Host and symptoms
'' Venturia effusa'' has a narrow host range, and its most economically significant host is the pecan, ''Carya illinoinensis''. The pathogen can also infect other trees in the genera ''Carya'' and ''Disease cycle
''Venturia effusa'' over-winters in plant debris, such as shucks, leaf petioles and stems, as well as in lesions on the tree from the previous season. It survives as stromata, a tight mat of fungal material that lives within tissue. In the spring time, hyphae in the stromata germinate to produce solitary or loosely fasciculate conidiophores which produceEnvironment
A conducive environment for the propagation of pecan scab requires wet weather. For the spores to inoculate the host, there must be a period of wet weather in order for the infection to occur. The amount of rainfall is not as important as the frequency of rainfall and length of time that the leaves and fruit stay wet. For example, the optimal time for a rain would be right before dusk because the plant will stay wet overnight, thus increasing the period of time the pathogen has to infect the plant. The temperature is not as critical of a factor in the environment.Vann, S. (n.d.). Home pecan diseases and control. Informally published manuscript, Division of Agriculture, University of Arkansas, Little Rock, AR, Retrieved from http://www.uaex.edu/Other_Areas/publications/PDF/FSA-7540.pdf The pathogen thrives in adequate temperatures of spring and summer ranging from 20–30° Celsius (68–86 °F). When there is a pecan orchard, cultural practices that increase the humidity levels also provide a good environment for the pathogen. These practices include close tree spacing, low limbs, and dense ground vegetation.Management
Although resistant varieties of pecan trees are available, such as the Elliot Pecan tree, historically the pathogen has overcome the resistance due to its ability to quickly change over time. Because resistance has been ineffective, the main control of the pathogen is fungicide application for commercial farming. Some commercial fungicides used are Pristine, Sovran, Quash, and Headline SC. For the fungicides to be effective, they must be sprayed before inoculation/infection period. The exact time for the first spray depends on the chemical used, but an example time periods is when the buds are bursting and the first leaves begin to show. The fungicide should be sprayed sufficiently in order to completely cover the tree. The rates per acre of application range from 2–5 fluid ounces on developing trees to 8–12 fluid ounces on mature trees. They are protectant fungicides, meaning that they work best to prevent disease from happening when sprayed before any disease is detected. The idea of "getting ahead" of the pathogen is key to preventing a large loss of production.Smith, D. (2010). Pecan scab update. Informally published manuscript, Entomology and Plant Pathology, Oklahoma State University, Stillwater, OK, Retrieved from http://entoplp.okstate.edu/pddl/pddl/2010/PA9-27.pdf After this initial spray, the plant tissue is still susceptible to infection making it necessary for multiple sprays to occur during a growing season. The number of applications during a season depends on the weather, amount and virulence of the pathogen, and susceptibility of the cultivars. Generally, the cultivars are sprayed about every two to four weeks of its growing season in spring and summer. This ends up being about 5–7 sprays per growing season. The fungicide needs to be sprayed in a concentration high enough to prevent infection and to completely cover the trees. The fungicides have not been shown to have any adverse effects outside of the warnings of all chemical control impacts on environmental and human health.European Union, Invasive Species Compendium. (2013). ''Fusicladium effusum''. Retrieved from CABI website: http://www.cabi.org/isc/?compid=5&dsid=13719&loadmodule=datasheet&page=481&site=144 Although fungicide application is effective in the commercial setting, the cost of the chemicals and equipment is usually not practical for noncommercial farming such as orchards and small farms. In the noncommercial case, good sanitation practices may help prevent the disease. Removal and destruction of all fallen leaves, shucks, and nuts each winter or early spring helps reduce tissue that is often involved in the primary infection. Also, selective pruning of infected tissue during the dormant season may help reduce the level of scab disease. A recent study has been done to test if phosphite, a chemical with the potential to induce systemically acquired resistance (SAR), could be used as a potential control for pecan scab. The results showed that in the early stages of the growing season, phosphite was comparable to current fungicide control, however may not be as effective in providing elongated protection in the later growing season.Importance
Pecan scab disease is the most economically damaging disease of pecans in the Southeastern United States, which is one of the major pecan producing regions in the world.Andersen, P. C. (2011). Pecan cultivars in north Florida. Informally published manuscript, Institute of Food and Agricultural Sciences, University of Florida, Gainesville, FL, Retrieved from http://edis.ifas.ufl.edu/pdffiles/HS/HS10600.pdf It infects the leaves, shucks, and nuts thus reducing crop yield. In addition, the cost of fungicide application reduces the profitability of pecan production. Because of the cost of fungicides, pecan scab also prevents small orchards and yard trees from producing high quality nuts. The average cost of pecans ranges from $2.86–$3.50 per pound.Rafanan, M. U.S Department of Agriculture, (2013).Pecan report (XXXI-7). Retrieved from Agricultural Marketing Service website: http://www.ams.usda.gov/mnreports/fvwtvpcn.pdf Prices may fluctuate slightly with demand. An average tree will yield 40–50 pounds of nuts, thus equating to roughly $135 per tree. At 27 trees/acre, the total profit would be $3,645 per acre.Call, R., Gibson, R., & Kilby, M. (2006). Pecan production guidelines for small orchards and home yards. Informally published manuscript, College of Agriculture and Life Sciences, University of Arizona, Tucson, AZ, Retrieved from http://ag.arizona.edu/pubs/garden/az1400.pdfNesbitt, M., & Wells, L. (n.d.). Estimation of pecan tree value. Unpublished manuscript, College of Agriculture and Environmental Sciences, University of Georgia, Athens, GA, Retrieved from http://www.caes.uga.edu/commodities/fruits/pecan/growers/documents/pecantreevalue.pdf Pecan scab is a devastating disease that can significantly impact the amount of quality pecans produced in a year. For example, during the summer of 2013, the estimated potential for the year was around 90 million pounds; however, the weather conditions were conducive for pecan scab growth and the actual total went down to 65 to 70 million.Thompson, C. (2013). Pecan crop hurt by summer rainfall. Unpublished manuscript, College of Agriculture and Environmental Sciences, University of Georgia, Athens, GA, Retrieved from http://georgiafaces.caes.uga.edu/index.cfm?public=viewStory&pk_id=4934 Pecans are valuable crops that are economically valuable to the farmer and also the farming industry. Devastating diseases like pecan scab can result in major financial loss that can hurt growers and also economic development.References
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