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© 2013 Plant Management Network.
Accepted for publication 29 January 2013. Published 28 March 2013.


An Epidemic of Downy Mildew caused by Peronospora destructor on Vidalia Sweet Onions in Georgia in 2012


Venkatesan Parkunan, Ronald D. Gitaitis, Bhabesh Dutta, David B. Langston, and Pingsheng Ji, Department of Plant Pathology, Coastal Plain Experiment Station, University of Georgia, Tifton, GA 31793


Corresponding author: Venkatesan Parkunan.  pcap@uga.edu


Parkunan, V., Gitaitis, R. D., Dutta, B., Langston, D. B., and Ji, P. 2013. An epidemic of downy mildew caused by Peronospora destructor on Vidalia sweet onions in Georgia in 2012. Online. Plant Health Progress doi:10.1094/PHP-2013-0328-01-BR.


The first reported occurrence of onion downy mildew (ODM), caused by Peronospora destructor (Berk.) on Vidalia sweet onions (Allium cepa L.) in Georgia was in 1999 (1). The term Vidalia refers to a group of sweet onion shortday, Granex-type varieties that are grown in 20 southeastern counties in Georgia near the city of Vidalia. However, the majority of Vidalia onions are grown in 13 counties surrounding Toombs and Tattnall counties. Although the disease is not common every year, two epidemics have occurred since 1999. We report here on the impact of a recent outbreak of ODM on the $100-million Vidalia onion crop in 2012.

Symptoms of ODM were first observed in early February 2012. Symptoms included elongated leaf lesions with purple-gray sporulation, tip dieback, and collapsed leaves (Fig. 1). Microscopic observation of symptomatic leaves showed numerous, dichotomously branched sporangiophores with pyriform to fusiform sporangia (Fig. 2) and non-septate mycelia. Based on the morphology and molecular identification (rDNA ITS sequencing), the pathogen was confirmed as P. destructor. Continuous periods of wet, cool weather with average low nighttime temperatures of 10.2 to 11.3°C, and a hard freeze followed by high humidity with foggy mornings in late February and early March in southeastern Georgia (Georgia Automated Environmental Monitoring Network, www.GeorgiaWeather.net) favored pathogen sporulation and disease development. Favorable weather persisting from late February through early March triggered repeated cycles of sporangial formation and resulted in severe and continued infection in the region.


 
A
 

B
 
C

Fig. 1. Symptoms of onion leaves (cvs. Sweet Carolina and Sapelo Sweet) infected by the downy mildew pathogen: (A) dark gray to purplish sporulation of P. destructor on lesions caused by Botrytis sp.; (B) tip dieback; and (C) death of leaves and plants.



   

Fig. 2. Dichotomously branched sporangiophore (A) with pyriform to fusiform sporangia (B) of onion downy mildew pathogen, Peronospora destructor.

 

It is uncertain where the initial sources of inoculum originated. ODM forms oospores in infested debris and bulbs in cull piles and these may survive for up to 5-6 years (2,3). A genetic diversity study on ODM populations from diverse locations in the Vidalia onion growing region in Georgia indicated that a single source of inoculum may have been involved (V. Parkunan and P. Ji, unpublished). Thus, it is possible that oospores from a previous epidemic in 2007 survived to initiate the epidemic.

The 2012 epidemic resulted in approximately 20-25% bulb loss based on estimates from the Vidalia Onion Committee with a reduction of 18.2 million kg of onions ($18.2 million) from the previous year and approximately the same acreage. Most of the affected onions were in the mid-bulbing growth stage and growers harvested bulbs two weeks earlier than normal, thus ending up with smaller bulb sizes. Disease incidences in affected fields ranged from 10-100%, with the greatest incidences occurring in Toombs and Tattnall counties (Table 1, Fig. 3). The average disease incidence in seven scouted fields in six counties was 59.3%. Disease severity was also assessed using 20 randomly sampled plants per field with a rating scale of 0-5 (0 = no disease, 1 = 1-20%, 2 = 21-40%, 3 = 41-60%, 4 = 61-80%, and 5 = 81-100% disease). The average disease severity in the scouted fields was 2.6 (Table 1). Some of the major varieties affected included ‘Sapelo Sweet,’ ‘Sweet Carolina,’ ‘Sweet Agent,’ ‘Sweet Harvest,’ and ‘Golden Eye.


Table 1. Incidence and severity of downy mildew on Vidalia onion in Georgia (March 2012).

Field County Onion
variety
Disease
incidence
(%)
Disease
severity
*
1 Candler Sweet Harvest 70.0 2.8
2 Bulloch Sapelo Sweet 75.0 4.0
3 Toombs Sweet Carolina 80.0 3.3
4 Tattnall Sapelo Sweet 50.0 1.4
5 Evans Sapelo Sweet 20.0 1.0
6 Appling Sapelo Sweet 60.0 2.7
7 Tattnall Sapelo Sweet 60.0 2.9

Mean  

59.3 2.6

 * Disease severity was quantified using 20 randomly sampled plants per field with a rating scale of 0 to 5 as: 0 = no disease symptoms; 1 =  1-20% coverage of lesions/plant; 2 = 21-40% coverage of lesions/plant; 3 =  41-60% coverage of lesions/plant; 4 = 61-80% coverage of lesions/plant; 5 =  81-100% coverage of lesions/plant or completely collapsed plant.


   

Fig. 3. Sampling locations of onion downy mildew outbreak regions in Georgia in March 2012:

1 = Candler Co.;
2 = Bulloch Co.;
3 = Toombs Co.;
4 and 7 = Tattnall Co.;
5 = Evans Co.; and
6 = Appling Co.

 

Acknowledgments

Authors would like to acknowledge Heath Paradise, AllTech Crop Science, Vidalia, GA, and Jason Edenfield, County Extension Agent, Toombs County, Lyons, GA, for their assistance in obtaining ODM samples.


Literature Cited

1. Langston, D. B., and Sumner, D. R. 2000. First report of downy mildew (caused by Peronospora destructor) of onion in Georgia. Plant Dis. 84:489.

2. McKay, R. 1937. Germination of oospores of onion mildew, Peronospora schleideniana W. G. Sm. Nature 139:758-759.

3. Schwartz, H. F., and Mohan, S. K. 1995. Compendium of Onion and Garlic Diseases. American Phytopathological Society, St. Paul, MN.