Beans: Irrigation can reduce the risk of stem rot.

Gray stem rot in bean Attention must be paid to irrigation management, especially between February and May, when many crops go through reproductive phases under high temperatures and periods of water deficit. Balancing the amount of water, frequency, and timing of irrigation helps reduce plant stress and disease severity.

Gray stem rot is caused by the fungus Macrophomina phaseolina, which thrives in warm environments with low water availability in the soil.

The pathogen survives in the soil and crop residues by means of resistant structures called sclerotia. These structures can remain viable for several years, awaiting favorable conditions to infect new roots.

Between February and May, many bean crops go through flowering, pod formation, and grain filling. During this period, high temperatures, irregular rainfall, and irrigation management failures can increase plant stress.

Plots with long periods between irrigations, poorly distributed water distribution, or episodes of waterlogging may also show a higher proportion of plants with symptoms, especially in sandy or compacted soils.

Gray stem rot is a soilborne disease that primarily affects the root system and the base of the stem. Macrophomina phaseolina has a wide host range, including soybeans, corn, sorghum, and cotton. The presence of these crops can favor inoculum accumulation in areas of intensive agriculture.

Infection occurs primarily through the roots, in micro-wounds or at the points where lateral roots emerge. Plants subjected to water stress exhibit greater susceptibility and a reduced capacity to react to infection.

After colonizing the tissues, the fungus can affect the vascular system, hindering the transport of water and nutrients. When the soil has low moisture, the wilting symptoms can intensify.

Sclerotia formation also increases in senescent tissues and in plants subjected to stress conditions such as drought, heat, and low fertility.

Water management is directly related to the level of water stress faced by the common bean plant. The disease tends to be more severe when plants experience moderate to severe water stress, especially during the reproductive phases.

Lack of water can reduce root growth and depth, concentrating them in the surface layers of the soil, where temperatures may be higher and water availability lower.

The deficiency also reduces the plant's turgor and defense capacity, in addition to promoting premature senescence of fine roots.

As a result, plants subjected to stress become more susceptible to colonization by the fungus and may more rapidly develop symptoms such as wilting, darkening at the base of the stem, and premature death.

Blade thickness and frequency need to be avoided at extremes.

Both a lack of water and an excess of water can hinder the management of gray stem rot.

According to data released in the technical material presented, insufficient water application and excessively long intervals between irrigations cause cycles of soil drying, increase the temperature in the root zone, and intensify water stress.

At the opposite extreme, excessively high water levels and short intervals can lead to episodic waterlogging. Although these conditions temporarily reduce the water deficit, they impair soil aeration and can cause root death due to lack of oxygen.

When the soil dries out again, these dead roots can serve as a substrate for fungal colonization.

Therefore, management should aim for balance, avoiding both prolonged periods of drought and waterlogging.

The way water is distributed over time affects the conditions of the root zone. More frequent irrigation, with moderate amounts of water, tends to reduce soil temperature fluctuations.

Irrigation that is too infrequent favors an increase in temperature during dry periods, a condition that can benefit M. phaseolina. Maintaining soil moisture in an intermediate range, close to field capacity, helps to simultaneously reduce thermal and water stress.

When heat, low water availability, and the presence of inoculum combine, gray stem rot can compromise crop development. Impacts include reduced effective plant stand, with plant death during pod filling, as well as a reduction in the number of pods per plant and grain weight. The disease can also cause uneven ripening, making it difficult to determine the ideal harvest time.

Losses vary depending on factors such as cultivar, soil, climate, and area history. The association between water stress and the presence of M. phaseolina can result in significant productivity losses.

The decision on when and how much to irrigate should not depend solely on a fixed schedule. Management should consider soil, plant, and climate indicators. Crop evapotranspiration can be used to estimate the amount of water consumed. Visual assessment of the soil, considering characteristics such as texture, color, and cohesion, can also contribute to decision-making.

When available, moisture sensors allow for more precise monitoring of soil moisture levels.

In the plant, signs such as slight wilting during warmer periods, partial stomatal closure, and loss of leaf shine can indicate the beginning of a deficiency. The better the monitoring, the lower the risk of subjecting the crop to repeated cycles of intense drought, especially between flowering and grain filling.

During the period from February to May, many crops enter or remain in sensitive reproductive stages. The priority should be to avoid significant water deficits during flowering and pod formation.

It is also important to reduce abrupt fluctuations in soil moisture during grain filling and to avoid waterlogging near physiological maturity. Management needs to consider rainfall forecasts, soil type, and the irrigation system used.

Since soilborne diseases are difficult to control once established, prevention plays a central role. Soil correction and preparation should promote structure and aeration, as well as reduce compaction problems.

Water application planning needs to consider the crop's water demand and the soil's water retention capacity, avoiding both excess and prolonged deficit. Maintaining mulch and soil cover can also reduce temperature fluctuations and surface evaporation, helping to decrease water stress.

Water management alone does not eliminate gray stem rot. Irrigation must be combined with crop rotation, the use of more tolerant cultivars when available, balanced nutritional management, and proper management of crop residues.

Crop rotation can help reduce the accumulation of inoculum in the soil. More tolerant cultivars may also have a greater capacity to withstand stress and infection by the pathogen.

Balanced nutrition also contributes to greater plant resilience, while proper crop residue management can reduce the amount of sclerotia returned to the soil.

Although the focus of management is on irrigation and prevention, complementary phytosanitary interventions should follow technical recommendations. Any use of chemical or biological products must respect the label, the package insert, and the agronomic prescription issued by a qualified professional. Correct diagnosis of the disease is fundamental to defining the management strategy.

Only products registered for the crop and target should be used, in accordance with current legislation, in addition to the necessary personal protective equipment during applications. Between February and May, water management can be crucial to reduce stress on the bean plant and limit the severity of gray stem rot.

The producer should check the area's history, monitor soil moisture, and avoid prolonged drought cycles, especially during flowering and grain filling.

The recommendation is to prioritize more frequent irrigation with moderate water depths, avoiding both excessive drying and waterlogging. Irrigation should also be integrated with crop rotation, balanced nutrition, crop residue management, and other phytosanitary practices.