PBLS over a period of several years. In 2004, there appeared to be a significant increase in the number of trees with symptoms. Through 2003, a total of 14 trees out of 396 had been identified with PBLS and infection confirmed by ELISA. In 2004, a total of 20 additional trees with symptoms in this block were identified. ELISA indicated that 17 of these were definitely infected. These results suggest that infection may increase slowly for several years but may also increase rapidly within one year's time. The method of natural spread of the pathogen in pecan orchards has not been identified.
Impacts
Pecan bacterial leaf scorch disease in pecan caused by the bacterium Xylella fastidiosa causes significant economic crop loss in some cultivars with severe disease. Identification of the means of transmission of the pecan bacterial leaf scorch pathogen to pecan trees including insect vectors and graft-transmission should provide some understanding of how the disease spreads from tree to tree within orchards. It may also lead to some useful methods to reduce the frequency of introduction of the pathogen into orchards and to slow the increase of disease within orchards where the disease already exists.
Publications
- Sanderlin, R. S. 2004. Cultivar susceptibility to pecan bacterial leaf scorch. Proceedings of Oklahoma pecan growers association 74:45-47.
- Sanderlin, R. S. 2004. Transmission of the pecan bacterial leaf scorch pathogen through scion wood. Citrus, Fruit, and Nut Louisiana State University Agricultural Center Exchange Groups Research Summary 157:40-41.
- Sanderlin, R. S. 2004. Pecan scab control with rates of fungicides used in fungicide mixture products. Citrus, Fruit, and Nut Louisiana State University Agricultural Center Exchange Groups Research Summary 157:42-44.
- Sanderlin, R. S. 2004. Leafhopper transmission trials of Xylella fastidiosa to pecan. Citrus, Fruit, and Nut Louisiana State University Agricultural Center Exchange Groups Research Summary 157:45.
- Graham, C. J., Sanderlin, R. S., Aulds, R., and Purdy, D. 2004. Viviparity of 'Cherokee' pecan in 2003. Citrus, Fruit, and Nut Louisiana State University Agricultural Center Exchange Groups Research Summary 157:37-39.
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Progress 01/01/03 to 12/31/03
Outputs
A test was initiated to determine if Xylella fastidiosa, the cause of pecan bacterial leaf scorch disease (PBLS), can be transmitted through scion wood. In January 2003, 200 pecan seedlings were planted at a spacing of 10 by 10 feet. In May, 100 of these were grafted with scion wood that had been collected during January 2003 from X. fastidiosa-infected cultivar Cape Fear pecan trees. Fifty of the seedlings were grafted with scions collected from Cape Fear trees not infected with the bacterium, and 50 seedlings were left ungrafted. Scion wood used in this test was collected and stored in a manner typical for pecan graft wood. Scion wood was prepared from wood that developed during 2002 or 2001. After the limbs were removed from trees, they were cut into sections of 15 to 20 cm length containing one to four leaf bud nodes. The cut ends were sealed with beeswax, and the scion wood was wrapped in moist paper towel, then enclosed in plastic bags to prevent drying and
stored at 4 to 7 degrees Centigrade until used about five months later. The plot was a randomized complete-block design with 10 blocks of 20 trees. Seventy-four percent of the grafted trees and 80 percent of the ungrafted seedlings were alive in October. Because PBLS-like symptoms had developed on some of the trees in September, all of the trees in the plot were assayed for X. fastidiosa by ELISA. Eleven of 76, 14 percent, of the trees grafted with scions from infected trees developed PBLS and were positive for bacterial infection. Because pecan cultivars are clonally propagated, these results suggest that transmission of X. fastidiosa through infected scion wood could be a significant source of the disease for the pecan industry. Using uninfected trees for scion wood sources may provide a useful level of disease control. Because PBLS is a chronic disease that has no treatment, commercial nurseries should make efforts to avoid infected scion wood. A test with the objective of
detecting insect transmission of X. fastidiosa in an orchard with a high intensity of PBLS was continued for the third year. Seedlings of cultivar Cape Fear were placed in the orchard for weekly intervals from 1 April through 16 July. The seedlings were observed for scorch development and assayed for infection by ELISA after being maintained in a greenhouse for at least two months after orchard exposure. As in the previous two years, no infection was detected and typical PBLS did not develop.
Impacts
It was demonstrated that the bacterium that causes pecan bacterial leaf scorch disease can survive a typical storage environment for pecan scion wood. Pathogen transmission and disease development occurred when scion wood was used from trees infected with the bacterium. Infected scion wood could be an important source of disease spread because pecan cultivars are clonally propagated. It may be possible to reduce the frequency of pecan bacterial leaf scorch in orchards by grafting with scion wood taken only from trees that are not infected with the bacterium.
Publications
- Sanderlin, R. S. and Heyderich-Alger, K. I. 2003. Effects of pecan bacterial leaf scorch on growth and yield components of cultivar Cape Fear. Plant Disease 87:259-262.
- Sanderlin, R. S. and Heyderich-Alger, K. I. 2003. Efficacy of fungicides tested for pecan scab disease, 2002. Fungicide and Nematicide Tests 58:STF016.
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Progress 01/01/02 to 12/31/02
Outputs
Effects of pecan bacterial leaf scorch (PBLS) disease on defoliation and yield components were examined in a 3-year study with cultivar Cape Fear. Leaflet defoliation, recorded in October, averaged 58 percent greater on the Xylella fastidiosa-infected trees than on uninfected trees. The weight of stem terminals with PBLS was significantly lower in 3 of 6 comparisons and was lower, but not significantly, in 2 of the other comparisons. This suggests a lower carbohydrate production from the heavily defoliated terminals. The mean number of in-shell nuts per pound and kernels per pound was 12 percent and 16 more, respectively, for the infected trees, over the 3-year period. Information on cultivar susceptibility and disease frequency was expanded by examing trees in orchards for disease symptoms and serological assay of symptomatic trees for X. fastidiosa.The serological assay in 2002 included 117 trees with PBLS symptoms, 95 of these tested positive for infection. This
disease and pathogen have been found in a total of 23 cultivars and several seedlings in commercial production orchards. Inoculation tests were started in 2001 to determine if multiple cultivars and seedlings could be infected with the bacterium by needle inoculation. All inoculum isolates were obtained from Cape Fear trees. Nine cultivars were inoculated in this study. Ten to 20 trees of each cultivar were inoculated with X. fastidiosa and 5 to 10 trees with water. A few trees developed disease in 2001, but because the inoculations were made in late summer there was not much disease development until summer 2002. A total of 117 trees were inoculated with the bacterium and 48 of these have been vertified to be infected, almost all of these developed PBLS. Eight of the 9 cultivars were infected and cultivar infection rate varied from 18 to 89%. None of 59 trees inoculated with water developed PBLS, and they assayed negative for infection. It appears that needle inoculation could be a
useful tool to screen for cultivar susceptibility. Seedlings may be even more susceptible to X. fastidiosa infection by needle inoculation. Seedlings of 6 cultivars were inoculated, and a high percentage of infection occurred in all seedling types, ranging from 60 to 100%. No infection was detected in 39 seedlings inoculated with water.
Impacts
Information on the effects of pecan bacterial leaf scorch on growth and yield components helps to provide some understanding of the economic effects of this disease. Information on the distribution of this disease across cultivars in commercial orchards and data on cultivar susceptibiity to infection and disease reaction provides knowledge on the overall economic importance of pecan bacterial leaf scorch.
Publications
- Sanderlin, R. S. 2002. Brown leaf spot (p53), Vein spot (p57),and Zonate leaf spot (p58). In Compendium of Nut Crop Diseases in Temperate Zones. T. J. Michailides, B. L. Teviotdale, and J. W. Pscheidt, eds. American Phytopathological Society Press, St. Paul, MN pp89.
- Sanderlin, R. S. and Heyderich-Alger, K. I. 2002. Evaluation of fungicides for control of pecan scab disease, 2001. Fungicide and Nematicide Tests 57: STF27.
- Sanderlin, R. S. and Heyderich-Alger, K. I. 2002. Effect of late season increases in scab disease on yield components. Citrus, Fruit, and Nut Agricultural Center Exchange Group Research Summary 139:53-54.
- Sanderlin, R. S, and Heyderich-Alger, K. I. 2002. Evaluation of fungicides for control of pecan scab disease, 2001. Citrus, Fruit, and Nut Agricultrual Center Exchange Group Research Summary 139:55-56.
- Sanderlin, R. S. and Heyderich-Alger. K. I. 2002. Effects of pecan bacterial leaf scorch on cultivar Cape Fear. Citrus, Fruit, and Nut Agriculural Center Exchange Group Research Summary 139:57-58.
- Sanderlin, R. S. and Heyderich-Alger. K. I. 2002. Xylella fastidiosa can cause pecan bacterial leaf scorch in multiple cultivars. Citrus, Fruit, and Nut Agricultural Center Exchange Group Research Summary 139:59-60.
- Sanderlin, R. S. and Heyderich-Alger, K. I. 2002. Test for evidence of insect transmission of Xylella fastidiosa to pecan. Citrus, Fruit, and Nut Agricultural Exchange Group Research Summary 139:61.
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