Summary of the 2007 Blue Mold Epidemic in North America
Thomas Keever
North American Plant Disease Forecast Center
NCSU, Department of Plant Pathology
The 12th year of forecasting for Tobacco Blue Mold epidemics in North America has concluded. Forecasts were produced three times per week in 2007 from March through mid-August. The forecasting website provided visitors with up-to-date information about the status of the epidemic and forecasts of potential epidemic spread to other regions from the known sources. We hope the forecasts assisted the growing community via improved crop management and assessment of the risk of developing tobacco blue mold.
The 2007 epidemic was mild and began in a historically unusual manner. For the fourth consecutive year, the first reports of blue mold in the U.S. growing regions did not come from northern FL or southern GA. For the 8 years prior to 2004, initial discoveries came from northern FL or southern GA in March, April, or May. In 2007, the disease season began late with the first report of blue mold arriving at the Forecast Center in early June from central KY.
The 2007 epidemic was heavily influenced by prevailing weather patterns. Most notable was the drought that persisted through much of the spring and summer, which affected regions from the middle and lower Mississippi River Valley eastward through the Ohio Valley, mid-Atlantic, and the Southeast. The majority of confirmed reports originated from the southern Appalachian Mountains and nearby counties of north-central NC. No blue mold was reported from the growing areas of northern FL or southern GA, nor from the eastern Carolinas or southern Ontario, Canada. Through August 31, 24 counties in 8 states had confirmed outbreaks of tobacco blue mold. Forecasts were produced on 70 days from early March through the middle of August.
The first report of blue mold came from Fayette County, Kentucky on June 1, 2007. Disease activity of recent origin was observed on a small portion of seedlings in a greenhouse. Plants from this greenhouse were set in the field in other counties before symptoms were observed. The Forecast Center applauds the aggressive action of the Kentucky growing community after the outbreak was discovered. All plants in the greenhouse were destroyed; field plants in 2 counties were treated in similar fashion. The several remaining field locations underwent frequent examination. These actions likely helped contain future spread of the epidemic in that region.
The first report from Tennessee arrived on June 8, 2007. Active infections were observed in a greenhouse in Greene County in the mountains of northeast TN. Observations indicated that disease may have been present for 2 to 3 weeks prior to the discovery. As in Kentucky, aggressive action here likely hindered future epidemic spread. Plants in the greenhouse were buried the day before the initial report.
Available information indicates that the TN outbreak was the first occurrence of disease in the U.S. growing areas in 2007. Origins of this outbreak remain uncertain despite thorough study. It appears quite unlikely that this outbreak was airborne. Post-discovery analysis revealed no evidence to support an airborne introduction. Potential points of origin were from areas where no tobacco was present, or from areas where no disease had been observed even though diligent scouting had occurred. The possibility remains, though highly unlikely, that an undiscovered or unreported source played a role, or that some other unknown mechanism was at work. Consideration of the disease reports indicates that the outbreak in KY arose from the introduction of infected transplants.
On June 14/15, blue mold was observed and confirmed in TN and KY in locations very close to the initial finds in those states. In KY, only a few plants in a field in Fayette County had symptoms and sporulation was weak. In TN, blue mold was found in a greenhouse and several fields in Washington County (adjacent to Greene County). Available information indicated that the field infections arose from plants from infected greenhouses.
The Forecast Center received no other reports of blue mold during the middle and later parts of June and early July. Weather conditions in the southern Appalachian Mountains and central KY during June and July became less unfavorable for disease development than they had been in previous months. Conditions for long-range transport and deposition from these areas were generally poor, though there were a few short periods during which conditions were more favorable.
The first report of blue mold in NC was from Yancey County on July 13. One lesion was observed in a field of burley tobacco in this county of the central NC mountains. Other fields nearby showed no symptoms. This outbreak was due to short-range transport and deposition from the sources in northeast TN.
On July 18 and 20, two separate reports showed the first occurrence of blue mold in PA, from Lancaster County. Both of the field outbreaks appeared to be of fairly recent origin. Unconfirmed reports from the region indicated that blue mold may have been present in the region since early July. The weather in southeast PA had been generally favorable for disease development over the past 2-3 weeks prior to the first report.
Introduction of inoculum to southeast PA appears most likely to have occurred during the transport events of July 4. Transport from either the southern Appalachian Mountains, central KY, or both was possible for the July 4 events. Trajectories from both known source regions moved directly over the southeast PA growing areas. Conditions were mostly unfavorable to mixed for survivable transport and deposition. Deposition of live spores may also have occurred from the July 10 events from these areas, though this is far less likely. Observations in southeast PA (75% disease incidence in one field, 50% in another) would seem to support an earlier initial infection date, as well. Figures 1 and 2 simulate the deposition from these events.
Figure 1: Simulated deposition from the July 4 transport event from northeast TN.
Figure 2: Simulated deposition from the July 4 transport event from central KY.
On July 23 and July 25, respectively, blue mold was reported from Person and Rockingham counties in north-central NC. Actual observations were made around July 19-20. Based on the information in the disease reports, these outbreaks were due to transport and deposition from the southern Appalachian Mountains on Friday, June 29. Tobacco plants in western and north-central NC were placed at Moderate to Strongly Moderate Risk for this event. An outbreak of blue mold in Surry County in western NC, reported later in July, may have been due to this same June 29 event or others later on July 11 and July 13. Plants in western NC counties were placed at High Risk for the July 11 transport events, Strongly Moderate for the July 13 events. Available information does not permit selecting one event over another for the Surry County outbreak. Figure 3 illustrates the deposition for the June 29 transport event.
Figure 3: Simulated deposition for the June 29 transport event from northeast TN.
Blue mold was discovered in Macon County, TN around July 27, 2007. Disease was widespread at light to moderate levels in a 300-acre field in this north-central TN county near the KY state line. Several days later the Forecast Center received reports of blue mold in 3 fields in Allen and Monroe counties in south-central KY. These counties are adjacent to Macon County, TN. Post-outbreak analysis and information in the disease reports support the July 6-7 transport events from central KY as the one that gave rise to these outbreaks. The weather was generally unfavorable for this event. However, all other possibilities are even less plausible. Figure 4 shows the modeled deposition for this transport event.
Figure 4: Simulated deposition from the July 6 transport event from central KY.
Washington County, in the extreme southwest portion of VA, was the site of the first report from that state. As of August 1, blue mold was active in many fields in that county. Disease development here was due to short-range spread from the other sources in the southern Appalachian Mountains.
Blue mold was confirmed in Hampden County, MA on July 31, 2007 in 2 fields of shade tobacco. The first report from CT, in the same growing region, came from Hartford County one week later. Recent weather in the CT River Valley growing region had been favorable. Observations indicated that blue mold had been present in this region for about 2 weeks. Post-outbreak analysis indicates that the first outbreak in MA was most likely due to the transport events of July 18-19. Airborne spores from southeast PA moved through southern New England during both events, and conditions were favorable for survivable transport and deposition in each case. Airborne spores from central KY may have played a role, though this is considered much less likely. The succeeding outbreak in CT most likely arose from short-range epidemic spread from the nearby MA source. However, given the information available, transport and deposition from the southeast PA growing region can not be ruled out as a possible factor. Figures 5 and 6 show the simulated deposition for these events.
Figure 5: Simulated deposition from the July 18 transport event from southeast PA.
Figure 6: Simulated deposition from the July 19 transport event from southeast PA.
The 2007 disease season also included a confirmed case of blue mold in southern Wisconsin. Blue mold was confirmed on August 10 in Dane County. Incidence of less than 1% was observed in a 2-acre field. Rigorous post-outbreak analysis has yielded no solid clues as to the origins of this outbreak. This seldom occurs; therefore, in such cases one begins to suspect the presence of undiscovered or unreported sources, or some other mechanism of disease introduction. However, absent any new information, there is no way to arrive at a good determination. At this time, the origin of this small outbreak remains unknown.
Other outbreaks occurred from mid-July through August near known source regions during periods of favorable weather. Two such outbreaks, both small, were confirmed in east-central KY in August. The most numerous reports came from the southern Appalachian Mountains, where scattered showers and storms occurred in July and August with more normal frequency than in past months. Six new counties in the NC mountains reported outbreaks during this period, as did Lee County, in extreme southwest VA. Localized to short-range transport and deposition from nearby sources were responsible for all of these outbreaks.
The 2007 epidemic was limited in both intensity and scope when compared to most epidemics of the previous decade. For the fourth year in a row, there were no reports from the growing areas of northern FL or southern GA. In previous years, the absence of disease from this growing region has had a significant impact on the ensuing epidemic spread to other areas. This was also the case in 2007. Additional dampening of the 2007 epidemic was due in large part to the extended drought in the southeastern 1/3 of the United States. The drought stifled field infections and the generally unfavorable weather provided few opportunities for long-range transport and deposition.
Airborne introduction of inoculum was the cause for the outbreaks in most counties. The transport events were either described in the forecasts and/or revealed themselves in post-event analysis. Movement of infected transplants was the culprit in several other outbreaks. In only two cases – the outbreaks in southern Wisconsin and northeast Tennessee – were plausible explanations lacking based on the available evidence. In all, 24 counties in 8 states reported blue mold to the Forecast Center.
The complete set of day-by-day forecasts can be reviewed on the NAPDFC’s website: http://www.ces.ncsu.edu/depts/pp/bluemold/
The NAPDFC thanks all the reporting coordinators for each state and adjoining countries for their participation during 2007. If you wish to become part of the forecast system, or need further information, contact Dr. Mina Mila or the NAPDFC directly. For a complete description of the NAPDFC, refer to the APSNet feature article, “Forecasting Long-Range Transport of Downy Mildew Spores and Plant Disease Epidemics” at http://www.aspnet.org/online/feature/forecast/
Dr. Mina Mila
Asst. Professor, Tobacco Specialist
mina_mila@ncsu.edu
Thomas Keever
Lead Forecaster / Meteorologist
thomas_keever@ncsu.edu