Anyone who spent the past winter in Ohio might wonder whether the winter was considered harsh or mild overall, because we saw periods of both extremes in temperature. After a very cold winter, we can expect to not have problems in sweet corn with Stewart’s bacterial wilt, but after a mild winter, we can expect to have problems in sweet corn with Stewart’s bacterial wilt. The severity of the disease is related to survival of the corn flea beetle, which vectors the causal pathogen, and which is adversely affected by cold temperature. Every year we make a prediction about how severe Stewart’s wilt will be by looking at the winter temperatures and using them to calculate flea beetle index values for several Ohio locations. The index is fairly crude but usually does reflect what we see in the field.
The index values for eleven Ohio sites in 2018 range from a low of 83 at Celeryville to a high of 101 at Piketon. The current winter was colder than the previous two winters (2017 and 2016) but not as cold as the two winters before that (2015 and 2014). Most Ohio sites fell in the disease-negligible category this year but there were several sites where wilt predictions are light to moderate, moderate to severe, and severe. Individual index values are shown in the chart below.
For a longer-term view, the 2018 values along with values from the previous 30 years are posted at a web site (https://cpb-us-west-2-juc1ugur1qwqqqo4.stackpathdns.com/u.osu.edu/dist/1/8311/files/2016/02/Corn-Flea-BeetleCumul2018r-1rcl18l.pdf ).
These days, most sweet corn hybrid seed is sold with insecticide treatment on the seeds; it can be difficult to find seed that is not treated. These insecticide seed treatments are effective at controlling the corn flea beetle on most hybrids. Systemic insecticide protection is provided on seed that has been commercially by Cruiser, Poncho, or Gaucho. Cruiser contains the active ingredient thiamethoxam (the same AI as in Platinum and Actara) and is made by Syngenta. Poncho contains the active ingredient clothianidin (the same AI as in Belay) and is made by Bayer. Gaucho contains the active ingredient imidacloprid (the same AI as in Admire) and is made by Bayer. Tests done at the University of Illinois when seed treatments were under development showed that incidence of Stewart’s wilt in susceptible varieties was reduced by about 70% by commercial seed treatment, and severity of symptoms was also reduced. Seed treatments are thus not products that alone will control corn flea beetle and Stewarts wilt.
For farms that are not planting insecticide treated seed, the cultural control of disease-resistant varieties should be used. Ratings for over 600 hybrids from Illinois as of 2010 are shown on a website (http://sweetcorn.illinois.edu/HybRxnSum/Hyb-Rxn-Summary-1984-2010.pdf). A few examples of hybrids that are most resistant to Stewart’s wilt are the Ambrosia and Nauset (bicolor se); Sumptuous, Merlin, and Miracle (yellow se); Argent, Celestial, and Denali (white se); Mirai 336BC, Obsession R, and Mirai 350BC (bicolor sh2); and Garrison, Overland, and SummerSweet 7650Y (yellow sh2).
If resistant varieties or commercially treated seed are not planted, it is important to protect seedlings of susceptible varieties from beetle feeding through the 7-leaf stage, especially on farms with a history of problems with this disease. An option is Latitude (imidacloprid plus fungicides), used as a hopper box seed treatment. Another option is systemic soil insecticide, Counter or Thimet, that can be applied to the soil at planting. A final option is to wait until seedlings emerge when they can be sprayed with Sevin, permethrin, or other non-systemic insecticide, but the foliar sprays are not usually as effective as the systemic seed or soil treatments.
-by Celeste Welty, Extension Entomologist