By Emmanuel Byamukama
If growers are seeing stunted yelling corn in patches, it could be due to corn nematodes, said Emmanuel Byamukama, Assistant Professor & SDSU Extension Plant Pathologist.
"Several plant parasitic nematodes infect corn leading to reduced plant vigor, stunted growth and yield loss," he said.
Plant parasitic nematodes are microscopic worm-like organisms that live in the soil. They have a "feeding straw-like" structure called a stylet that they use to injure the plant roots and suck nutrients from the plant cells.
"Some of the nematodes feed from the outer surface of the root without entering the root (ectoparasites), whereas other types enter the root and feed from within the root (endoparasites)," Byamukama explained. "Infected roots have reduced water and nutrient uptake and wounds created by nematode feeding can be entryways for fungal pathogens."
Byamukama added that nematodes, in general, are slow movers.
"They are spread through tillage and water movement within the soil," he said. "This is the reason corn plants with severe nematode infection appear in patches (Figure 1)," he said.
Nematode infection in corn usually goes unnoticed or can be mistaken for other diseases such as root rots or nutrient deficiency. Yield loss due to nematode infection can still occur without necessarily observing above ground symptoms.
Sampling for Corn Nematodes
The first step to effective nematode management, Byamukama said is diagnosing the type and density of corn nematodes in the soil.
"Since corn nematodes can be inside the root and also on the surface of the root, diagnosis of these nematodes requires sampling both soil and corn roots," he explained.
For fields suspected to have corn nematodes, four to six plants should be carefully dug out without injuring the roots when corn is still young (before V6).
The stalk can be cut off and only the root mass sent to the Plant Diagnostic Lab: SDSU Plant Diagnostic Clinic, SPSB 153, Box 2108, Jackrabbit Dr., Brookings SD 57007.
To increase chances of determining if nematode infection is causing the symptoms being observed, Byamukama encourages growers to collect another set of four to six plants from parts of the field with no symptoms.
To sample the soil, use a probe or a shovel to obtain 20 cores of soil between 6 to 8-inches within the root zone.
Soil from non-symptomatic areas should be collected separately to determine population densities in symptomatic and non-symptomatic areas.
Up to 2 cups of soil for each location within a field can be mailed or dropped at the SDSU Plant Diagnostic Clinic.
Corn Nematode Management
Every corn field may, to some extent, harbor corn nematodes.
"What determines the need to apply corn nematode management practices is the type and density of nematodes infecting corn in a given field," Byamukama said.
For instance, he explained, the threshold for needle nematode is 10 nematodes/ 100 cubic centimeters of soil whereas for spiral nematode, the threshold is 1000 nematodes.
"That is why it is important to have the soil and corn plants tested in the lab to determine the type and density of different nematodes infecting corn," Byamukama said.
The most common management practice is crop rotation. However, some the nematodes that infect corn can also infect other crops such as soybean.
"Therefore, this practice alone may not be effective against certain nematodes that have a wide host range," Byamukama said.
Nematicide seed treatments are another corn nematode management practice.
The commercially available nematicide seed treatments include Aveo, Avicta Complete, Nemastrike, and Poncho Votivo.
Figure 1. Stunted and yellowing corn plants in a patch.