The North American Blue Mold Forecast Center at North Carolina State University completed its fourth year of service to tobacco producers, industry, and extension services throughout North America. The forecasts were issued on Monday, Wednesday, and Friday each week from March throught August. Additional forecasts were provided during peak epidemic periods. Timely information on the geographic occurrence of blue mold and the future movement of inoculum (fungus spores) throughout the North American continent was important to tobacco producers in managing this destructive and fast moving plant disease epidemic. Users have found the maps and other information useful in determining the time of arrival of spores and assessing the risk outlook for their production areas. We are always interested in your comments on the system!
Blue mold occurred in all the major production regions of North America during the growing season of 1998. Continental forecasting began on March 2 with disease sources reported to the Forecast Center in western Cuba and Mexico. A total of 451 forecasts were made on 72 different days. These forecasts were the result of analyzing and evaluating the disease development potential of over 1200 individual spore transport events. Forecast production ended on August 21, 1998. At that time some disease was still active in the Ohio Valley, southern Appalachian Mountains, southern Ontario, and southern New England. During the height of the blue mold epidemic during June and July, more than ten active and geographically distinct source regions of blue mold were being monitored and tracked for their disease risk potential to uninfected target regions. Blue mold samples submitted to the various laboratories for strain testing were largely insensitive (resistant) to metalaxyl.
Blue mold was first reported within the United States on March 31st at Alachua and Columbia counties in northern Florida. The most likely spore transport and deposition events for this outbreak occurred March 17 through March 19. Spores were moving northward from Cuba under favorable conditions each day. Disease development in the area continued during April as approaching cold fronts brought favorable weather every three or four days. Blue mold was reported in southern Georgia on the 20th and 22nd of April. Live spores may have been deposited in this area several times. However, the events of April 8 and 9 were the most probable ones for the first outbreak in Georgia. Conditions were particularly favorable on these dates and the stage of blue mold at discovery corresponded with the estimated transport time frame.
A very significant situation occurred during the middle of April when blue mold was reported in north-central Tennessee and south-central Kentucky. Based upon firsthand observation and diagnosis, these outbreaks could be traced to import of infected transplants from southern Florida. The ensuing local spread resulted from local distribution of infected transplants to growers in nearby counties. Increasing reports of blue mold to the Forecast Center from growers and extension agents in this region continued for several weeks. Most of these reports were linked directly to the infected transplants.
The progressive movement of blue mold into the Southeast and the introduction of blue mold to the Ohio Valley region occurred from mid-April through May 10th during a month of generally favorable weather. Spread due to airborne spore showers was slowed in the Ohio Valley region due to unfavorable nighttime temperatures. Blue mold continued to spread in the South. Eight counties in southern Georgia reported active blue mold by May 4.
Following a weekend of highly favorable weather May 8-10, the following ten days were hot and mostly dry throughout the southern and eastern production regions. However, a period of mostly favorable weather began May 22 and lasted about a month. Temperatures in the target growing regions shifted to the optimum range (18C - 24C) for infection and disease development. During this interval blue mold was reported present in most of the remaining production areas in eastern North America.
Blue mold was reported May 22 in southern Indiana, western Kentucky, and the mountains of eastern and northeast Tennessee. This initial outbreak in Indiana occurred due to spore transport and deposition on May 6 and 7 from the existing TN/KY border sources. The source origin of the first blue mold in the southern Appalachian Mountains of eastern Tennessee is uncertain. Though the report arrived at the Forecast Center on May 22, later information indicated that blue mold had been discovered in Blount County, TN on May 4 and Sevier County, TN on May 8. It is possible that infected transplants were imported from elsewhere, or airborne inoculum arrived from the active sources to the west.
The first report of blue mold in South Carolina occurred on May 26. Estimated date of first infection was on or before May 12. Of the few possible scenarios, the transport episode of May 7 stands out. Conditions ahead of a cold front approaching from the west were favorable, and copious amounts of inoculum were being generated by well-established sources in southern Georgia and northern Florida.
Extreme southwest Virginia was the first locale in that state to report blue mold, on May 29. Short range transport and deposition from east and/or northeast Tennessee were probably responsible for this outbreak.
Blue mold was discovered in southeast Pennsylvania on May 28. This outbreak was puzzling because the weather during the previous two weeks had been unfavorable for disease devleopment. Thus, initial infection was estimated to have occurred on or before May 14. Only two episodes could have accounted for spores coming close to the Pennsylvania growing areas. One possibility occurred on May 1 from the TN/KY source; the other occurred on May 7 from the southern GA/northern FL sources.
These are significant because both cases would represent two-day transport events. Of the spore transport episodes resulting in disease development during the past three growing seasons, the majority involve deposition of viable spores within 24-30 hours, frequently less. In most cases, the weather during an event doesn't remain favorable for survivable transport and/or deposition the second day. Other times the direction of the trajectory changes and veers away from the target growing regions.
However, conditions persisted and were favorable during both of these forecast trajectories. While very unusual, these proposed events were certainly possible. There were no other short transport events that could reasonably explain the Pennsylvania outbreak. Spore transport and deposition apparently occurred over the necessary time span and distance to infect southeast Pennsylvania tobacco in early May. Following initial infection, the disease turned dormant during unfavorable conditions from May 10 to May 21, then developed sufficiently to be noticed and reported by May 28. Either or both events appear likely to have been responsible for the Pennsylvania outbreak.
The first report of blue mold from North Carolina came from the burley-growing area in the western mountains on June 3. Possible spore introduction episodes include May 2 or May 8 from the TN/KY sources to the west or a short-range event from the TN mountains early in May. Any or all of these could have occurred, corresponding to the estimated time of initial infection of early May.
Blue mold was not reported from the flue-cured production areas of North Carolina until June 9. As was the case with the burley tobacco in the western mountains, several spore transport episodes may have brought live spores to Greene and Lenoir counties in eastern North Carolina. The two most likely events originated from east-central South Carolina on May 27 and on May 30/31. These events fell within the estimated spore introduction period of May 27 through June 2. No other events appeared likely.
New outbreaks were reported in southern Maryland and southern Ontario, Canada, on June 8. Maryland's blue mold may have resulted from transport episodes occurring May 7 from Georgia/Florida or May 26 from the southern Appalachian Mountains. Both of these were extended events, especially the one of May 26. For the Canadian outbreak, the most likely event was the airborne explanation of May 7 related to a transport scenario from Georgia/Florida! Additional trajectories were examined from the southern Appalachian Mountains for May 7. Conditions were favorable for survivable transport. The trajectory moved north toward the southern shore of Lake Erie with the centerline coming close to the southern Canada growing areas. Spores from the southern mountains, if present, could have been deposited. However, the only known blue mold source in the southern Appalachian mountain region at that time had just been discovered May 5. Another source was found May 8. Disease activity levels at those locations are unknown, but it seems unlikely that these new sources could have produced enough airborne inoculum to threaten southern Ontario. Trajectories from stronger sources along the Tennessee/Kentucky border moved north also, but did not approach the Canadian production region. Other than transport from some unknown and unreported source, the only explanation we can offer is that spores from the Georgia/Florida region caused the outbreat in southern Canada.
In retrospect, the spore transport episode of May 7 from the infected Georgia/Florida production areas was very significant. Initial disease outbreaks in South Carolina, Maryland, and southern Canada were all most likely the result of inoculum deposited during this one event. Introduction of blue mold to south-central Virginia and southeast Pennsylvania could have occurred from this episode as well. Disease development had been occurring locally in southern Georgia and northern Florida for some period and blue mold was well-established. Large quantities of spores were being produced. The weather conditions were prime for long-range transport. A slow-moving front that stretched from Canada south to the Gulf Coast was approaching from the west. Cloudy conditions ahead of the front provided protective cover for transport of spores. On May 7 and 8, occasional bands of showers moved through many areas along the Great Lakes region and U.S. eastern seaboard. Though unlikely and highly unusual, this very long-range transport and multiple-target deposition event was entirely possible and explainable. Indeed, it provides the only plausible explanation for several of the new disease outbreaks.
Local spread and repeated outbreaks near existing sources continued to be reported. The explosive rapid-spread phase of the 1998 epidemic culminated on June 17, when blue mold was reported from Massachusetts. Trajectories toward this region were few during the time of suspected initial infection. The most likely episode occurred on June 2, originating in south-central Virginia. One blue mold source had recently been reported in Chesterfied County. Though unreported at the time, there were two other sources reported later in Prince Edward and Mecklenburg counties that had been present for two to three weeks. Inoculum was being released and conditions were reasonably favorable for survivable transport and deposition.
Following a two-week period of favorable weather in southern New England, Connecticut growers reported blue mold on June 29. This outbreak was almost certainly the result of spore transport and deposition from the nearby Massachusetts source reported earlier in the month.
The last ten days of June and the remainder of the growing season saw a change in the predominant weather patterns. Low pressure systems and fronts tended to pass through the more northern growing areas and move less frequently into the southern production areas. Variable, unsettled weather occurred in the northern production areas. Periods of dry weather alternated with periods of partly cloudy skies and occasional afternoon thunderstorms in the southern regions.
Since blue mold had already spread throughout the major production regions, there was a corresponding shift from interest in long-distance potential transport to outlooks related to localized spread. After late June in the south, blue mold slowed down or essentially stopped in eastern areas of the mid-Atlantic states. The disease remained active in parts of the Ohio Valley and southern Appalachian Mountains which were subjected to periodic showers. Extended periods of hot, dry weather suppressed disease activity. Epidemic activity continued unabated in southern Ontario and New England, where frontal systems were passing consistently, bringing favorable conditions.
Ohio on July 10, Wisconsin on August 17, and Missouri on August 21 reported new occurrences of blue mold. The Ohio outbreak was the result of spore transport episodes on June 29 and 30 from Kentucky. Introduction of blue mold to Wisconsin occurred from western sections of Kentucky on August 3. This was probably the only period during the entire growing season when the winds and weather were favorable for that event to occur! The Missouri outbreak is difficult to explain. Estimated time of initial infection was August 1. However, there were no observed trajectories towards the Missouri growing areas near that date. The most likely time seems to have been on July 14, when spores from western Kentucky would have been moving in that direction. Unfortunately, data for forecast trajectories was unavailable at that time, and only a postseason analysis determined when spores may have been introduced. The Forecasting System was discontinued for the season on August 21.
The 1998 blue mold season was very active. The long-term and widescale spread of the 1998 epidemic was greatly influenced by two factors: (1) two 30-day disease-favorable periods in eastern North America, from April 10 to May 10 and from May 22 to June 22, and (2) the inadviseable movement of infected transplants into the Kentucky/Tennessee/Ohio Valley region in mid-April. The combination of these two events set the stage for a two-pronged, long-range blue mold assault on the eastern North American production regions. The result was the occurrence and development of blue mold in every major production area before July 4. Incredibly, a single transport episode on May 7 was likely responsible for outbreaks stretching from South Carolina to southern Canada! This was one of the most interesting and significant episodes since blue mold forecasting began in 1995.
The complete set of day-by-day forecasts, archived by year, date, and location, can be reviewed via the Internet website: 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 1998. If you wish to become part of the Forecast System, or need further information, contact C. E. Main or the Forecast Center directly.