The North American Plant Disease Forecast Center at North Carolina State University completes its eighth year of service to tobacco producers, industry, and extension services throughout North America. The forecasts are issued on the Internet three times each week from March through August. Additional forecasts are provided during peak epidemic periods. There is also a toll-free telephone "hot-line" that provides timely forecasts to growers who don't own computers. Timely information on the geographic occurrence of blue mold and on the impending movement of inoculum (fungus spores) across the North American continent is important to tobacco producers in managing this destructive and fast moving plant disease epidemic. Users find the maps, disease outlooks, and other information helpful in determining the time of arrival of spores and assessing the risk for their production areas. As always, we are interested in your comments or suggestions on the forecast system!
Blue mold occurred in most of the tobacco production regions of North America during the 2003 growing season. We received reports again this year from the Dominican Republic. No official reports were received from Maryland, Wisconsin, southern Canada, or the flue-cured areas of North Carolina (!). Continental forecasting began on March 3 with disease sources in western Cuba, the Dominican Republic, and Mexico. 380 forecasts were made on 67 days. The last forecast was made on August 29, 2003. Disease activity was still present in a few regions at that time. Nearly all disease samples submitted to the various laboratories for pathogenic strain testing were insensitive (resistant) to the fungicide metalaxyl. This is consistent with our experience since 1991, when metalaxyl-insensitive strains of Peronospora tabacina became dominant during the yearly epidemics in North America.
In direct contrast to 2002, the eastern U.S. experienced a wet growing season in 2003. Large-scale weather patterns in the middle and upper levels of the atmosphere featured a ridge of high pressure in the western U.S. and a trough of low pressure in the eastern United States. This weather pattern, which is common in winter but much less common in the later spring and summer, persisted during much of June and July. The effects of this situation were most pronounced in the mid-Atlantic, southern Appalachian Mountain, and Ohio Valley regions. Crops were set in the field later than usual due to soggy ground conditions. Fields in many areas were frequently too wet to scout effectively for blue mold. In many cases, reports of blue mold outbreaks described situations in which the initial incursion of airborne spores occurred a number of weeks prior to disease discovery.
The first report of blue mold came on March 21 from Suwannee County in northern Florida. Georgia's first report arrived just 3 days later on March 24. Blue mold was discovered in the southern Appalachian Mountains in late May. Disease first occurred in the Ohio Valley in June and became widespread in July and August. The first report from the southern New England region arrived in mid-July. Blue mold was first reported from the mid-Atlantic region (in southeast Pennsylvania) on August 11.
The southern Appalachian Mountains, the Ohio Valley, and southern New England were hit hardest by blue mold in 2003. These regions had frequent rains through the months of June, July, and much of August. Disease activity levels were highly variable with strong activity reported in many areas. Some growers in the southern New England region were forced to destroy their fields. The majority of the tobacco-growing counties in Indiana, Ohio, Tennessee, and the mountains of North Carolina and Virginia reported blue mold. In Kentucky, 96 counties had confirmed cases of blue mold by the end of August (there are 122 counties in the entire state).
Southern Ontario, Canada did not report blue mold in 2003. Field scouting was quite diligent in this region. It was far more likely that there was no blue mold there and much less likely that disease was present but not discovered. A review of the 2003 trajectories showed very few opportunities for airborne spores to be transported toward the southern Ontario growing areas. Only one trajectory (from central Kentucky on July 25) moved over southern Ontario. Conditions for survivable transport were unfavorable and this was rated a Low Threat / Low Risk event. There was another event in early August in which trajectories from the Ohio Valley moved near to but south of the Ontario growing region. In that case, heavy rain was expected near the Ohio Valley source areas. This resulted in a large washout of airborne spores early in the event and limited the southern Canada growing areas to a Low Risk.
The reasons for the lack of transport events that were threatening to southern Canada were fairly straightforward. Due to the prevailing weather patterns for most of the growing season, cold fronts approached the eastern U.S. / southern Canada from the northwest. The orientation of the fronts was typically southwest to northeast. This positioning led to winds ahead of the fronts blowing mostly from southwest to northeast. A review of the trajectories out of the Ohio Valley confirmed this. A large majority of these trajectories moved to the ENE or northeast. In addition to frontal approach and positioning, the weather during most of the growing season was more dynamic than is usually the case. With few exceptions day-to-day weather features did not remain static for any extended periods. Winds directions at any given location changed fairly frequently. Thus, the chances for any airborne spores to move toward (and stay on course toward) the Canadian growing region were quite limited.
In 2003, blue mold was reported from most of the tobacco-growing regions. Disease arrived in the United States in late March. Once the epidemic spread to the southern Appalachian Mountains (late May) and the Ohio Valley (mid-June), it flourished in those regions due to persistently favorable weather conditions. A similar scenario was played out in southern New England after disease first occurred in mid-July. One surprising aspect of the 2003 epidemic was the nearly complete lack of disease reports from the flue-cured areas of North Carolina and Virginia.
This represents a short report describing the 2003 North American blue mold epidemic. A more complete report will be available at a later date following detailed analysis of all disease reports, meteorological records, and state impact data. The complete set of day-by-day forecasts can be reviewed on this Internet site: http://www.ces.ncsu.edu/depts/pp/bluemold/.
The North American Blue Mold Forecast Center thanks all the reporting Coordinators for each US state and adjoining countries for their participation during 2003. If you wish to become part of the Forecast System, or need further information, contact C. E. Main or the Forecast Center directly. For a complete description of the Plant Disease Forecast Center, refer to the APSNet feature article "Forecasting Long-range Transport of Downy Mildew Spores and Plant Disease Epidemics" at http://www.apsnet.org/online/feature/forecast/top.htm
First Reports of Blue Mold to the North American Blue Mold Forecast Center in 2003 (a)
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Suwannee County, FL |
March 20 |
Cuba |
? |
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Lowndes County, GA |
March 23 |
Cuba |
? |
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Greene County, TN |
May 27 |
Southern GA / northern FL? |
? |
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Florence & Colleton Counties, SC |
June 9 |
Southern GA / northern FL |
? |
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Lee County, VA |
June 23 |
Northeast TN |
? |
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Perry and Estill Counties, KY |
June 24 |
Southern Appalachian Mountains? |
? |
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Madison County, NC (burley) |
July 1 |
Southern Appalachian Mountains |
? |
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Hartford County, CT |
July 11 |
undetermined |
? |
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Adams and Scioto Counties, OH |
July 28 |
One or more sources in KY and TN |
? |
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Jefferson, Switzerland, Ripley, and Ohio Counties, IN |
Last 10 days of July |
Neighboring areas of Ohio Valley |
? |
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Lancaster County, PA |
August 11 |
Undetermined, most likely Ohio Valley and/or southern Appalachian Mountains |
? |
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Franklin and Hampden Counties, MA |
August 20 |
CT |
? |