Bob Mulrooney, Extension Plant Pathologist; bobmul@udel.edu
The Stewart’s wilt prediction for 2012, as you can imagine with a very mild winter, is severe for all three counties in Delaware. We have not had such a mild December, January and February since the winter of 2001-2002 when the Index was 123.1 for Georgetown and 123.0 for Newark. The following table tells it all.
Winter Temperature Index For Predicting Stewart’s Wilt in Delaware Sweet Corn: 2002-2012
Average monthly temperatures in °F at Georgetown, DE, REC: 2002-2012
| 2011-2012 | 2010-2011 | 2009-2010 | 2008-2009 | 2007-2008 | 2006-2007 | 2005-2006 | 2004-2005 | 2003-2004 | 2002-2003 | |
| December | 43.2 | 31.3 | 37.9 | 41.8 | 39.7 | 43.5 | 36.2 | 38.9 | 38.6 | 36.7 |
| January | 39.5 | 31.0 | 32.7 | 31.0 | 36.8 | 39.7 | 43.0 | 34.9 | 29.5 | 28.9 |
| February | 40.4 | 39.6 | 31.1 | 39.2 | 39.9 | 30.1 | 37.4 | 36.7 | 35.2 | 33.8 |
| INDEX | 123.1 | 101.9 | 101 .7 | 112.0 | 116.4 | 113.3 | 116.6 | 110.5 | 103.3 | 99.4 |
Average monthly temperatures in °F at Newark, DE, Experiment Station: 2002-2012.
| 2011-2012 | 2010-2011 | 2009-2010 | 2008-2009 | 2007-2008 | 2006-2007 | 2005-2006 | 2004-2005 | 2003-2004 | 2002-2003 | |
| December | 41.3 | 30.8 | 34.9 | 37.1 | 37.5 | 42.5 | 34.0 | 35.5 | 34.0 | 33.5 |
| January | 37.0 | 28.7 | 31.6 | 28.0 | 35.5 | 37.3 | 39.5 | 31.0 | 26.4 | 27.1 |
| February | 39.5 | 35.2 | 31.0 | 35.8 | 36.5 | 27.8 | 34.5 | 34.2 | 33.1 | 29.5 |
| INDEX | 117.8 | 94.7 | 97. 5 | 100.9 | 109.5 | 107.6 | 108.0 | 100.7 | 93.5 | 90.1 |
Severity Index: < 90, usually absent; 90-100, intermediate; >100, usually severe.
The index is used to predict overwintering flea beetle populations that vector the Stewart’s wilt bacterium, Pantoea stewartii.
Prediction for 2012
| Average Temperature for Dec, Jan & Feb | ||||
| Location | Prediction | 2011-2012 | 2010-2011 | |
| Georgetown | 123.1=Severe | 41.0°F | 34.0°F | |
| Dover | 121.0=Severe | 40.3°F | 32.5°F | |
| Newark | 117.8=Severe | 39.3°F | 31.6°F | |
Control Strategies
For processing and fresh market growers the high risk of Swetart’s wilt means that if you are planting susceptible or moderately susceptible hybrids, flea beetle control is very important. A number of strategies are available including seed treatments, granular insecticides at planting and/or foliar applied insecticides after emergence. For foliar applied insecticides treat susceptible cultivars at spike stage when 5% of the plants are infested. See the 2012 Delaware Commercial Vegetable Production Recommendations for control suggestions.
Note: Weather records from University of Delaware Carvel REC, Georgetown, DE, DE State Fire School, Dover, DE and University of Delaware Ag Experiment Station Farm, Newark, DE. Data records found online at http://www.deos.udel.edu/