Randy S. Sanderlin and Rebecca A. Melanson
A new look at an old problem
Pecans are one of the few economically important horticultural crops native to North America. Louisiana is within the native U.S. range, which is primarily the flood plain of the Mississippi River and its tributaries. Louisiana produces about 12.5 million pounds of pecans each year, which contributes to the average U.S. value of about $245 million. Pecans are harvested from either ungrafted trees or from grafted varieties. As with any agricultural crop, numerous management problems, including diseases, affect production potential.
For more than two decades there was uncertainty about the cause of a common pecan disease referred to as leaf scorch. Symptoms consist of pecan tree leaflets turning tan to brown, beginning at the tip and edges and advancing toward the middle of the leaflets. Affected leaflets drop shortly after symptoms begin. The disease may appear on only one limb or throughout a tree. Symptoms and leaflet loss may begin as early as May but tend to become more prevalent as the summer progresses.
Researchers were able to distinguish this problem from several other problems such as nutritional imbalances, scorch mites and physiological problems during the mid-1970s. Their evidence pointed to a pathogen. The pathogen’s identity, however, remained unclear until 1998, when the scientists discovered that this disease was caused by a bacterium, Xylella fastidiosa. This pathogen lives in the water-conducting tissue of a plant and infects more than 100 species of plants including grapes, peaches, almonds, coffee and citrus. When the bacterium infects hardwood trees, such as pecan, it typically produces leaf scorch-type symptoms. On pecan, the disease is referred to as pecan bacterial leaf scorch, or PBLS. As the bacterium multiplies, the water-conducting vessels become plugged, resulting in death of pecan leaflets. Effects of PBLS
During a three-year study from 1999-2001, severely infected trees of the pecan variety Cape Fear had an average of 58 percent more defoliation by the end of the growing season compared with noninfected Cape Fear trees in the same orchard. The Cape Fear variety was very popular during the 1970s and 1980s and was widely planted in Louisiana and across the southeastern states. Most of the commercial orchards planted during this period have some trees of Cape Fear in them, and they will be around for a long time. However, Cape Fear is no longer recommended for Louisiana orchards because it has several production problems including PBLS.
Nuts from diseased terminals had about 12 percent lower in-shell weight and 16 percent lower kernel weight compared to nuts from trees without PBLS. The growth of current season stems on infected limbs was reduced and early tree growth appeared to be stunted by the disease. PBLS occurs across the commercial production areas of Louisiana. Pecans are grown throughout Louisiana. These data indicate that PBLS is a significant problem for commercial pecan growers resulting in annual economic loss over the lifetime of infected trees.
PBLS has been identified in more than 20 grafted pecan varieties and in ungrafted trees. See the list in Figure 1. The disease has been identified in most of the varieties recommended for the southeastern United States and in some of the old standard varieties widely planted in this region. Of the popular southeastern varieties, Cape Fear is the most frequently diseased and has the most severe symptoms. The disease within a variety, however, can vary from mild to severe. The factors that determine the level of disease severity are not known. Graft Transmission
Once a tree is infected, PBLS is usually chronic, occurring annually. There is no effective treatment to eliminate the bacterium from infected plants, so methods to manage disease caused by X. fastidiosa must be focused on preventing infection. Pecan trees can be infected by passage of the pathogen through grafting with either infected scion wood or infected rootstock. Pecan trees have to be propagated by grafting to maintain their variety type. Scion wood is taken from a tree of the variety to be propagated and grafted onto a rootstock. The rootstock is a small tree grown from a nut and develops the root system of the tree. The scion produces the upper portion of the tree and determines the variety type.
In tests, grafting with scion wood taken from infected trees resulted in about 10 percent of the newly developing trees having PBLS, and more than 80 percent of healthy scions grafted onto infected rootstocks became diseased within one growing season. Graft transmission may be a significant factor for introduction of the pathogen into pecan orchards because all pecan varieties are propagated through grafting. Avoiding the use of infected scion wood by identifying infected trees through symptom recognition during late summer before collecting scion wood in the winter may be one way of reducing the infection of young trees. Insect Transmission
The primary method of pathogen spread in other plant hosts is by insect vectors. All of the insects capable of transmitting the bacterium to plants feed only in the water-conducting tissue of plants. The primary culprits are leafhoppers and spittlebugs. LSU AgCenter scientists recently determined that adult pecan spittlebugs can transmit the pathogen from infected pecan trees to noninfected trees. Commercial pecan growers can reduce infestations of spittlebugs through insecticide application. The scientists have also found evidence that some leafhopper insects can transmit the pathogen to pecan trees. Management techniques for leafhoppers on pecan, however, have not been developed.
In orchards, PBLS has been observed to sometimes remain static for several years and then exhibit rapid increases in the number of infected trees. This is likely related to the sporadic occurrence of insects that can transmit the pathogen. Observations from orchards suggest that much of the pathogen spread occurs from infected pecan trees within an orchard; however, because the pathogen has such a wide host range, it is probable that some insect vectors that feed on multiple hosts can acquire the bacterium from other species and transmit it to pecan. Pathogen Movement
LSU AgCenter researchers at the Pecan Research & Extension Station in Shreveport
are trying to determine how the pathogen moves into orchards, the pattern and rate of pathogen spread within orchards, the identity of insect vectors that feed on pecan, the frequency of occurrence of the disease in pecan nurseries, the relationship of the pecan pathogen to subspecies of the bacterium from other hosts and the susceptibility of new cultivars to PBLS. They also hope to evaluate a potential method of eliminating the pathogen from scion wood.(This article was published in the winter 2007 issue of Louisiana Agriculture.)