Soybean and corn disease update
Bacterial diseases in soybean and corn fields got a head start this growing season due to the wet spring and hail events occurring throughout South Dakota.
"The wet spring and also a few hail events have created conducive environment for bacterial diseases to develop in soybean and corn fields," said Emmanuel Byamukama, SDSU Extension Plant Pathology Specialist.
Several soybean fields inspected throughout the state had bacterial blight. Byamukama explained that bacterial blight is characterized by small water-soaked spots on the lower leaves. These spots later turn yellow then brown to black in the center, with a yellowish-green halo around the spots. The brown spots may coalesce into blotches that may tear and fall out after windy weather, giving the leaves a ragged appearance.
Cool (less than 80° F), wet weather favors the spread of the disease. Dry weather halts the spread of the disease up the plant.
"The bacteria survive on residue, therefore crop rotation and tillage may help reduce the severity of this disease," Byamukama said.
Goss's wilt and blight
Goss's wilt and blight were found in two counties: Faulk and Brown. This disease is caused by the bacteria, Clavibacter michiganesnsis subsp. nebraskensis. The pathogen causes two types of symptoms: systemic wilt of the entire plant and leaf blight.
The leaf blight symptom is the most encountered and is characterized by the dark spots that resemble freckles. The leaf blight lesions are large and longitudinal and can resemble other corn disease lesions like northern leaf blight. The presence of water soaked lesions and freckles are distinct symptoms for Goss's wilt.
"When the bacteria infects the vascular system, it blocks water-conducting tubes leading to wilting of the entire plant," Byamukama said.
The bacteria overwinter on infested corn residue on soil surface and enter the plants through wounds created by hail, sand blasting, high winds and wounds created by insect feeding. Goss's wilt can be managed by selecting corn hybrids that are tolerant to this disease. If the field has history of Goss's wilt, selection of resistant/tolerant cultivars is the first step.
"Because the Goss's wilt pathogen survives on residue, tillage and crop rotation will reduce the inoculum. Fields at high risk are corn following corn and no-till/minimum till fields," he said. "Some weeds like foxtail, shattercane, and barnyard grass are hosts of the bacteria; therefore, early weed control is important to eliminate further sources of inoculum."