Rhizoctonia ear rot

The primary symptom of Rhizoctonia ear rot is the development of a white to cream-colored fungal mycelium on the surface of the corn ear husks. As the infection progresses, this mycelium may darken and become more dense.

A distinctive feature of this disease is the formation of small, hard sclerotia on the husks and between the husks and the ear. These structures are typically dark brown or black and look like small, irregular nodules.

When the husks are pulled back, the kernels often show signs of discoloration, becoming dull and greyish. In severe cases, the kernels appear rotten, brittle, and are easily crushed, showing a clear breakdown of the plant tissue.

The fungus produces a web-like mat of mycelium that binds the husks tightly to the ear. This characteristic matting prevents the ear from drying naturally and promotes further decay of the corn kernels.

Advanced stages of the disease can result in the entire ear being covered by fungal growth, leading to significant structural damage and loss of grain quality throughout the ear.

The causative agent of this disease is the fungus Waitea circinata (the anamorph stage is Rhizoctonia zeae). It is a soil-borne pathogen that survives as sclerotia or mycelium in crop residues and soil.

This fungus belongs to the class Basidiomycetes and is known for its ability to persist in the environment for several years. It has a broad host range, which contributes to its survival in agricultural ecosystems.

Infection is primarily initiated by the fungus splashing from the soil onto the plant during rainy weather. Once established, the fungus spreads rapidly throughout the ear under favorable moisture conditions.

The biology of the pathogen is characterized by aggressive colonization of host tissues. The sclerotia of the fungus are highly resistant, allowing the pathogen to withstand harsh winter or dry conditions until the next season.

Research indicates that the fungus can also infect roots and stalks, making it a persistent and multifaceted threat to corn production in many geographical areas.

Rhizoctonia ear rot thrives in environments with high humidity and moderate to warm temperatures. Extended periods of rain during the silking and grain-filling stages are the most critical factors for disease outbreaks.

Temperatures ranging from +20°C to +30°C are optimal for the rapid development of the fungus. Within this range, the mycelium grows aggressively, colonizing the ear tissues and husks effectively.

Monocropping of corn significantly increases the risk of infection. By growing corn in the same field repeatedly, farmers allow the inoculum levels of Waitea circinata in the soil to reach levels capable of causing severe damage.

Dense planting patterns can exacerbate the problem by reducing airflow and keeping the lower parts of the corn plants, including the ears, moist for longer durations after precipitation.

Physical injury to the corn ear, often caused by insects like the corn borer, provides an easy entry point for the pathogen, significantly increasing the severity of the infection.

The primary impact of Rhizoctonia ear rot is a substantial reduction in both grain yield and quality. Infected kernels are lighter in weight and have poor nutritional value, which negatively affects market price.

Grain affected by this fungus is prone to further deterioration during storage. High levels of infected kernels can lead to the heating of grain in silos and increase the risk of contamination with harmful mycotoxins.

The disease reduces seed vigor and germination rates, making infected grain unsuitable for use as seed for the following season, which can lead to poor plant stands.

Economic losses are compounded by the increased costs associated with cleaning and drying the grain, as well as the need for fungicide applications during the growing season.

In cases of early-season infection, the entire ear can be lost, rendering the affected stalks economically unproductive and contributing to significant financial losses for the grower.

Crop rotation is the most effective management strategy to disrupt the life cycle of Waitea circinata. Rotating corn with non-host crops reduces the population of the pathogen in the soil over time.

Deep tillage helps to bury crop residues, which facilitates the decomposition of the fungal sclerotia by soil microorganisms and reduces the primary inoculum source for the next season.

Choosing hybrids with high levels of resistance or those known to have tight husk coverage can provide better physical protection against fungal infection during the sensitive development stages.

Fungicide applications during the flowering stage may be used in high-risk areas if weather conditions favor disease development, though they must be applied at the correct time to be effective.

Managing insect pests that damage ears is essential for disease control. By keeping insect populations low, growers can minimize the number of open wounds that the fungus uses to gain access to the kernels.