Stalk rots are possible threat to corn

Sept. 10, 2020

Many corn fields were affected by gray leaf spot because of high temperatures and humidity. Leaf diseases predispose plants to stalk rots, so if a field had leaf diseases, it probably will have stalk rots… unless leaf diseases were aggressively controlled with fungicides.  The descriptions below can help identify problems.

Anthracnose leaf blight and stalk rot is caused by the fungus Colletotrichum Graminicola which overwinters in corn debris. In the leaf blight phase, it produces long, wavy lesions and can move to stalks. Microscopic spores give the rind a black shine like shoe polish, making this disease is easy to identify by shiny black spots near the base of plants. They eventually move up to several internodes. Affected stalks start to decay inside and eventually lodge. Premature plant death interrupts grain-fill and causes yield losses. It is one of the most important diseases in the Corn Belt.

Diplodia stalk rot is caused by the fungus Diplopia Maydis which also causes ear rot. Infections can occur within two weeks after silking. Ear rot usually starts at the base of ears and, eventually, the whole ear may rot. When husks are peeled back, a white mold is visible between kernel rows. Shanks and husks of affected ears look bleached and whitish. The base of the stalk produces small black dots called pycnidia. They’re embedded in the rind and can’t be scraped off.

Fusarium stalk rot is caused by the fungus, Fusarium Moniliforme. Gibberella stalk rot is caused by Gibberella Zeae. In the field, it’s difficult to distinguish between Fusarium and Gibberella. Both can infect ears and stalks. Pathogens overwinter in old corn roots and stalks. Ears are infected at the tips first. Pathogens attack the base of stalks through roots. Inside tissues and fibers deteriorate and tissue is soft and pink-to-reddish.

Charcoal rot is caused by the fungus Macrophomina Phaseolina which infects plants near maturity. It begins as a root-rot and moves to lower internodes. Stalks have black fungal bodies known as sclerotia that look like charcoal dust. Stalks die and lower internodes disintegrate.

Pythium stalk rot is caused by Pythium Aphanidermatum and other Pythium species. This disease occurs during hot and wet weather during the summer.  It usually infects a single internode near the soil line, which collapses.

To determine if stalks are severely infected, conduct the pinch or push test. If using the pinch test, simply pinch stalks one after another and determine how many are soft and have lost integrity. For the push test, push stalks to the side. If they bounce back, they’re still healthy. If they remain leaning or fall over, they’re likely infected with stalk rot.

For either test, sample 100 stalks in a row. Then reverse direction and test 100 stalks in the opposite row. For example, if 10 of 100 stalks fail the pinch or push test, then infection is 10 percent. Repeat the test at random locations within the field.