Summary of the 2008 Blue Mold Epidemic in North America
Thomas Keever
North American Plant Disease Forecast Center
NCSU, Department of Plant Pathology
The 13th year of forecasting for Tobacco Blue Mold epidemics in North America has concluded. Forecasts were produced three times per week in 2008 from March through 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. Forecasts were produced on 75 days from early March through the end of August.
The 2008 blue mold epidemic was the weakest since full-time forecasting began in 1996. Only 11 counties in 6 U.S. states and 1 Canadian province reported outbreaks. Regionally, these included northern FL / southern GA, eastern VA, the southern Appalachian Mountains, north-central KY, southeastern PA, and southern Ontario. Many of these occurred during the latter portion of the growing season (mid-July or afterward). There were no reports from most of the major growing regions. Impacts from this year’s epidemic were anticipated to be quite minimal, though in a few isolated cases more substantial damage may have been experienced. Persistent drought over a broad area of the southeastern United States during the Spring and early Summer likely played a large role in limiting the extent and severity of the 2008 epidemic.
The first report of blue mold came from Alachua County, FL on March 19, 2008. A plant bed had been severely impacted by what initially appeared to be frost / cold damage but upon further analysis was diagnosed as blue mold. This was an early introduction of blue mold to the United States when compared to previous years. The find in northern Florida also ended a 4-year stretch in which the first report arrived from another growing region.
Examination of transport events from western Cuba prior to the start of official forecasts showed that viable spores were most likely introduced to the northern FL region during the transport events of February 20-22 from that region. Available information indicated that there was a significant problem with blue mold in the Cuban growing regions during their growing season (which overlaps with the U.S. season in late Winter and Spring). Conditions during these events were not particularly favorable. Had these events been part of regular forecasting the risk to the plants in the U.S. would have been Low or Weakly Moderate, mostly due to spore death expected in the afternoon hours after release. However, passage of airborne spores over northern FL / southern GA did coincide with rainfall produced by a weather system moving through the region. See Figures 1 and 2 below.
Figure 1: Multi-level trajectories from western Cuba for February 20.
Figure 2: Multi-level trajectories from western Cuba for February 21.
Rainfall that was much below normal, combined with aggressive grower action, led to very low disease activity at the initial outbreak site. These factors kept the epidemic from spreading for some time. No new reports of blue mold were received for well over two months after the Alachua County, FL report.
In early June, blue mold was reported in 2 counties (Pierce and Lowndes) in southern GA. The first report from Virginia, Nottaway County in the east-central portion of the Commonwealth, arrived on June 9. The Georgia outbreaks were in the field; the Virginia outbreak was in a greenhouse. Observations in both GA and VA led to estimation that symptoms first appeared sometime in mid to late May at both locations. Clear indications were absent at the scene, leading to the large window of opportunity for inoculum introduction.
The GA outbreaks arose from airborne transport from the northern FL source. Deposition may have occurred in conjunction with transport events on May 11 or May 16. It is also possible that infection may have occurred during a period of unsettled weather from April 26-28. Tobacco plants in southern GA were at Moderate risk of disease development for the event of Sunday, April 27. In this case, the initial infections would had to have remained dormant or slowly simmering during the following two weeks of dry weather, until another period of favorable weather in mid-May allowed for renewed disease activity. All of these possibilities are plausible, yet the available evidence is not sufficient enough to indicate a definitive choice.
The greenhouse outbreak in east-central VA was similarly imprecise. Airborne introduction scenarios were few. Introduction may have occurred from one or two events in early May from northern FL. The transport event of May 15 from both northern FL and southern GA was also a possible candidate. This possibility would imply the GA sources were introduced in late April. It is also noted that the GA sources were undiscovered at the time during which they may have been involved.
The transport events mentioned above involve factors that make them poor candidates for the eastern VA infection. All of the FL and GA sources would have been weak during this time. Also, all of the above transport events were characterized by unfavorable conditions for survivable transport. This resulted in forecasts of Low Risk to tobacco plants for these events, not only for VA plants but also for those in the Carolinas that any airborne spores would have been required to pass over. No blue mold was reported this season from any applicable parts of the Carolinas. Though highly unlikely, it is within the realm of possibility that one of these transport events encountered enough cloud cover so that a few surviving spores may have been blown into the VA greenhouse while missing other locations. Ultimately, however, airborne explanations for the eastern VA outbreak remain unsatisfactory.
The next report of blue mold was from Suwanee County, FL on June 16. Symptoms were estimated to have appeared around June 3. This outbreak resulted from localized / short-range transport from the nearby Alachua County infection. Deposition may have occurred during one or more opportunities during the latter part of May.
The next reports of blue mold did not arrive at the Forecast Center for nearly a month. The first report from Pennsylvania in 2008 came in on July 11 from Lancaster County. The first report from the southern Appalachian Mountains arrived on July 14, from Washington County, VA. Information from PA indicated that symptoms likely first appeared around July 5. It was unknown how long disease had been present in southwest VA, though observations from the field showed that the infection was not recent, and thus may have been there undiscovered for some time.
The origins of the southwest VA outbreak likely lie in the June 11 transport event from eastern VA with possible contribution from the June 13 event from southern GA / northern FL. Plants in southwest VA were at Moderate Risk in each case. While this time period is a month removed from the disease report, various factors point to these events as being the most likely to have led to the infection. These include the weakness of the proposed sources, corresponding light amounts of introduced inoculum, the time needed for disease activity to increase during favorable weather, and the lack of any other plausible transport events. See Figure 3 below.
Figure 3: Multi-level trajectories from eastern VA for June 11, 2008.
The mid-June infection date for southwest VA also fits when considering the origins of the southeast PA outbreak. The eastern VA outbreak was aggressively attacked soon after its discovery and report on June 9. It became a very weak source of inoculum within days. At the time of the Lancaster County, PA confirmation, several fields in the area were affected with observations indicating infection taking place in late June or early July. While it’s possible for the eastern VA source to be responsible for the southeast PA outbreak (Moderate Risk from the June 22 event, Weakly Moderate Risk from the June 29 event), it seems more likely for there to be another factor contributing to the situation observed on the ground in PA. That would be the source in southwest VA while it was still undiscovered. Analysis of trajectories from mid-June through early July reveal two transport events that could have brought airborne spores from the southern Appalachian Mountains to southeast PA. These were on June 28 and July 2. Conditions during these events were probably mixed (or better) for survivable transport and effective deposition, as they were associated with organized weather systems. The data do not support the affirmation or denial of any of the possible origin events from eastern or southwest VA over one another. It is possible that more than one event impacted the growing areas of southeast PA.
Figure 4: Simulated deposition from the June 28 transport event from southwest VA.
Yet another month passed before the next report, the first from Kentucky in 2008 in Shelby County, on August 18. Blue mold was discovered a few days later in nearby Oldham County. Despite thorough analysis, an airborne explanation for the KY outbreak remains problematic. Symptoms likely appeared during the first week of August. However, there is a complete lack of evidence in support of an airborne introduction of spores during what should have been the appropriate time frame. The only potential transport event to central KY from the known sources was one on July 15 from the southwest VA source, which was characterized by unfavorable conditions for survivable transport and deposition. It has been speculated that blue mold may have spread into KY from points east via small, weak jumps that went undetected until disease was observed in mid August. However, there is no objective evidence available to support this speculation.
The outbreak in southern Ontario, Canada was reported on August 26. Infections were estimated to have been present 7 to 10 days, implying that introduction of inoculum occurred during the first two weeks of August. Again, as in the KY outbreak, there is no evidence supporting transport and deposition of spores from any of the known sources.
The last report of blue mold in 2008 came from the mountains of western NC, in Haywood County, on September 5. This outbreak is likely the result of short-range transport and deposition from the southwest VA source during times of favorable weather during the last two weeks of August.
The 2008 blue mold epidemic was the lightest on record. Most of the major growing regions of the U.S. and Canada reported no outbreaks. Only 11 counties in the U.S. and southern Canada had reported infections. The epidemic was characterized by an early start in the traditional U.S. entry region of northern FL. Most sources began weak and remained that way, though some became moderate in strength. Future epidemic spread was extremely slow, isolated in time and space, and heavily influenced by drought in the southeast U.S. during the Spring and Summer. Other themes prevalent during the disease season complicated the tracking of the 2008 epidemic. These included sporadic disease activity, predominance of Low Risk transport events, new outbreaks that were weak / isolated / hard to detect, mixed / inconclusive signals from the field evidence, and unknown sources that contributed to epidemic spread.
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 2008. 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