Stewart’s Wilt disease is caused by the bacterium Pantoea stewartii, which survives the winter in the digestive system of the corn flea beetle. Upon emerging from protected overwintering sites, the beetle spreads the disease to corn seedlings (plants less than one foot tall) in the spring and summer. The adult corn flea beetle passes the winter hidden under leaf litter and other plant debris along the hedgerows, treelines and margins of fields. Further south, Stewart’s wilt is a common disease due to the mild winter temperatures which allow high numbers of corn flea beetles to survive. If the winters are exceptionally warm, Stewart’s wilt may become a problem on sweet corn as far north as southern New England.
Stewart’s wilt is usually absent in years when the sum of the mean temperature for the months of December, January and February are below 90°F. Connecticut growers see some infected plants when the index is between 90°F and 100°F. (See Table 1) The last time this occurred in Connecticut was in 1991. I was working with 10 sweet corn growers on the UConn IPM program. Notes from my scouting records for June 1991 indicate that most growers experienced losses due to Stewart’s wilt that ranged between 8% to 70% of the plants in some blocks, depending upon the tolerance of the variety, and if it was at a susceptible stage when the flea beetle generations occurred. Since all Stewart’s wilt infected corn either dies or fails to produce a marketable ear, that equates to an average yield loss of approximately 20% to 25% for susceptible varieties.
Stewart’s wilt first appears as long yellow strips that extend the length of the leaf on plants in the early-whorl to pre-tassel stage of growth. The yellowing is caused by the bacterium accumulating and clogging the vascular system of the plant which interrupts the upward flow of water and nutrients. Infected plants appear stunted and wilted. The yellowed tissue dies within days or a week to form large stripes of dead (necrotic) tissue. The plant often succumbs to the disease soon after leaves show necrotic lesions. A good field test to confirm the presence of the disease, involves making cross-wise cuts through the stems at the soil line of a few plants showing yellow leaf discoloration. The presence of a brownish cavity and soft rot in the pith indicate a severe stem infection due to Stewart’s wilt.
Floating row covers used to enhance crop earliness, prevent the transmission of the disease by excluding the corn flea beetles from the plants during the seedling stage.
Many varieties are now available with a range of resistance to Stewart’s wilt. (See Table 2) You can also scout for the beetles between the spike and early-whorl stage of development. The beetles enter the fields during the day and migrate out for the night, so higher numbers may be found on plants closer to the borders. They prefer to fly and feed in warm, sunny weather, so spring scouting should be limited to the early afternoon when temperatures are the warmest. Keep the sun in front of you so that your shadow does not scare the beetles before you are close enough to see them. The beetles will hop from the plants (like fleas) as you approach, so count the beetles on plants several feet in front of you. Check 100 plants in groups of 20 at several locations around the field. If 5% of the plants have flea beetles on them, a couple of insecticide applications at approximately five-day intervals may help minimize disease transmission. Refer to the New England Vegetable Management Guide for effective flea beetle insecticides.
|Table 1. Mean Monthly Temperatures for Connecticut in 1997-1998|
|Bradley Airport / Windsor Locks||Mount Carmel / Hamden||Research Farm / Storrs|
By Jude Boucher, Vegetable Crops IPM Program Leader, University of Connecticut. Updated 2007. Reviewed 2012.
Originally published in Grower, New England Vegetable and Small Fruit Newsletter, Vol. 98-3. March 1998. p. 1-2.
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