Beans: crop rotation helps control nematodes

The lesion nematode, Pratylenchus brachyurus, is one of the main phytosanitary problems in... bean and can compromise root development and crop productivity. Present in virtually all producing regions and with a wide range of hosts, the organism requires medium- and long-term planning, with crop rotation among the main strategies to reduce its population in the soil.

Beans are cultivated practically year-round, with different sowing times and production systems. This characteristic exposes the crop to varying conditions of temperature, humidity, and crop rotation, favoring the occurrence of plant-parasitic nematodes.

Among the main problems is Pratylenchus brachyurus, a migratory species that attacks the roots and causes lesions capable of compromising the absorption of water and nutrients. The nematode can also favor the entry of opportunistic fungi and bacteria into the root tissues.

In densely populated areas, bean plants may exhibit uneven development, yellowed leaves, wilting on hot days, and darkened, short, and poorly branched roots. These symptoms can be mistaken for nutritional deficiencies or localized water shortages, delaying diagnosis.

Because it remains in the soil and uses different species as hosts, the nematode population does not decrease immediately. Therefore, management needs to be planned over several cycles, combining crop rotation with other strategies.

Pratylenchus brachyurus lives inside and outside roots and feeds on the cells of the cortex. Unlike root-knot nematodes, it does not cause the formation of nodules, but causes necrotic areas distributed along the roots.

The process begins when juveniles and adults penetrate young roots, primarily in the elongation region of fine roots. The organism perforates the cells, feeds on the cellular contents, and migrates through the tissues.

With the death of the cells, darkened and necrotic areas appear. The females lay eggs, which give rise to new juveniles and continue the cycle. Part of the population remains in the roots, while another part returns to the soil and can reach new roots of the same crop or the next crop.

The lesions reduce the volume of functional roots and impair the absorption of water and nutrients. They can also create entry points for root pathogens, such as Fusarium and Rhizoctonia, worsening the damage and reducing plant vigor.

The impact on productivity varies depending on the initial nematode population, the susceptibility of the cultivar, soil and climate conditions, and the history of host crops in the area.

Among the effects are reduced vegetative growth, decreased leaf area, fewer pods and grains per plant, and the formation of smaller grains. In more severe situations, stand failures or plants with low grain formation may also occur.

The presence of nematodes can further increase the susceptibility of beans to drought periods, especially in sandy soils or soils with low water retention capacity.

In addition to reduced productivity, the problem has an economic impact by increasing the need for inputs and corrective operations. In areas managed in monoculture or in succession with host species, the nematode population tends to increase over the years.

Crop rotation is one of the main strategies to interrupt this process. The goal is to replace highly susceptible crops with less susceptible or non-host species for one or more growing seasons.

By reducing the availability of food for P. brachyurus, the strategy can cause a gradual reduction in the population with each cycle. With a lower nematode load in the soil, subsequently cultivated beans tend to find more favorable conditions for developing their root system.

However, planning needs to consider the production calendar throughout the year. Harvest and off-season crop windows, irrigated or rainfed systems, and models such as no-till farming, conventional cultivation, and crop-livestock integration all influence crop selection.

The first step is to diagnose the area. Collecting soil and root samples for nematological analysis allows us to identify which species are present and determine the population density.

The diagnosis also shows whether P. brachyurus is predominant or if there is a combination with other important species, such as root-knot nematodes.

With this information, the technician can assess the risk to the bean crop and determine the intensity of management actions. In areas with very high populations, it may be necessary to use less susceptible species for more than one cycle.

To plan crop rotation, it is important to differentiate crops according to the nematode's ability to multiply. Good host plants allow for high reproduction rates and can rapidly increase the population in the soil.

Poor or less susceptible hosts allow for limited multiplication, while non-hosts do not serve as a food source and favor population reduction throughout the cycle.

The reaction of each species can vary depending on the cultivar and the soil and climate conditions. Therefore, the choice of crops should consider results from regional trials, technical bulletins, and research recommendations.

Simply alternating crops does not guarantee control. Many annual crops used in succession with beans can be good hosts for P. brachyurus. If two susceptible species are used in sequence, the nematode population may remain high.

Green manures and cover crops can be integrated into the system as long as they have a low capacity for nematode multiplication. Furthermore, these species can contribute to biomass production, improved soil structure, and increased organic matter content.

One example of planning begins with an area that has a history of bean cultivation in succession to potentially susceptible crops and a high population of P. brachyurus.

In the summer crop season, the strategy may prioritize a crop that is less susceptible or not a host, using cultivars that have been previously evaluated for their reaction to the nematode.

During the off-season, a green manure or cover crop with low organism multiplication capacity can be included. In addition to contributing to the formation of mulch, this strategy can help improve soil conditions and suppress weeds.

During the off-season, cover crops or a vegetated fallow with non-host species can keep the soil protected, reduce erosion, and limit nematode multiplication.

Beans can be reintroduced into the system according to the region's growing window, preferably after one or two cycles with less susceptible or non-host species. The intention is to start the new crop in soil with a lower population of P. brachyurus.

The strategy needs to be adjusted to the climate of each region, the availability of cover crop seeds, and the economic objectives of the property.

Crop rotation should also be combined with other practices. When available, bean cultivars with better tolerance can contribute to management in infested areas.

Weed control is another important point because many spontaneous species can act as hosts for P. brachyurus. Management should continue throughout the year, including during the off-season.

Correcting soil acidity, managing fertility, adding organic matter, and maintaining mulch can also promote root health and the activity of beneficial organisms, increasing the crop's resilience to damage.

Biological control, when carried out with registered agents, can be integrated into crop rotation programs and contribute to reducing nematode pressure in the soil.

Proper care of machinery and equipment is also part of pest management. Moving soil and debris from infested areas to locations that are still free of the problem can promote the spread of the nematode. Cleaning and disinfecting equipment helps reduce this risk.

To verify if the rotation is working, monitoring should continue over the years. Soil and root samples can be collected before planting beans and after harvesting important crops within the rotation strategy.

Comparing population density across cycles allows us to identify whether the population is decreasing, remaining stable, or increasing.

These results should be evaluated along with data on productivity, plant vigor, and the occurrence of symptoms in the roots. If the population remains high, it is necessary to reassess the species used in the rotation, the presence of host weeds, and possible sources of nematode introduction into the area.

The choice of species and cultivars for rotation should consider research recommendations for each region and an economic analysis of the property. The selection should also take into account the specific reaction of the materials to P. brachyurus.

Any use of chemical or biological products must follow the label and package insert recommendations and the agronomic prescription issued by a qualified professional.

Therefore, controlling lesion nematodes in beans depends on planning that goes beyond a single crop season. Identifying the population present in the soil, choosing less susceptible or non-host crops, controlling weeds, and continuous monitoring form the basis of an integrated management strategy.

Crop rotation, when planned according to farm conditions and combined with other practices, can gradually reduce the population of Pratylenchus brachyurus and create more favorable conditions for bean establishment and productivity.