By Alison Robertson, Department of Plant Pathology
Infection by a number of pathogens is favored by flooded conditions. Excess soil moisture and anaerobic soil conditions also favor the development of certain diseases.
Over the past week, I have received a number of reports of crown, mesocotyl and root rot of corn. Although root rots of corn occur to some extent every year, under wet conditions, they can cause economic losses.
Oxygen deficiency as a result of flooded conditions predisposes plants to infection by root rot pathogens (Pythium graminicola, Fusarium spp. and Exserohilum pedicellatum) that thrive in these conditions. Root rots develop further into mesocotyl and crown rots under wet conditions and thereby make a bad situation worse, especially if this occurs prior to V6. Seed treatment trials completed in 2007 by my graduate student showed a positive correlation between the incidence of mesocotyl rot at V4 and crown rot at V6.
Remember, prior to growth stage V6, a healthy kernel, seminal root system and mesocotyl are vital for seedling survival. This is because a developing corn seedling relies on the kernel endosperm and seminal roots for nourishment until the nodal root system has fully developed, usually around the six-leaf stage.
Thus, the mesocotyl acts as the “pipeline” for translocation of nutrients from the kernel and seminal roots to the seedling stalk and leaf tissues. Therefore, as the flood waters recede and you start to assess plant stands, it is important to dig up seedlings every now and then to check below ground, as well as crown health.
Pythium and Bacterial Stalk Rot
Pythium stalk rot and bacterial stalk rot are favored by warm, wet conditions. Unlike most other stalk rots that occur after tasseling, these stalk rots can occur at any time during the season. Symptoms caused by both pathogens are similar: the rind and pith of the first internode above the soil becomes soft, brown and water soaked (as seen in the photo below). Affected stalks often twist and fall over. A foul odor is associated with bacterial stalk rot.
Pythium stalk rot in a corn plant
Infection by the crazy top pathogen (Sclerophthora macrospora) occurs during periods of flooding when infective spores are produced. This fungus survives as oospores in the soil. Under saturated conditions, oospores germinate to produce sporangia within which are borne zoospores. Swimming zoospores infect the growing point of young corn plants. The most characteristic symptom of crazy top is a leafy proliferation instead of a tassel. Other symptoms include short or long internodes, narrow, strap like leaves, excessive tillering and compete lack of ear formation.
Corn plant exhibiting symptoms of crazy top
The frequent rains we have had have also been favorable for sporulation and dispersal of the anthracnose fungus Colletotrichum graminicola. There have been a few reports of symptoms of anthracnose leaf blight starting on the lowest leaves of corn seedlings, particularly in corn-following-corn fields.
Corn plant showing symptoms of anthracnose leaf blight
At this stage of the growing season it is difficult to predict if other common foliar diseases (gray leaf spot, common rust, northern corn leaf blight) will be a problem on corn this year. Foliar disease severity depends on numerous factors, the most important being weather and hybrid susceptibility. If the weather continues to be wet, there is increased risk of disease especially since planting corn was delayed this spring, and the cool condition thus far has slowed crop development.
Alison Robertson is an assistant professor in plant pathology with research and extension responsibilities in crop diseases.