The North America Blue Mold Forecast Center in Raleigh, NC provided the tobacco industry and growers with timely documentation on the occurrences (new outbreaks) and potential for further spread (transport of fungal spores) of the pathogen (Peronospora tabacina) during the 1999 growing season. The Forecast Center is located within the Department of Plant Pathology at North Carolina State University. A total of 527 separate forecasts were prepared on 87 days from March through September. Forecasts were posted on the World Wide Web three times each week, and more often when necessary. The forecasts attempt to document new occurrences, gauge the potential for sporulation at the source, evaluate the chances of survival and deposition during long-distance transport of spores, and provide a risk assessment for each separate forecast. New reports and disease status are monitored daily via a reporting network of coordinators in U.S. tobacco-producing states, Mexico, and Canada. Coordinators are in daily contact with the Forecast Center and all other coordinators.
Blue mold occurred in almost all the tobacco production areas of eastern North America in 1999. First report of blue mold in commercial tobacco was in three locations in southern Georgia on March 12. Due to drought conditions in the major production regions through the growing season, there was less disease activity in 1999 when compared with recent years. Fewer areas reported blue mold, and in those areas that had disease it was characterized by light to moderate levels of activity. With few exceptions, blue mold occurrences were dominated by metalaxyl-resistant strains of P. tabacina.
A table showing the geographic sequence of first occurrences by state is included at the end of this summary. A short executive summary (table included) is available elsewhere on the homepage. Growers without Internet access to the Forecast Center Homepage could use toll-free telephone forecasts via 1-800-662-7301 (NC only) and/or 1-888-835-BLUE (2538) for all other states and foreign countries. The URL for the North American Plant Disease Forecast Center is http://www.ces.ncsu.edu/depts/pp/bluemold/.
Pervasive and continuing drought in the Southeast, Mid-Atlantic, and Ohio Valley regions of the United States was the prime governing factor in moderating this year's epidemic. Extended periods of favorable weather in any one region were very unusual. Disease activity tended to be highly variable and was often hindered or arrested by lack of moisture and/or hot summertime temperatures. Blue mold infections were reported from most growing regions and became scattered across several of them. However, some areas that normally report the disease experienced no blue mold this year. In addition, light-to-moderate activity levels were typical; severe cases were few in number. Most outbreaks were generally dependent on locally favorable conditions, such as shade from trees or irrigation, rather than large-scale, consistently favorable weather.
The first reports of blue mold in the United States came from three sites in southern Georgia on March 12 and northern Florida on March 19. All reports were from plant beds or greenhouses. The source of these outbreaks was spores from western Cuba, on or about February 16-17. This would have been a Low Threat / Low Risk forecast. If infection occured under this scenario, it would likely have resulted in very few lesions, which could have gone undetected until the pathogen had cycled one or two times. Virtually all the other trajectories originating from Cuba during late February and early March were directed well away from the U.S. growing areas. Also, conditions were frequently unfavorable for transport. The mid-February event provides the most reasonable scenario.
(The Threat of blue mold is associated with those disease-development factors at or near the source. Included among these are geographical extent of blue mold, recent disease activity, and amount of cloud cover near the source region. Risk is associated with disease-development factors occurring near the source and along the forecast pathway. More emphasis is placed on weather conditions than disease status. Cloud cover along the forecast track, the potential for washout, and the timing and location of rainfall along the pathway are important factors. For any specific transport episode (source + trajectory + conditions), Threat will be fixed, but Risk may vary along the forecast trajectory. Elevated Threat levels can lead to elevated Risk levels. However, there is not a strict one-to-one relationship. )
Disease activity increased in greenhouses and plant beds across southern Georgia, becoming common by early April, and then declined during the remainder of the month. Field activity was low during most of March and April due to dry conditions coupled with cool overnight temperatures. No other report of new outbreaks of blue mold was received during March or April.
Blue mold became established in irrigated fields of southern Georgia in mid-to-late April. Two short periods of favorable weather in late April and early May were followed by unfavorable weather for several weeks. Blue mold was slowly increasing in northern Florida. Georgia field activity increased as well, becoming moderately active and scattered over the entire Georgia growing belt by May 21.
Late May through mid-June was dry with occasional one-day or two-day periods of favorable weather for some areas. First reports of blue mold for areas north of Georgia/Florida came during this period. The initial report from flue-cured areas of North Carolina (Jones County) occurred on May 26. First report from South Carolina (Williamsburg County) was on May 28. Both of these outbreaks occurred in the field. Blue mold was reported in southeast Kentucky on May 31. The discovery was made in Clay County, in outdoor float beds and five greenhouses. Additional reports from south-central North Carolina and Shelbyville, KY were received on June 2.
The initial outbreaks in eastern North Carolina, eastern South Carolina, and southeast Kentucky most likely originated with spore trajectories on May 5 and May 6 from southern Georgia / northern Florida. Conditions across the Southeast were favorable for two days ahead of a slow-moving cold front, and the date corresponds reasonably well with the estimated time of initial infection. Trajectories from the southern Georgia / northern Florida region were moving north and northeast each day during this period.
Few other possibilities exist for these outbreaks. Unfavorable weather dominated the Southeast for three weeks after May 5 and May 6. The only other situation that stands out is the transport episodes of April 26 and April 27. The weather situation and the trajectories were similar to those of May 5 and May 6. However, infection during this event appears less likely when disease source activity levels are considered. According to our blue mold reporting network, infected greenhouses and plant beds in southern Georgia no longer presented a threat by the last week of April. Overall field activity in the source region was low prior to late April, though disease was present and moderately active in irrigated fields in at least four counties. By Monday, May 3 (just before the favorable weather of May 5-6), Paul Bertrand reported that disease activity in the fields of southern Georgia was increasing. While it may be possible that some of these infections occurred in late April, the more plausible explanation involves transport after the increased southern-Georgia field activity resulting from favorable late April weather. Greater spore production would be expected during the next week. These spores were then transported north once the regional weather conditions became favorable on May 5-6.
The closest resemblance to a period of widespread favorable weather occurred from mid-June to July 2nd, interspersed with an unfavorable period from June 18 through June 23. Due to slow-moving cold fronts, conditions remained favorable for several days at a time over much of the eastern United States. Disease activity increased in Kentucky and eastern North Carolina. Blue mold was reported from the central North Carolina mountains (burley) on June 16. Blue mold was reported in central Tennessee on June 22.
First reports of blue mold in the North Carolina burley region were from fields in Madison County. Estimated time of initial spore arrival and infection was mid-May. Both old and new leaf lesions were present. Although there was an outside chance of mid-May infection from eastern Kentucky (from then unreported hotspots in tobacco greenhouses), the most likely scenario is May 5-6 as described above. Conditions were favorable and trajectory centerlines moved very close to the Madison County area.
The Tennessee report was from a field location in Montgomery County. No old lesions were found. Estimated date of infection was June 14; rain occurred that day and no rain had fallen on this field for the previous two weeks. With minimal opportunities for infection and the lack of old lesions, confidence in this infection date is high. Determining a source for the spores responsible is difficult. No trajectories from known sources approached this site. No definite conclusions can be drawn, but this outbreak was probably the result of a short-range transport and deposition event (or events) from an undiscovered or unreported source in Kentucky.
All other U.S. areas affected in 1999 reported outbreaks in July, following the late June period of favorable weather: Connecticut on July 2, Pennsylvania on July 9, Maryland on July 16, southwest Virginia on July 19, Massachusetts and Indiana on July 23, and Ohio on July 30. After July 2 and through the month, typical summertime weather patterns became firmly established. Hot, dry conditions were dominant with sporadic short periods of disease-favorable weather in some locations. Conditions were favorable for transport and infection more frequently in the southern Appalachian Mountains than other areas resulting in significant increases in disease activity. Blue mold was present at low levels throughout much of Kentucky, with more intense disease activity in certain locations. Disease activity diminished in many regions as the generally unfavorable weather continued.
The East Coast July reports were probably the result of long-range transport, while the other outbreaks were most likely due to short-range disease development from locally known sources. Spores released from eastern Kentucky on June 2 were the likely culprits for the outbreak in southeast Pennsylvania. Conditions were very favorable. Heavy washout of airborne spores near the source region was anticipated. However, it appears that some spores remained airborne and were rained out over Pennsylvania the following day. This was unlikely, but no other plausible explanation presents itself.
The last important long-range transport episode occurred during the weekend of June 25-27. Spores released from eastern North Carolina traveled north each day. Conditions in the Mid-Atlantic and Northeast were favorable ahead of a slow-moving cold front. On one of these days, probably June 26 or 27, transport and deposition events resulted in infections in St. Mary's County, Maryland and Hartford County, Connecticut. This time frame corresponds well with the estimated dates of first infection given by the observers at these sites. Trajectories moved near the Maryland location. The approach to the Connecticut growing region was less obvious yet well within the realm of possibility. Few other scenarios exist to explain these outbreaks.
Outbreaks in southwest Virginia, Ohio, Indiana, and Massachusetts were most likely the result of short-range spread during a period of favorable conditions. Old and new leaf lesions were discovered on field plants in Lee County and Washington County, Virginia. Estimated time of initial infection was late June to early July. Disease development was likely due to transport from neighboring western North Carolina sometime during the last two weeks of June. Blue mold in Jefferson County, Indiana arose from short-range transport around July 10 from nearby parts of Kentucky. Likewise, the outbreak in Brown County, Ohio was due to infected fields in northern Kentucky. Infection occurred during the last week of June. Transport and deposition of spores from northern Connecticut resulted in the initial infection in Hampden County, Massachusetts. Estimated time of initial infection was July 9, which corresponds well with favorable conditions and likely localized spread on July 10.
Conditions in August and September were largely a continuation of those established during July. Blue mold remained active in southern New England, the southern Appalachian Mountains, and parts of the Ohio Valley. These areas received occasional showers. The disease slowed or stopped in the Mid-Atlantic states. Forecasting was discontinued on September 23.
There were several significant transport episodes in 1999. The first occurred during mid-February, when viable spores from western Cuba were deposited in the southern Georgia / northern Florida region. The second significant transport event occurred on May 5 and May 6 during a short period of favorable weather. Spores from the southern Georgia and northern Florida region were likely responsible for first outbreaks in eastern NC, eastern SC, western NC, and eastern KY. The Pennsylvania outbreak probably arose from the episode of June 2, from Kentucky. Blue mold cases in Connecticut and Maryland were likely due to a transport episode (or episodes?) from eastern North Carolina the weekend of June 25-27. Other outbreaks in Tennessee, Virginia, Massachusetts, Indiana, and Ohio were the result of local and/or short-range transport events.
The last significant transport events are ones that did NOT result in blue mold. These events could have brought blue mold to southern Ontario, Canada. Blue mold was never reported in this region during 1999 despite frequent scouting of the fields. This was most likely a case of good fortune. Only two forecasts showed trajectories threatening to this region, one on May 31, the other one on August 12. Southern Ontario was at Moderate Risk both times. In each case, conditions at the threatening source region had been unfavorable for days beforehand. Thus, it was likely that the density of airborne inoculum was less in those instances.
Disease activity in 1999 was limited in scope and intensity when compared to the 1996 through 1998 epidemics. This was both directly and indirectly related to the extensive drought in the eastern United States. The predominantly sunny weather made spore survival during transport unlikely in many cases, thereby limiting the potential for long-range threat to distant production areas. Dry, hot conditions stressed the tobacco plants per se and reduced infection and sporulation. The lack of extended periods of favorable weather within any one region prevented the buildup of huge spore loads experienced in other years and decreased explosive, damaging disease development. The result was a fairly widespread but mostly less intense blue mold season in 1999.
First Reports of Blue Mold to the North American Blue Mold Forecast Center in 1999 (a)
|
County/State |
Date Reported |
Probable source (b) |
Impact (c) |
|---|---|---|---|
|
Grady, Echols, and Colquitt Counties, GA |
March 12 |
Cuba |
? |
|
Columbia County, FL |
March 19 |
Cuba |
? |
|
Jones County, NC (flue-cured |
May 25 |
S GA / N FL |
? |
|
Williamsburg County, SC |
May 26 |
S GA / N FL |
? |
|
Clay County, KY |
May 31 |
S GA / N FL |
? |
|
Madison County, NC (burley) |
June 16 |
S GA / N FL |
? |
|
Montgomery County, TN |
June 22 |
Kentucky |
? |
|
Hartford County, CT |
July 2 |
Eastern NC |
? |
|
Lancaster County, PA |
July 9 |
Kentucky |
? |
|
St. Mary's County, MD |
July 16 |
Eastern NC |
? |
|
Lee and Washington Counties, VA |
July 16 |
Western NC |
? |
|
Hampden County, MA |
July 23 |
Connecticut |
? |
|
Jefferson County, IN |
July 23 |
Kentucky |
? |
|
Brown County, OH |
July 29 |
Kentucky |
? |
We will continue to update this summary as additional reports and impact information come in to the Forecast Center. Check our Web site periodically for the updates and new technologies as they develop.
The North American Blue Mold Forecast Center in Raleigh was pleased to provide this important service to the tobacco growers of the eastern United States and Canada. We would especially like to thank the state coordinators, extension agents, and other persons who reported blue mold to the Center. You are the eyes and ears of the reporting system! Thanks goes to the Center staff who help put out the forecasts on schedule. Of special note is Thomas Keever, our Forecaster and Meteorologist for the past five years. Without his expertise, dedication, and hard work the system would not operate. In this connection, we also thank Dr. Roland Draxler, Dr. Barbara Stunder, and staff at Air Resources Laboratory in Silver Spring, Maryland whose production of trajectory maps and expertise we depend upon.
Comments on the forecasts, the Forecast Center, and the Homepage (or its contents) are welcome. Address your comments to C. E. Main, Center Leader, at ce_main@ncsu.edu or Department of Plant Pathology, Box 7616, N.C. State University, Raleigh, NC 27695.
Web page last updated by Thomas Keever on 16 November 1999.