Tri-State Neighbor: South Dakota Farm Newspaper

In this update, we're looking at stalk rots. Fortunately, with a little extra effort and some keen observations, problem fields can be detected and management adjustments to reduce the impact on the bottom line can be made.

Conditions Leading to Stalk Rots

To compensate for reduced nutrient uptake, plants tend to cannibalize the roots and stalks for nutrients in order to fill the ear later in the season. This provides soil-borne stalk rots an ideal opportunity to infect the roots and ultimately, the stalks.

Areas with the most stress will see the biggest problems. Not only will weather and soil conditions affect this, but also leaf diseases such as gray leaf spot, leaf blights and corn rust can exasperate the problem. European corn borer, along with other tissue damaging insects, can contribute to the problem, as well as hail and lack of nutrients applied to the field, most notably, nitrogen.

Scouting Procedure

Scouting for stalk rots should be done at or before harvest. There are two common methods: the squeeze test (the hard way) and the push test (the easy way).

The squeeze test is done by squeezing the lower internodes of the corn stalk and checking for pith deterioration. If the stalk is easily compressed between the thumb and forefinger, stalk rot may be present.

The push test consists of pushing the stalk at ear height 5-10 inches from vertical. If the stalk breaks between the ear and the lowest node, stalk rot is likely present.

You may also want to split some stalks to look at the stage of pith disintegration, discoloration and other symptoms that aid in the identification of the disease present.

Make sure you check several areas in the field and enough plants to be confident in your findings. Five locations with 20 plants per location should be sufficient. Observing 25 percent or better of the plants with stalk rot is an indication that early harvest is warranted.

Diagnostics General Symptoms

The first symptom you may see is discoloration of the plants, not unlike frost damage. Plants die prematurely and early development of these diseases may lead to poor grain fill and chaffy ears. The lower stalk may turn tan to dark brown and the pith disintegrates.

Stalks may break, lodge or drop ears, which leads to harvest losses. You may see several symptoms in one plant, which is relatively common. It is more important to identify stalk rot, not the specific strain, as the disease management for all strains is the same. Also, visual symptoms can be confusing, inconsistent and nonexistent or in a complex that defies singular identification.

Send a sample to the South Dakota State University Plant Clinic to have a specific pathogen identified.

Anthracnose - a shiny black discoloration of the stalk is characteristic of this disease. The discoloration may be in the form of streaks, spots or large blotches. The pith will disintegrate and turn a dark brown color. A strong connection exists between corn borer tunneling/feeding and Anthracnose stalk rot infections. This disease may be associated with Anthracnose leaf blight. The disease is favored by excessive rainfall in August and September and above normal temperatures.

Diplodia - both ear and stalk, historically, this is our most common and important stalk rot. It is related to soil or root injury issues, like compaction, nutrient losses, “wet feet” and rootworm feeding. Later development of the disease shows raised black specks on or near the internodes that cannot be scraped off with the thumbnail. Wet and warm conditions also favor Diplodia. On the ear, it will appear as a white mold starting at the base and working up causing significant quality loss.

Gibberella - both ear and stalk. There may be black specks on the stalk that are easily scraped off. A pink to reddish rot disintegrates the pith and may be readily visible in nodal and root tissue. The reddish color may not be diagnostic since other secondary fungi also exhibit that same discoloration. This is favored by wet and warm conditions. On the ear, it will be a pinkish mold.

Fusarium - both ear and stalk. There may be a pink discoloration of leaf sheaths. To further confuse things, there may be a pink discoloration to the inner stalk tissue. This is a tough one to identify in the field. Again, this disease is favored by warm, wet weather late in the season.

Charcoal Rot - diseased stalks may show grayish streaking on the surface of the lower internodes. The disintegrated pith will have a grayish-black coloration as if the pith was sprinkled with charcoal. This disease differs from the others by being favored by hot, dry conditions and high soil temperatures late in the growing season.