Beans: Learn how to combat collar rot.

The rot of the colon in bean It is a soilborne disease caused by the fungus Sclerotium rolfsii and can cause rapid plant death and severe stand failures, especially in hot and humid soil conditions. The risk requires special attention between February and May in many producing regions, with integrated management from area selection to crop monitoring.

According to data released by Embrapa in technical publications on bean management, collar rot is caused by a fungus capable of surviving for long periods in the soil through resistant structures called sclerotia. The pathogen mainly attacks the collar region, at the transition between the root and the aerial part of the plant. The infection can also spread to the roots and stem near the soil.

Sclerotium rolfsii is considered a polyphagous pathogen because it can develop on different plant species of agricultural importance. This characteristic makes strategies based exclusively on crop rotation difficult. The sclerotia remain viable even in the absence of the bean plant and represent an important source of inoculum for the following crop.

The cycle begins with the survival of sclerotia in the soil or on crop residues. Under favorable conditions of moisture and temperatures around 25°C to 30°C, these structures germinate and produce mycelium. The mycelium can grow on crop residues, weeds, and fresh organic matter. From these materials, the fungus reaches the base of bean plants.

According to data released by Embrapa, the infection occurs mainly in the contact area between the plant and the soil, where the pathogen penetrates and degrades the tissues. As the disease progresses, the plants wilt, dry out, and may die. New sclerotia form on the necrotic tissues and return to the soil, restarting the cycle.

The disease usually appears initially in patches, with groups of plants showing sudden wilting. Another sign is the darkening and rotting of the collar area. Depending on humidity conditions, the tissues may appear waterlogged or dry.

On humid days, dense, cotton-like white mycelium can be observed at the base of the plants and in the soil near the collar. Small spherical sclerotia may also appear. Initially white, they turn yellowish or brownish and may adhere to the collar tissues or remain on the soil. As the roots and basal region are compromised, leaf drop and complete plant death occur. Dead plants can be easily pulled up due to the damage caused to the root system.

Between February and May, many producing regions experience conditions favorable to the development of the disease, with high temperatures combined with frequent rainfall or intensive use of irrigation.

This period may also coincide with the planting or development of late-season or second-crop beans, as well as a greater presence of crop residues left by previous crops, such as soybeans and corn. The warm, humid soil favors the germination of sclerotia and the growth of the mycelium of S. rolfsii. Therefore, crops grown during this period require attention from area planning to adjusting irrigation and plant density.

The losses caused by the disease vary according to the inoculum pressure in the soil, environmental conditions, and the susceptibility of the cultivar. In situations of high infestation and favorable climate, plant death reduces the stand and causes gaps in the planting row. The reduction in the number of plants also impairs row closure and light interception by the crop. When the disease occurs at stages such as flowering and grain filling, plant death directly interferes with pod formation and final productivity.

Furthermore, the producer may experience reduced efficiency in the use of fertilizers, irrigation, and pesticides, since some plants fail to complete their life cycle. In areas with a recurring history, the problem can also increase production costs due to the need for changes in the cultivation system, residue management, irrigation adjustments, and possible fungicide applications.

According to data released by Embrapa in technical recommendations for bean cultivation, the phytosanitary history of the area should be considered before planting. Plots with frequent occurrences of collar rot or other soil diseases require greater attention. Acidity and fertility correction should be based on chemical analysis. The goal is to promote root development and plant vigor. It is also important to avoid areas with waterlogging, poor drainage, or subsurface compaction. These conditions can increase water retention at the collar and favor disease.

Crop rotation is one of the strategies that can be integrated into the management of collar rot. When possible, it is recommended to introduce crops less susceptible to S. rolfsii for more than one growing season, helping to reduce the inoculum in the soil over time. However, this practice needs to consider the fungus's ability to infect different plant species.

Continuous successions of highly susceptible crops, especially legumes, should be avoided in areas with a history of the disease. Weed control is also important, as host species can serve as a substrate source for the fungus and contribute to maintaining the sclerotia population.

Managing crop residues from the previous crop also affects the environment surrounding the plants. In areas with a high incidence of the disease, it is advisable to avoid the accumulation of very large and poorly distributed crop residues along the planting row. Uniform straw distribution can be improved by adjusting the harvesters used in previous crops.

Another point of attention is the excessive presence of fresh organic matter in direct contact with the seeding row, a condition that can favor the development of the fungus. According to data released by Embrapa, the history of crop residue and diseases in the area should be considered, especially in no-till planting systems.

The use of certified seeds with high physiological and sanitary quality contributes to rapid and uniform emergence. Seed treatment with fungicides registered for bean cultivation can also be part of preventive management. Application must strictly follow label and package insert recommendations and the agronomic prescription. The goal is to protect seedlings against early soil pathogens and promote crop establishment.

Water management is another important aspect of controlling collar rot. Irrigation rates and intervals should be adjusted to avoid excess water in the soil, especially during warmer periods. Applying very high volumes at once can cause soil saturation near the plant collar. In certain situations, dividing the water application into smaller applications may be more appropriate. Moisture can be monitored using tensiometers, sensors, or careful manual assessment. The goal is to avoid both waterlogging and prolonged periods of drought that lead to the need for heavy corrective irrigation.

Adjusting plant population helps prevent excessive row closure. Very high populations can reduce air circulation and increase relative humidity near the soil. Therefore, density should be appropriate to the cultivar, sowing time, and local conditions. Choosing materials adapted to the region and planting season, with good initial vigor and a well-developed root system, is also part of preventive planning.

The use of fungicides should be evaluated based on the area's history, the risk of occurrence, and symptom monitoring. According to data released by Embrapa in technical publications on integrated management of bean diseases, chemical measures should be integrated with other strategies and not used in isolation. When technically indicated, control may involve localized soil treatments in the planting row or targeted applications at the base of the plants.

The product selection should only consider fungicides registered for bean cultivation and for the target crop, following the dosage, timing, application interval, maximum number of applications, and other recommendations outlined in the product label. The agronomic prescription issued by a qualified professional must be followed.

During the period from February to May, crop inspections should be intensified, especially in areas with a history of the disease. Inspections are important after periods of heavy rain or irrigation. When finding suspected plants, the producer can carefully check the collar region and adjacent soil for white mycelium and sclerotia. The location of the affected areas should also be recorded. This information helps to monitor the evolution of the problem and plan management measures for future crops.

Bean stem rot requires a combined strategy. Key measures include proper site selection, crop rotation, weed management, uniform distribution of crop residues, and the use of quality seeds. Adjusting plant density and irrigation also helps reduce conditions favorable to fungal development. In high-risk areas, monitoring should guide any necessary localized chemical interventions, always with technical supervision.

Complementary cultural measures include avoiding mechanical damage to the root collar caused by cultivation operations, poorly conducted weeding, or the passage of machinery on very wet soil. In areas with severe outbreaks, removing highly affected plants to reduce local sclerotia production may also be considered. The adoption of this measure should take into account the cost-benefit ratio in each situation.

Collar rot exhibits a cumulative effect because the pathogen can survive in the soil through sclerotia. Therefore, decisions made in one crop season can directly influence the risk of its occurrence in subsequent seasons.

According to data released by Embrapa in technical materials on integrated disease management in beans, long-term planning should include area selection, crop rotation, moisture management, residue control, and weed control. The strategy aims to reduce the amount of inoculum and avoid environmental conditions that favor infection.

Before and during cultivation, the producer must check the area's history and plan crop rotation with less susceptible species. It is also necessary to correct soil and drainage limitations, use good quality seeds, and apply registered fungicide treatment when indicated. Plant density should be adjusted to avoid excessive canopy closure, while irrigation management must prevent excess moisture near the base of the plant.

Another important consideration is the proper distribution of crop residues and weed control. Finally, the crop should be monitored regularly to identify affected areas and, with technical guidance, determine if localized chemical interventions are necessary.

All decisions involving fungicides must follow an agronomic prescription issued by a qualified agricultural engineer. The producer must strictly adhere to the label and package insert instructions, including dosage, application time, safety interval, and application technologies. The use of personal protective equipment is mandatory during the handling and application of pesticides, in accordance with current legislation.

According to data released by Embrapa in technical publications on crop management, chemical measures should be part of an integrated control strategy, associated with cultural practices and monitoring. Bean stem rot has complex management, but the combination of area planning, crop rotation, weed control, residue management, seed quality, adequate irrigation, and crop monitoring can reduce risks and limit damage.