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2010 annual CDM summary

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Summary of the 2010 Cucurbit Downy Mildew Epidemic in North America

Thomas Keever
Forecaster / Meteorologist, CDM ipmPIPE
North Carolina State University

 

2010 was the third year of Cucurbit Downy Mildew (CDM) epidemic forecasting within the CDM ipmPIPE initiative. The CDM ipmPIPE is a joint effort of academia, the USDA, and industry headquartered in the Plant Pathology Department at North Carolina State University. The CDM ipmPIPE provides opportunities not previously available for cucurbit downy mildew forecasting. A network of carefully monitored sentinel plots located in southern Ontario and the central and eastern United States (plus 2 in CA) was available to aid the tracking of epidemic spread. Collaboration with the NC State Climate Office yielded improvements in the website, increased efficiencies in the reporting and presentation of outbreaks, and provided additional tools for analysis and evaluation of the forecasts.

The most notable change for 2010 was a format change for the forecasts. Rather than showing atmospheric trajectories originating from each source or source region, a comprehensive risk prediction map was shown for each day’s transport events from all disease sources. The number of risk levels was reduced from 5 to 3. The risk to cucurbits was color coded as red (High risk), orange (Moderate risk), and yellow (Low risk). The simplification of risk levels and the addition of the easily-readable map resulted in a more pleasing and understandable presentation. Please consult our website at http://cdm.ipmpipe.org/ for additional and more specific information.

 

Sentinel Plots

Though the number was reduced from previous years, a network of sentinel plots was again part of the CDM forecasting framework in 2010. Forty-nine sentinel plots were planted in 20 states and 1 Canadian province. Each plot contained cucumber, squash (2 varieties), cantaloupe, watermelon, and pumpkin. Plots were scouted weekly for signs of disease. Thirty-nine of the 49 sentinel plots reported CDM; disease was not observed in 10 sentinel plots. Most of the sentinel plots where disease was not observed were in areas that were a large distance from other outbreaks or in regions where they were part of a mix of weak and/or widely scattered outbreaks.

Thirty-six of the 39 sentinel reports in 2010 occurred first on cucumber, or concurrently with another cucurbit host (cantaloupe or squash). Disease then spread to other hosts in the plot during periods of favorable conditions. Only 5 sentinel plots that reported CDM had infection on a single host and most plots had 3 or more hosts infected by CDM. As in past years the additional hosts varied. This was likely due to other factors in addition to the CDM (weather favorability, host susceptibility, condition/growth stage of the plants, presence of other diseases, etc).

 

Tracking of the 2010 CDM epidemic was enhanced by the presence of the sentinel plot network. Roughly one-third of the initial reports from new states or new regions came from the sentinel plots. Monitoring the evolution of a plant disease epidemic is a cooperative effort that requires timely information from the field. Diligent scouting at regular intervals is essential. The sentinel plot system provided both of these related elements.

 

Epidemic Overview

The CDM ipmPIPE Forecast Center received 176 unique reports from 132 counties in the United States and southern Canada in 2010 (“unique reports” excludes sentinel plot updates). This included 24 states east of the Rocky Mountains, southern Ontario, and California. There were 39 reports from the sentinel plots, 92 from commercial fields, 29 from other research plots, and 19 from home gardens. The numbers for unique reports and counties were a decrease from 2009 values and quite close to the counts from 2008.

The large majority of outbreaks occurred from the Great Lakes into the Northeast and along the growing regions of the Eastern Seaboard in 2010. Outbreaks were clustered in the Northeast, TX, and CA. Isolated outbreaks were reported from the Gulf Coast into the Ohio Valley.

Michigan had the most reporting counties in 2010 with 25. North Carolina had 14. NY had 10 counties with confirmed CDM. Ohio had 9; FL had 8. PA, TX, CA, and Ontario reported 6 outbreaks each. Five counties in AL and SC reported CDM. GA and WI recorded 4 outbreaks. The remaining states reported 3 or fewer counties with CDM in 2010.

 

Hosts

As has been the case since 2004, the 2010 epidemic was characterized by widespread infections on cucumber (164 reports, ~2/3 of all reports). Every region that reported disease had confirmed CDM on cucumber. Only 3 of the 39 sentinel plots that reported CDM did not have a cucumber report during the growing season. In addition, a large majority of ‘first reports’ from states or growing regions occurred on cucumber. Forty-five reports involved squash, mostly from the South or East. Cantaloupe was mentioned in 33 reports, mostly from the North or East. Pumpkin appeared in 27 reports from various regions. There were 19 reports on watermelon; most came from TX or SC. (Note: reports in this section include sentinel plot updates)

 

Other Planting Types

In the commercial fields, there were reports on squash from the Southeast. Most field reports of CDM on cantaloupe were across the northern tier. Pumpkin reports arrived from NY, IL, TN, and KY. Most of the watermelon field reports were from the TX growing region, though there was one report from SC. Reports of CDM on cucumbers in the field came from every growing region affected.

Most reported outbreaks in home gardens were from the more northern areas (MI, OH, PA, ON, NY). There were also a small number of home garden reports from locations in the South and East. Cucumber, squash, and cantaloupe were the affected hosts, with cucumber cited most frequently.

Reports of CDM in research plots arrived mostly from the northern areas, the southern states, and from the Carolinas. CDM on cucumber dominated these reports, though each host was mentioned at least once in this group.

 

Epidemic Chronology

The 2010 forecasts began on March 26 from confirmed field infections on squash in Miami-Dade County, FL.  The epidemic moved slowly for the next 2 months. New reports in FL arrived from Hillsborough County on May 7 and Collier County on May 12.

The first report from GA came from Brooks County on May 17. CDM was confirmed on cucumber in a large commercial field. Estimated date of first symptoms was May 14. Analysis reveals that the most likely time for viable spore introduction was during the transport events of April 30 / May 1st from FL. The initial infection may have been light, with intensity increasing due to rain that occurred approximately one week later. The forecast (map?) for April 30 is shown below. CLICK HERE for the complete forecast.


 

No new reports or updates were received for the next 10 days. On May 28, however, CDM was reported in both NC and TX for the first time in 2010.

CDM was confirmed on cucumber in a field in Columbus County, NC. Severity was high at 45%. The estimated date of first occurrence was May 23. The origin of this outbreak remains unclear. None of the transport events from the known sources affected southeast NC during the appropriate timeframe. It is possible that live spores were deposited during unsettled weather that occurred about a week prior to first symptoms.

CDM was confirmed in a large field of watermelon in Frio County, TX. First symptoms were estimated at May 24. This outbreak likely resulted from transport from the then-undiscovered source in Hidalgo County in deep south TX. That source, a large commercial cucumber field, would be reported later on June 7 at 30% severity. Symptoms in Hidalgo County were roughly estimated to have first appeared around May 7. The origin of the Hildalgo County outbreak probably lies with infections somewhere in Mexico, though this suspicion cannot be confirmed.

Disease activity began to increase during the first half of June in and around the known sources. There were new reports from northern FL, southern GA, and eastern NC. Much of this and later epidemic spread was due to a period of favorable weather from May 28 through June 6.

The first report from SC also arrived during this time, on June 14. CDM was confirmed on cucumber in the sentinel plot in Charleston County. First symptoms were estimated at June 13. The second report from SC was from Clarendon County from a much larger source (45 acre cucumber field, severity of 80%). First symptoms here were estimated at June 12, very close to the Charleston County date. Both of these outbreaks almost certainly occurred during the unsettled, disease-favorable period from June 2-6. Transport events were moving through the same areas on multiple days under mixed or favorable conditions. In addition, some undiscovered sources were likely providing more airborne spores to the mix (Toombs County, GA for example) but were not yet a part of the forecasts. The events for June 2, which may have introduced live spores, are shown below. There were likely others. CLICK HERE for the complete forecast.



The first report from the growing areas of southern Ontario, Canada came on June 18. CDM had been confirmed on cucumber in a small commercial field in Hamilton Division. The estimated date of first symptoms was June 16. As has been the case in recent years, no satisfactory explanation for an airborne introduction of inoculum to this region can be found.

With disease becoming better established in the Southeast / lower mid-Atlantic and the advent of the new source in southern Ontario, the situation was set for more rapid advance of the 2010 epidemic. Epidemic spread could occur along the East Coast due to the sources to the south and across the northern areas due to the source(s) around the Great Lakes. This is a common scenario. Fueled by periods of favorable weather, infections spread along the Eastern Seaboard, through the Great Lakes region, and into the Northeast from mid-June through August. The CDM epidemic spread both westward and eastward from the increasing number of sources in the Great Lakes. Epidemic spread to the northeast occurred along the East Coast. Additional outbreaks sprang up in parts of the South due to short-range transport and deposition.

Ohio’s first report of CDM came from a small commercial field in Holmes County. The report was made June 23, the same day as estimated first symptoms. Severity was 20% in this field.

CDM was confirmed on cucumber in commercial fields in Erie and Niagara Counties, NY for that state’s first reports. Severity was low. The report arrived on June 30; first symptoms were estimated at June 29.

Both the OH and NY initial infections likely began via transport of airborne spores from southern Ontario.

The first report from Maryland arrived on July 5-6 from Caroline County. CDM had been confirmed in a small commercial field of cucumber. Symptoms were seen at harvest July 1. Disease may have been present for some time.

Disease reports on July 7 included the first reports from MI and WI. CDM was confirmed on cucumber in a large commercial field in Bay County, WI. Severity was 10%. Reports of CDM in 5 additional counties in MI came in to the Forecast Center 2 days later. Columbia County, WI was the first county in that state to report CDM. Cucumber plants in a research plot were infected. First symptoms in both of these outbreaks were estimated to have appeared within 2 days of the report.

The sentinel plot in DE went positive for CDM on cucumber on July 9. Symptoms were estimated to have appeared July 7.

The first report in AL arrived on July 9, from a small commercial field of cucumber in Geneva County in the southern portion of that state. Incidence was 100%, severity was 15%. Estimated date of first symptoms was July 1. This outbreak is noteworthy in that it originated from sources in GA and the Carolinas located to the east. High pressure to the north of the southeast U.S. produced west to southwest winds and conditions were mixed to favorable for survivable transport and deposition for 2 consecutive days. The risk prediction map for June 30 is shown below. CLICK HERE for the complete forecast.


 

Pennsylvania’s first report was from the sentinel plot in Centre County. The report came in on July 23. First symptoms on the cucumber were estimated at July 21.

CDM was reported in a 5 acre commercial cucumber field in Cumberland County, NJ on July 30. Incidence and severity were both 20%. Estimated date of first symptoms was July 29. This outbreak likely originated from nearby sources to the southwest. The sentinel plot in Hunterdon County, NJ went positive for CDM a few days later. The Cumberland County outbreak (and perhaps both) likely was due to the transport events of July 21. Moderate risk was painted for these counties. The risk map is shown below. CLICK HERE for the complete forecast


 

The first report from IN arrived on August 6. The cucumber in the sentinel plot in LaPorte County (far northern IN) had low incidence and low severity. First symptoms were estimated at August 1.

The spread of CDM into the New England region was documented by the reports of August 11. CDM was confirmed on cantaloupe and cucumber in a commercial field in Sullivan County, NH. Severity was 80% in this field. The estimated date of first symptoms was July 15. Massachusetts’ first report was for CDM on cucumber in a small commercial plot in Norfolk County. Severity was 75% in this case. First symptoms were estimated at August 3. Viable spores may have come from locations to the south or west to produce these outbreaks.

Regional epidemic spread was illustrated nicely by the outbreaks in eastern NY. CDM was reported in Fulton and Schoharie Counties on August 27, with first symptoms estimated at August 5. A transport event that originated from the numerous sources to the west swept toward the east on July 28. Conditions were mixed to favorable for epidemic spread. These infections likely occurred toward the end of this event. The counties in question were painted with Moderate Risk. The map is given below. CLICK HERE for the complete forecast.


 

The first report from CA arrived on August 18. CDM was confirmed in a 25 acre commercial field in Santa Clara County. Incidence was 30% and severity 10%. The estimated date of first symptoms was July 30. CDM spread to 4 other nearby counties in the central CA growing region during the late summer.

Of particular note in 2010 was the observation of CDM in Minnesota. Reports from this area are infrequent. CDM was confirmed on cucumber in a research plot in Ramsey County. Incidence and severity were quite high (100% and 70%, respectively). The estimated date of first symptoms was August 20. This plot was on an organic research and teaching farm. The infected plants were removed and destroyed. This outbreak likely originated from nearby infections in WI.

The 2010 CDM epidemic moved into the Ohio Valley region in August. The first reports from KY and TN arrived on September 3. CDM was confirmed on cucumber in a 3 acre field in Harrison County, KY. Incidence was 80%, severity 15%. Estimated date of first symptoms was August 12. Tennessee’s first report was on pumpkin. CDM was confirmed in a 10 acre field in Cumberland County. Incidence and severity were low. Estimated date of first symptoms was August 25. A report from southwest IN arrived on September 8. The Knox County sentinel plot was positive for CDM on cucumber. First symptoms were estimated at August 30. CDM spread to a few other locations in TN and KY during September. Most infections were light.

The disease season began to wind down as September turned into October. Outbreaks that had become less frequent during September became even more so during October. The growing season was ending or had ended for the northern areas. The first reports from LA (Tangipahoa Parish, tiny field of squash) and MS (Copiah County, squash and pumpkin in a home garden) arrived during this time.

Forecasts ended on the 29th of October, 2010.

The 2010 CDM epidemic affected nearly all of the major growing regions. The epidemic increased in the Southeast / lower mid-Atlantic in early June during a period of favorable weather. Rapid epidemic spread occurred during the summer months from sources along the East Coast and in the Great Lakes, a common theme. Infections were discovered in the Ohio Valley region later during the growing season. The CDM ipmPIPE center received 176 unique reports from 132 counties in the United States and southern Canada in 2010. This represents a decrease from 2009 but is very similar to 2008. Counties with confirmed infections were clustered more heavily in the Mid-Atlantic and Great Lakes. Reports of CDM were scattered in the Northeast. Counties reporting CDM in the Ohio Valley and South were frequently more isolated though small groups were present in some cases. The general prominence of reports from the Great Lakes and Mid-Atlantic has been consistent in recent years.