CRIS Project No.: 6207-21000-005-00D
Title: Development of Genetic Resistance to Diseases and Insects in Pecan
Scientists: Dr. T. E. Thompson, Research Geneticist, GM-14, .20
Dr. L. J. Grauke, Research Horticulturist, GS-12, .10
OBJECTIVES:
Develop improved pecan cultivars with resistance to scab, vein spot, downy spot, and shuck diseases. Develop improved pecan cultivars with resistance to yellow aphids, black aphids, and leaf and stem phylloxera.
NEED FOR RESEARCH:
U.S. commercial pecan production today is in or near the geographic area of species evolution. Co-evolved diseases and insects are numerous and potentially devastating. Chemical control is expensive both economically and environmentally, even when it is effective. Without host plant resistance (HPR), there would be no pecan industry east of Central Texas since some diseases cannot be chemically controlled on susceptible cultivars. The current level of resistance must not only be maintained, but improved, as chemical control measures lose efficacy or become restricted.
The majority of U.S. pecan production is in the humid Southeast and is plagued by disease. The arid western U.S. production area is relatively free of disease problems. The dominant disease problem on pecan is scab (Cladosporium caryigenum). Scab attacks the foliage, developing stems, and nuts. It is capable of completely eliminating production, and rendering susceptible trees unthrifty with reduced growth, early defoliation, and debilitating refoliation.
In Georgia, the Cooperative Extension Service estimates that over a recent 3 year period, an average of 5.7% of the crop was lost each year to scab (12% loss in 1991). No other disease accounted for even 1% loss. In Georgia, the Cooperative Extension Service estimates that $8 million is spent annually in efforts to control scab (based on 6 sprays per year on 150,000 acres at $9 per acre). Sprays in Georgia directed at the control of all other pecan diseases combined totaled $4.5 million.
Some cultivars have maintained resistance to the scab fungus for a number of years. Among these cultivars are Elliott introduced in 1925, and Gloria Grande in 1923. Since scab resistance is present in available germplasm, it is possible to transfer this resistance to future improved cultivars through standard breeding techniques.
In some cases, host resistance to scab has been overcome by the scab fungus. A noteworthy example is Stuart which originated in 1874 and was planted on a wide scale in the Southeast in the early 1900's. At that time, it was considered resistant to scab. It was not until 1956 that Cole and Gossard reported that it was susceptible to scab in Mississippi. Now it is susceptible in most locations in the southeastern states. Even so, Stuart is still resistant in areas such as Oklahoma and retains a useful level of general resistance throughout the Southeast. It is still being recommended for planting in many southeastern states. The main limitations of Stuart are its lwo yields and poor nut quality.
It has been demonstrated that several races of the scab fungus exist, and it is probable that a very large number of different races exist. The existence of races complicates the breeding process. How effective scab breeding can be, largely depends upon how mutagenic the pathogen is. The perfect or sexual stage of scab has never been found, and this may be indicative of the slow genetic change in this pathogen. Indeed, we should expect a slow rate of genetic change over time in the pathogen, since the host has a generation time of perhaps 100 years in the wild.
Besides specific (vertical) resistance, there seems to be general (horizontal) resistance in the host. For instance, there are obvious levels of resistance among cultivars. This may be the most valuable form of resistance.
Genetic aphid resistance is also present in pecan since USDA has released what appears to be one of the most susceptible cultivars (Cheyenne) and the most resistant cultivar (Pawnee). Currently, this is the major insect problem on this crop worldwide, since it is almost impossible to control chemically and since it probably has developed resistance to specific insecticides. Pawnee possesses a useful level of aphid resistance by preventing rapid population buildup and is possibly damaged much less by those populations. This resistance mechanism needs to be elucidated and exploited as an economical control strategy in future cultivars.
Genetic resistance exists to other diseases and insects also. The frugal use of limited resources does not allow stringent selection for these resistance traits. More selection pressure here would detract greatly from the selection pressure for yield and nut quality. Most of these diseases and pests can be chemically controlled.
APPROACH AND RESEARCH PROCEDURE:
The USDA-ARS Pecan Breeding Program is based on the strategy of combining selected parents and screening the progeny. Parents are selected on the basis of production, quality, and other performance considerations, recognizing that different geographic regions of the pecan industry have different requirements. Pecan production in the Southeast depends upon scab resistance. Cultivars having a high level of resistance to scab will be selected as parents for crosses targeted for use in the Southeast.
To facilitate the recognition of disease resistant cultivars, pecan trees in the National Clonal Germplasm Repository (NCGR) are evaluated for disease expression. In addition to historic cultivars, collections of the NCGR at Brownwood and College Station include seedlings grown from seed collected throughout the range of the species, permitting maintenance of genetic diversity for characters such as disease resistance which may not be associated with other horticulturally valuable traits. These collections form a valuable resource in characterizing the genetic basis of disease resistance. Evaluation data are entered into the computer data-base of the repository, contributing to the ease of recovering information concerning prospective parents for breeding cycles.
When possible, trees used as pollen sources or as mother trees for crosses are those located at the USDA-ARS Pecan Breeding and Genetics facilities in Brownwood or College Station, Texas, since the identity of those trees has been carefully verified and is documented with voucher nut specimens. Controlled pollinations are made using techniques developed by Smith and Romberg . Seed of controlled crosses are harvested and measured as a normal part of the breeding program.
Field screening.
Progeny seedlings are grown in a non sprayed, high-density environment in the Basic Breeding Program (BBP) at College Station to accentuate disease development. During the period of rapid vegetative growth which accompanies juvenility, progeny seedlings are screened for foliage diseases. Scab is evaluated on leaves using methods outlined by Hunter and Roberts. Five rating classes are recognized with 1 being no scab and 5 being greater than 50% of the leaf surface infected. The reliability of vein spot evaluation on juvenile phase leaves may be obviated by the ubiquitous occurrence of the disease under conditions conducive to disease expression. Dr. R. R. Sanderlin (LSU Pecan Research-Extension Station, Shreveport, LA) (personal communication) has suggested that juvenile phase leaves tend to be more severely influenced by the disease that adult phase leaves. This may be related to leaf vestiture since trichome assortment and density change as trees achieve bearing status. Such questions will themselves become the subject of study when resources permit. The rating system for vein spot developed by Dr. Sanderlin is used. This system is based upon the maximum number of lesions on any petiole and rachis. Lesion size and coalescence of large lesions are possible sources of error in this system.
Severity of downy spot is determined on all clones under test in the BBP or NPACTS as differences become apparent. Other foliage diseases, such as fungal leaf scorch complex, liver spot, zonate leaf spot, leaf blotch, brown leaf spot, bunch disease, crown gall and tumor disease are monitored as genetic differences manifest themselves in the breeding nurseries. In the absence of a more sophisticated rating system, a 1-5 scale is used, with 1 being the most resistant or least affected by the disease.
As trees begin to bear, the incidence of disease on the fruit is evaluated. Nut scab is monitored using the methods of Hunter and Roberts. Data on the incidence of disease in the breeding nurseries is evaluated to determine parental effects. Recognition of parents that impart resistance to their progeny contributes to increased recovery of resistant clones in future breeding cycles. Crosses between proven parents should facilitate development of more robust resistance.
Individuals found in the BBP to be disease resistant and which possess other horticulturally valuable traits are further tested to insure a high level of disease resistance. Ten container trees are grafted with each advanced selection for evaluation under the high disease environment of Monticello, FL. Advanced selections found to have high levels of resistance under those conditions are considered elite clones and enter the National Pecan Advanced Clone Testing System (NPACTS) at Byron, GA.
In addition to scab, other nut diseases such as the shuck disease complex, powdery mildew, and pink mold are monitored, time and resources permitting.
Insects
Many of the observations of insect predation on pecan are anecdotal, rather than the result of systematic observations in carefully designed tests. Since reported differences in insect predation can usually be discounted due to the influence of other factors, little reliable information is available on insect resistance to direct breeding efforts. In the past, diseases have been much more economically important than insects. This is partially due to the availability of effective, relatively inexpensive insecticides, and the development of efficient methods of both timing and delivering pesticide applications. Pesticides are being removed from the market at an increasing rate, while new materials face increased restrictions to qualify for release. This increases the need for developing host plant resistance to insects. Accurate characterization of the occurrence of variable insect predation as a function of genotype is recorded. NPACTS orchards are the ideal sites for this research.
We have reported yellow aphid resistance in pecan in the USDA cultivar Pawnee. This resistance was verified by counting insects per leaf every two weeks throughout the growing season. This complete-season approach is needed since aphid populations may peak on different clones at different times in the summer. We continue to rate NPACTS clones and standard cultivars in an effort to more fully characterize resistance to these important insects and to more fully understand mechanisms of resistance.
The possibility of genetic resistance to black aphids is also investigated by counting the number of insects per leaf in replicated NPACTS tests.
Differences in resistance to leaf and stem phylloxerae sometime become apparent under heavy insect infestation. We rate susceptibility as genetic differences become apparent using the published system of recording the number and diameter of galls.
Rootstock development
Biotic activity in soils includes not only plant growth, but fungi and other microorganisms, such as nematodes. The wet, warm conditions of the southeastern U.S. are especially conducive to soil microorganisms. The role of fungi and nematodes should not be overlooked in assessing pecan rootstock performance. Nurserymen have noted reduced growth of seedlings if nursery sites were replanted to pecan. The extent of fungus and nematode involvement should be determined in such instances. Several species of nematode have been reported in association with pecan,with Meloidogyne species being most directly associated with pecan roots. Recovery of both galls and egg masses of Meloidogyne apparently differ between seedstock families. Researchers have reported that the ring nematode [Criconemoides quadricornis (Kirhanova) Raski] and Pythium irregulare Buism. were the most common soil microorganisms associated with pecan groves in Georgia. Pythium spp. reduced the root weight of container grown pecan seedlings up to 65%, compared to uninoculated seedlings. Poorly drained areas are more subject to Pythium infestation than well drained sites. There is an apparent interaction of the ring nematode and two fungi [Pythium irregulare and Fusarium solani (Mart.) App. & Wr.] on growth of pecan seedling roots. There is a marked reduction in root growth of pecan seedlings grown in soil infested with either fungus or with combinations of Criconemoides and fungi, but not where seedlings were grown in association with the nematode alone.
PROGRESS:
Parental cultivars in the National Clonal Germplasm Repository, and controlled cross progeny in the Breeding Program have been evaluated for scab disease, and the heretability of resistance determined.
Procedures for screening controlled cross pecan seedlings for scab resistance have been implemented, in cooperation with Dr. Bruce Wood and Simpson's Nursery, Monticello, FL. Controlled cross seedlings from the Breeding Program, as well as putatively scab resistant cultivars from diverse geographic regions have been propagated in an orchard under extreme scab pressure in Alabama. Disease resistant controlled cross selections from that orchard should have reliable scab resistance, and will enter NPACTS testing to determine production characteristics. Patterns of disease resistance in cultivars from that orchard may reflect genetic resistance related to geographic distribution, and could influence strategies of parental combinations.
Seasonal patterns of aphid buildup were monitored on cultivars in an NPACTS orchard in Burleson County, TX, revealing reduced aphid population levels on Pawnee, with elevated levels on Stuart.
PLANS:
All material in the Basis Breeding Program and in NPACTS tests in areas prone to disease will continue to be evaluated for resistance . This will insure the continued availability of disease resistant cultivars for the Southeastern U.S. Yellow aphid resistance will continue to be monitored in NPACTS orchards to determine levels of resistance.
Funding has been sought to develop RAPD markers in controlled cross families of pecan. The immediate objectives of the proposal are to:1) tag genes responsible for scab resistance in pecan with DNA markers; and 2) test the use of tagged DNA markers as aids to selection in the breeding program. Our ability to pursue that strategy will depend on competitive grant funding, due to budget cuts to this program.
The mechanism of aphid population reduction in Pawnee should be elucidated in carefully controlled tests. Budgetary and personnel constraints prevent the logical continuation of that work.
The influence of nematodes and fungi on pecan seedling rootstocks should be evaluated in cooperation with Southeastern nurserymen. Limitations of time and money prevent initiation of that work. Both of the above projects will be suggested research areas for prospective graduate students, and resources to support the work will be pursued by grant proposals as time and resources permit.
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COOPERATORS:
Dr. Bill Goff, Dr. Marvin Harris, Dr. Randy Sanderlin, Dr. Morris Smith, Dr. Bruce Wood
Publications (Last 3 years)
1. Goff, W.D., Campbell, L., Thompson, T.E., Bannon, J.S. and Latham, A.J. 1993. Scab occurrence on pecan clones in Alabama in a year of high disease incidence. Fruit Var. J. 47(1):47-51.
2. Nesbitt, M.L., Storey, J.B., Lyda, S.D. and Grauke, L.J. 1993. Evaluation of fungal infection of pecan roots with high pressure liquid chromatography. HortScience 28(4):262. (Abstract)
3. Thompson, T.E. and Grauke, L.J. 1994. Genetic resistance to scab disease in pecan. HortScience 29(9):1078-1080.
4. Thompson, T.E., Goff, W.D., Nesbitt, M., Grauke, L.J., Wood, B.W., Smith, M.W. and Smith, M.T. Breeding scab resistant cultivars. Proc. Southeastern Pecan Growers Assn. 88:162-167. 1995.
5. Thompson, T.E. and Grauke, L.J. 1995. Breeding and genetics of scab resistance. Proc. Tex. Pecan Grow. Assoc. 67:25-28.
6. Grauke, L.J. and Thompson, T.E. Pecans and Hickories. IN: J. Janick and J.N. Moore (eds.) Fruit Breeding: Nuts. Purdue U. Press, W. Lafayette, Ind. (In press)
LJ Grauke , Research Horticulturist & CuratorUSDA-ARS Pecan Genetics
Route 2 Box 133
Somerville, TX 77879
tele: 409-272-1402
fax: 409-272-1401
e-mail:ljg@tamu.edu
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