Soybean and corn disease update

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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."

Some products are being marketed for Goss's wilt control; however, Byamukama said there is insufficient data in the region on the efficacies of these products.  

Holcus spot

Holcus spot, another bacterial disease was seen in one corn field in Brown County, the same field also had Goss's wilt. Holcus spot is caused by a different bacteria, Psedomonus syringae pv. sringae. Holcus spot is a rare disease and even when it occurs, seldom will it cause yield loss. Byamukama explained that the pathogen also survives on residues and is favored by rainy and windy weather early in the season.

Common smut and rust

Common smut on corn leaves was seen in a few fields that had hail damage in the northeast counties. The fungal pathogen infects young, actively growing parts usually through wounds and forms galls. The fungus survives on crop debris or soil and can remain viable for several years. If the spores land on the silk, the fungus will infect the developing kernels resulting in galls on ears.

"Most corn hybrids have good resistance to common smut, however, corn on corn, no-till, and hail damage conditions may increase the risk for common smut infection on leaves," Byamukama said.

Another disease beginning to develop on corn is common rust. Several fields scouted had trace levels of this rust developing.

"This disease rarely develops to high levels to cause yield loss because most hybrids have good tolerance to this pathogen. The common rust pathogen does not survive in South Dakota; the spores are blown in from the southern states in spring. . Therefore residue management or crop rotation will not affect common rust or any other rust disease, for that matter", he said

If growers plan to apply fungicide, Byamukama said a general note on fungicide application on corn is that several research reports show that increase in yield from fungicide application happens when disease severity on flag leaf at R5 is greater than 5 percent. He encourages growers to review a publication published by Iowa State University, at see: http://www.extension.iastate.edu/CropNews/2010/0706muellerandrobertson.htm

"Most of the corn scouted across the state looked very clean with no disease developing. Corn following corn or corn on no-till may have an elevated risk for significant disease to develop, depending upon the cultivar planted and weather conditions," he said. "Applying a fungicide at tasseling in this case may be beneficial."  

The corn plant pathology working group published a list of fungicides that are effective for several fungal pathogens on corn. This table can be found here: http://www.uky.edu/Ag/GrainCrops/Briefs/CDWGCornFungicideEfficacy_Table_2013_FINAL.pdf

Byamukama said fungicide application should be done when all corn has fully tasseled to avoid arrested ear syndrome, a physiological disorder that is caused by nonionic surfactant (NIS) fungicide additives when applied before tasseling. To learn more about this visit, www.extension.purdue.edu/extmedia/BP/BP-85-W.pdf. 


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