Anthracnose in soybeans: Efficient prevention

Anthracnose is responsible for serious damage to soybean crops. Its management requires effort and rationality from producers. Preventive applications, before flowering, have become essential in chemical control. But to do so, it is necessary to consider the actual or potential presence of the inoculum of Colletotrichum truncatum on the farm

Among the various phytopathogenic microorganisms (fungi, viruses, bacteria and nematodes) associated with soybean cultivation, causing some type of damage, the fungus Colletotrichum dematium (Pers. Ex Fr.) Grove var. truncata (Schw) Arx (syn. C. truncatum (Schw.) Andrus & Moore, which causes anthracnose, is one of the most important. It affects all phases of development, from crop establishment to grain filling, constituting an important problem in the cerrado of the Center-West, North and Northeast of Brazil.

The greater intensity of anthracnose in the cerrado has been attributed to the high rainfall and high temperatures that occur in crops during the harvest season. However, other aspects such as excess plant population, continuous soybean cultivation, use of infected seeds, infestation and damage by caterpillars and stink bugs, in addition to nutritional deficiencies, mainly potassium, are also responsible for the higher incidence of the disease.

Since the 2011/2012 harvest, an increase in the occurrence of C. truncatum in soybean seeds, which contribute substantially to an increase in disease pressure in the early stages of crops. In such situations, producers are obliged to apply early control measures in the vegetative phases, avoiding irreversible losses during the reproductive phase of the crop.

Epidemiology and symptomatology

the fungus C. truncatum It has a saprophytic phase, surviving in the organic matter of the soil and in cultural remains. It is believed that C. truncatum can survive on host plants other than soybeans. However, the most efficient way of introducing and spreading the disease occurs through infected seeds. The pathogen is transported mainly in the internal tissues of soybean seeds and, as it is an infective pathogen in the aerial part of the plant, it is easily carried, during the germination process, to the first cotyledonous and trifoliate leaves in the lower third of the plant. The progress of the disease can be rapid if the environment is favorable to the epidemic.

Under conditions of high precipitation, high temperatures, dense planting, inadequate supply of nutrients and continuous cultivation of soybeans, the pathogen present in seeds or crop residues can infect soybeans since the crop establishment phase.

The dynamics of infection by Colletotrichum truncatum in soybeans it presents an almost exclusive development pattern. Plants infected by the inoculum present in the seeds may not present visible symptoms, and the pathogen travels along the stem reaching the pods. In this case, the systemic and asymptomatic nature of the disease is characterized.

When the cotyledons fall, the fungus housed in these tissues survives saprophytically in the soil until the microclimate conditions in the lower third of the plant are favorable. Thus, high humidity and dense crops allow the manifestation of C. truncatum on older leaves, carried by wind or rain splashes. From the infection on the leaves, the pathogen reaches petioles and pods, ensuring its transmission to new generations. In the presence of C. truncatum in soybean pods does not necessarily indicate its presence in the seeds.

The sequence of these events within the pathogenic process of soybean anthracnose is not mandatory, since the pathogen originating from soil inoculum or carried through wind and rain can directly infect pods or petioles without being previously identified in leaves or cotyledons. Therefore, crops free from anthracnose in the vegetative phase are not free from infection in the reproductive phase. This fact becomes important when evaluating the tactics to be adopted in the management of this disease,

The cotyledons of infected seeds have depressed dark brown spots. This necrosis can extend to the hypocotyl, causing tipping over or the establishment of less vigorous plants. After closing the crop, the symptoms on the lower leaves of the plant are characterized by reddish and discontinuous necrosis of the epidermis of the veins. This symptom, which occurs quite irregularly, can be confused with the occurrence of phytotoxicity caused by systemic herbicides.

Another characteristic and extremely common symptom of the disease is the necrosis and strangulation of the petioles, observed mainly in the shaded part of the crop. This symptom may occur on petioles of asymptomatic leaves. In conditions of high humidity, early defoliation of the lower and middle thirds is observed during the reproductive phase of the crop. Strangulation of petioles without corresponding progressive necrosis (chlorotic petiole) is a symptom that may be related to nutritional deficiency.

Infected pods at the initial stage of formation turn dark brown to black in color, are twisted and do not show grain formation. At this stage, necrosis of the peduncle and pods that are still in formation may occur. During the grain filling process, the lesions begin with anasarca streaks and evolve into black, depressed spots. Under high rainfall, the pods open, with consequent premature germination and rotting of the grains. In more susceptible varieties, the sharp drop in the number of pods can lead to the symptom of leaf retention and green stem. In conditions of high humidity and cloudiness, reproductive structures of C. truncatum they can be observed on infected tissues in the field or incubated for 48 hours in a controlled environment.

Control

The reduction in damage caused by anthracnose in soybeans in the Cerrado will be greater the greater the number of control tactics to be adopted within the integrated management program for this disease.

The cultivation of soybeans in the direct planting system and in succession or rotation with cotton and corn is widely advantageous from a phytosanitary point of view, allowing the reduction of the inoculum of several fungal pathogens with a saprophytic phase in the soil and aggressive to the crop.

When installing the crop, the use of healthy seeds and/or treated with a mixture of systemic and contact fungicides registered with the Ministry of Agriculture, Livestock and Supply (Mapa), is an essential measure in controlling anthracnose in the initial stages of plant development. . Seeds with high inoculum potential C. truncatum should be avoided even under chemical treatment, as registered products have reduced control efficiency in the presence of high rates of occurrence of the pathogen.

The cheapest and most efficient way, therefore, the preferred way to control explosive fungal pathogens in the aerial part, is the use of varieties with a high degree of genetic resistance. The literature does not present varieties genetically resistant to anthracnose for cerrado conditions, where intermediate levels between susceptibility and resistance have been frequently observed, and should therefore be a technology integrated with other available control techniques.

Research on the use of nutrients, which has been carried out in several teaching and research institutions in Brazil, has pointed to promising results regarding the success of the application of foliar fertilization integrated with the use of fungicides to control anthracnose. This measure aims to increase the efficiency and reduce the costs of chemical control of the disease.

Chemical control of anthracnose in soybeans has been evaluated under cerrado conditions. Satisfactory results have been achieved with systemic fungicides (benzimidazoles, triazoles, triazolinthiones and strobilurins) and protective fungicides (dithiocarbamates) applied in different mixtures. The use of isolated active ingredients is not a recommended practice as it allows the selection of less sensitive populations. Thus, the commercial mixture of active ingredients enhances the mode of action of each active ingredient and reduces the risk of selecting resistant populations. A large number of research studies focused on the chemical control of diseases in soybeans seek to evaluate the efficiency of new mixtures, concentrations, times and application programs for active ingredients that have already been used for some time.

Always keeping in mind the reduction of the initial inoculum from contaminated or infected seeds, decomposing matter in the soil and crop residues, preventive applications, considered here as those carried out before the crop blooms, have become essential in chemical management. of fungal diseases in soybeans, especially anthracnose. This practice must consider the actual or potential presence in the crop of inoculum of C. truncatum.

Experiment report

With the objective of evaluating the efficiency of mixtures of the active ingredients azoxystrobin + tebuconazole (120 + 200g/L) and picoxystrobin + tebuconazole (120 + 200g/L) in controlling anthracnose in soybeans, an experiment was conducted in the municipality of Campo Verde, Mato Grosso, in the experimental area of ​​Assist Consultoria e Experimentação Agronomia (coordinates 15°31'36"S, 55°18'31"W, altitude of 763m in relation to mean sea level), during the period 3/11/ 2013 to 22/3/2014. The Pioneer XB90C13 variety, reported as susceptible to anthracnose, was used, sown in plots of five 5m long rows at a density of nine plants/m (200 thousand plants/ha).

Eight treatments were evaluated in four applications (Table 1) after the closure of soybean lines (preventive applications) or at the beginning of flowering (curative applications). From the first and each application of treatments, the severity of anthracnose (percentage of infected tissue), the area under the disease progress curve (AACPD), defoliation (at the end of grain filling) and crop productivity were recorded. culture (bags/ha).

The best control of anthracnose, resulting in lower severity (less than 5% of infected tissue until the beginning of grain filling and less than 10% of infection in pods – Graph 1) and lower AACPD (Graph 2) were provided by: application preventive (before closing soybean lines) and curative (during grain filling) of azoxystrobin + tebuconazole (120 + 200g/L) interspersed with picoxystrobin + tebuconazole (120 + 200g/L) during flowering and pod formation; applications in flowering and pod formation of picoxystrobin + tebuconazole (120 + 200g/L) followed by azoxystrobin + tebuconazole (120 + 200g/L) during grain filling; azoxystrobin + tebuconazole (120 + 200g/L) in four applications during flowering, pod formation and grain filling; picoxystrobin + cyproconazole (200 + 80g/L) in four applications during flowering, pod formation and grain filling.

Applications of azoxystrobin + tebuconazole (120 + 200g/L) and picoxystrobin + tebuconazole (120 + 200g/L) interspersed between the vegetative and reproductive phases of the soybean crop also provided the best yields, exceeding 51 bags of soybeans/ha, representing gains above 25% in income compared to untreated plots (Graph 3).

These results are important, as they provide security in the preventive management of soybean diseases and offer greater flexibility to the producer in choosing mixtures of active ingredients different from those traditionally used in soybean crops in the Cerrado, helping to reduce the selection of populations of resistant fungi.

* By Lucas MA Marinho, Daniel Cassetari Neto, Rafael da Silva Juliani, Douglas LG Mosque, Marllon Darlan S. Almeida, Pedro L. B. Lacerda (Federal University of Mato Grosso); Adriano Melo e Danilo Silva (Adama Brazil)

Article published in December 2014, in issue 187 of Cultivar Grandes Culturas.