Anthracnose in soybeans planted in the second crop

In recent harvests, in addition to not implementing crop rotation, some farmers have been growing soybeans in off-season conditions. Off-season is the second planting season, when corn or sorghum would normally be grown.

Although the greatest technical concern is with Asian rust, which has required several sprays, there has been a significant increase in diseases caused by necrotrophic pathogens, however, with incidence from the initial stages of development.

The higher the initial inoculum, the greater the likelihood of early disease incidence, the greater the damage caused and the costs of control, which is commonly achieved by applying fungicides.

Among the various diseases affecting crops, anthracnose, caused by the fungus Colletotrichum truncatum, has occurred in different regions of the country, being more important in places with temperatures above 26ºC and high rainfall. Work conducted by Klingelfuss and Yorinori (2001) demonstrates that Colletotrichum truncatum was one of the necrotrophic fungi present in most leaflet and stem samples collected in the field, even when there were no apparent symptoms of the disease (asymptomatic plants). For these authors, latent infections are common in this pathosystem and the greatest colonization initially occurs in the lower part of the plants, where there is less incidence of sunlight and greater humidity.

Since 1997, Dhingra and Acuña have reported that in Central Brazil there has been a considerable increase in the incidence of anthracnose, reaching levels above 50%.

The disease has an aggregated distribution in crops, popularly known as reboleiras, and symptoms can affect any part of the plant, such as stems, pods, petioles and leaf veins (photo 1). Field observations and experiments conducted by Agro Carregal have suggested that when Colletotrichum is the causative agent of the disease, symptoms are observed in at least two distinct locations (photo 2).

Symptoms that occur exclusively in pods and in the entire area are not related to anthracnose. The soybean plant can abort pods naturally due to different types of stress, whether biotic or abiotic, such as the incidence of diseases, pests, thermal, water or even nutritional stress. The fungus Colletotrichum may present a saprophytic phase in its cycle, developing in already damaged tissues. It is common to identify this fungus in pods that were aborted by other types of stress. Photo 3 illustrates the evolution of anthracnose symptoms in soybean pods originating from natural infection by the pathogen.

The occurrence of aggregated forms in the field is mainly due to the type of conidia and the spread of the fungus in the aerial part of the plant. Conidia are produced in structures called acervuli, where they are aggregated in a mucilaginous mass, which is made up of polysaccharides, glycoproteins and enzymes. The main functions of the formation of conidia in this structure commonly called matrix are protection to prevent the conidia from germinating prematurely, in addition to maintaining their viability in periods of low humidity and protecting them from extreme temperatures and ultraviolet rays.

Recent research has verified the incidence of Colletotrichum and other fungi in samples collected from plants suspected of having “pod anomaly”, mainly in the state of Mato Grosso. Identifications have been carried out both by morphological and molecular analyses. It is important to emphasize that there is no effective definition of the cause of this new problem, but different researchers have identified the incidence of fungi of the genera Colletotrichum, Phomopsis, Fusarium e cercospora, suggesting that this fungal complex would be the etiological agents. This situation that farmers have faced in recent harvests further interferes with crop yield, increasing losses.

For Manandhar and Hartman (1999), the loss caused by the fungus can vary from country to country and from region to region; in Thailand it is estimated to be between 30 and 50% and in Brazil it can cause 100% losses. Studies conducted at the University of Rio Verde (UniRV) since 2002 have shown that, under experimental conditions and with artificial inoculations, relative losses reach 10 to 20%, that is, losses of six to 14 sc/ha have been observed.

According to Silva et al. (2013), anthracnose requires the adoption of several control practices, such as seed treatment (avoids the introduction of new races), balanced fertilization (potassium has been important in reducing symptoms), crop rotation (reduction of initial inoculum), use of less susceptible cultivars and chemical control. In the field, the main alternative adopted by producers is based on the use of fungicides.

According to the main recommendations, chemical control should be initiated as a preventive measure even before flowering or as soon as the first symptoms are detected (Silva et al., 2013). Nowadays, applications from 25 to 30 days after emergence have been more effective. However, as occurs with any living being, natural evolutionary processes cause genetic variability in the pathogen and the selection pressure exerted by the constant and incorrect use of fungicides can select naturally resistant individuals.

Results published by Rogerio et al (2022) showed mutations in the fungus Colletotrichum truncatum for multiple resistance to fungicides belonging to the chemical groups of strobilurins (mutation G143A) and benzimidazoles (mutations E198A and F200Y). For these authors, the chemical groups of triazoles and carboxamides were effective in the in vitro control of the tested isolates. Field experiments carried out at Agro Carregal until the 2022/23 harvest corroborate the results published by Rogerio et al (2022).

In the in vitro experiments conducted at Agro Carregal, among the triazoles tested, products containing difenoconazole, propiconazole and epoxiconazole stood out as the most effective. Among the carboxamides evaluated, benzovindiflupir also stood out with high control efficacy (photo 4). It is important to note that in these experiments some active ingredients were not tested under in vitro conditions, such as the triazoles mefentrifluconazole, flutriafol and tetraconazole; and the carboxamides bixafen, pidiflumetofem, impirfluxam and fluindapir.

For effective disease management, farmers must always consult an agricultural engineer and keep up to date with quality information.

By Louis Henry Carregal, Agro Carregal Research and Plant Protection

Article published in issue 291 of Cultivar Grandes Culturas Magazine