THE 1996 FORECAST OF SPORE TRANSPORT AND SPREAD OF TOBACCO
BLUE MOLD. C. E. Main, J. M. Davis, T. A. Melton, T.
Keever, and P. B. Shoemaker, Dept. Plant Pathology, North Carolina
State University, Raleigh, NC 27695.
Blue mold (mildiou) of tobacco (Pernospora tabacina) is a foliar disease disseminated long-distance by winds in the planetary boundary layer of the atmosphere. A NCSU Blue Mold Forecast system went operational from March 5 to August 8, 1996 providing county agents, growers, industry and the media with 344 forecast outlooks on 63 days via the Internet site http://www.ces.ncsu.edu/depts/pp/bluemold/. The North American forecasts provided timely information on the intra-continental movement of inoculum (spores) south of the 30th latitude prior to May 6 and within the eastern US and Canada after the first occurrence May 9.
Blue mold was present on commercial tobacco in the Caribbean, Mexico and Latin America, and on wild Nicotiana repanda in SW US during the winter and early spring months. The first confirmed report of blue mold on U.S. commercial tobacco occurred on May 6, 1996 in Columbia and Union counties in Northern Florida. The most probable source of inoculum was spores released from infected fields in Western Cuba on or about April 29. During early to mid-May the weather was dry in the Southeast and the risk of sporulation and further spread was limited. Blue mold was reported in Kentucky on May 21 (confirmed May 24), but indications were that the disease could have been present in transplant greenhouses and outdoor float beds for several weeks. After May 24 a stationary weather front became established in the mid Atlantic region beginning a month-long period of favorable weather for blue mold.
With the exception of late June and early July, favorable weather conditions continued to persist for disease development in the eastern U.S., especially in the higher altitude burley production areas. By June 19, blue mold was present in 31 counties in central Kentucky. Blue mold was reported in southeast Georgia on June 5, south central North Carolina on June 10 and eastern North Carolina on June 26. Northeast Tennessee reported the presence of blue mold July 3, western North Carolina on July 8 and northeastern Georgia on July 10. Blue mold was discovered as far north as Delhi, Ontario Canada by early July (probably from inoculum originating in Central Kentucky either June 5 or June 13).
Blue mold distribution and losses were generally light during 1996 in much of the U.S. and Canada. However, during late July and early August very favorable weather occurred in burley areas and damaging epidemics resulted. Because of blue mold, the supply and quality of 1996 burley leaf was seriously threatened. An estimated $200 million loss was reported in the Kentucky burley crop due to blue mold. More than 100 counties reported blue mold in August. Burley tobacco in Ohio and Indiana experienced blue mold in mid-June. Southeast Virginia, Pennsylvania (August 6), Maryland (July 3 1) and Missouri also reported outbreaks. On July 26, a serious epidemic was reported in Vernon and Dane counties, Wisconsin, however the disease likely occurred in early July but went undetected.
Of 166 early season Forecast Outlooks prepared March through May, 85 percent were of a low risk category and 15 percent moderate risk. During June, July and August, when favorable weather occurred, 30 percent of the forecasts were high risk, 40 percent moderate and 30% low risk.
The 1996 forecast system successfully predicted the first outbreak May 6 in Florida from inoculum arriving from Cuba. Other success examples are the first outbreak in south central North Carolina on June 10 by spores arriving from northern Florida, and the first occurrence in the Dehli region, Ontario, Canada as predicted from spores released in central Kentucky on or about June 5. Late in the season (August 8) spores from infected Wisconsin tobacco fields were being transported southward into Indiana and Kentucky. The forecasting system was discontinued after August 8.
A new mesoscale atmospheric model (Mesoscale Atmospheric Simulation System by NffiSO, Inc., Troy, NY) will be used in 1997 to supplement the line trajectory forecasts produced by the HY-SPLIT model. The MASS model accommodates multiple sources, terrain features, and carries remaining atmospheric spores over from one day to the next. The output provides a 3-dimensional map showing wind arrows and atmospheric spore cloud concentration(s) from combined sources. The input requirements are source location, source strength, physical data on spore size/weight, settling velocity, etc. Sources are considered as 45 km x 45km grids (2025 sq. km) of land surface where sporulating fields are located. Source strength is expressed as kg spores released per grid; atmospheric concentrations are expressed in kg spores/2 . The MASS model shows promise in simulating spore cloud movement and dispersion.
Blue mold is a unique problem. Inoculum can be transported great distance in a few hours. Successful management of mildew diseases like tobacco blue mold will require a coordinated, intra-continental approach since the pathogen is highly mobile and is not known to overwinter north of the 30' latitude. Late in the season, inoculum can also be transported southward from the US toward the Caribbean and Mexico where their winter tobacco crop can be infected. The pathogen has become established (endemic) on several wild species of Nicotiana throughout Mexico and Latin America representing a continuing source of spores to the United States.
The NCSU Forecasting System could have application in forecasting a wide number of other foliar diseases such as potato late blight, movement of Thrips in the southeast carrying the Tomato Spotted Wilt Virus, transport of Telicia indica, (movement of the Karnal bunt pathogen of wheat in the US). The system could serve as a model for many plant and animal diseases involving long-distance transport of inoculum, aerobiology, and mesocale disease management.
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