Is Stewart’s Wilt a Game Changer for 2012?

Mild winter conditions could be a game changer when selecting varieties and seed treatments for sweet corn crops this year.  The corn flea beetle is a border-line pest for most parts of the province. Prolonged cold weather from December-March typically kills a large proportion of the over-wintering population. Even in South-western Ontario, outbreaks of this pest have been sporadic in the past. However, mild winter weather conditions could potentially result in higher populations this spring.

Corn flea beetles are small (2 mm), shiny black beetles that move very quickly when disturbed.  Adult beetles feed on corn plants causing small circular holes or elongated scratch marks on the leaves.  The beetles typically emerge in mid-spring and populations peak in late-June.  Feeding injury rarely causes economic losses; however corn flea beetles are the primary vector of the bacterial disease, Stewart’s wilt.

Stewart’s wilt is a potentially devastating disease of sweet corn. It can cause yield losses ranging from 10-100%, depending on the time of infection and the variety.  When susceptible varieties are infected prior to the 5-leaf stage; the plants become wilted, turn brown and die. Later infections cause pale green or yellow stripes on the leaves, running parallel to the veins.  These plants remain stunted and often fail to produce a marketable cob.

There is a wide range of varietal tolerance for this disease. Dr. Jerald Pataky, University of Illinois, researches disease susceptibility in sweet corn varieties.  There is an extensive summary of his research at http://bit.ly/swtcrn-var-tol.  This article summarizes disease reactions of 800 commercially-available and pre-commercial hybrids since 1984. It also includes information on varietal tolerance for other important sweet corn diseases and herbicides.

The highly mobile nature of the corn flea beetle makes in-season control using foliar insecticides very difficult.  Where susceptible varieties are being grown, insecticidal seed treatments are a valuable tool to prevent losses due to this disease.  Systemic insecticides such as Gaucho 480 FL, Cruiser 5FS and Poncho 600 FS, provide good protection from flea beetle feeding during the early stages of crop growth.

The actual impact of the corn flea beetle and Stewart’s wilt on the 2012 growing season is yet to be seen. The mild winter is a concern.  However, the risk of infection will also depend on; the population levels leading into fall 2011, spring soil conditions and heat accumulation during the early growing stages. Where possible, avoid growing susceptible varieties for early and mid-season plantings. Where susceptible varieties are the preferred option, consider using a systemic seed treatment to prevent infection.