Anthracnose: sneaky advance

Soybean anthracnose, a physiological consequence or cause of abortion of vegetables. The topic is part of a current discussion in soybean crops, where in recent harvests, vegetables without grains, stuck to the plants, have been observed more frequently. In an attempt to estimate the damage and identify the causes of this symptom, the disease, caused by a fungus of the genus Colletotrichum, has been considered.

In the current 2017 harvest, anthracnose has emerged as the second most important disease in the minds of soybean farmers when making decisions about chemical control, behind only rust. High temperature and humidity conditions, combined with potassium-poor soils, especially in the cerrados, favor its occurrence. Despite C. truncatum being the taxon most commonly associated with soybean anthracnose, other species of the genus are also involved, such as C. destructivum (Lehman and Wolf), C. gloeosporioides (Spauld; Schrenk) and C. graminicola (Sinclair and Backman, 1989).

An experiment recently published by professors from USP/Esalq (Barbieri, 2017) who collected stems from soybean plants, from January 2012 to November 2014, with irregular brown necrotic lesions in commercial crops in the cerrado, found among the isolates the species Colletotrichum cliviae. Pathogenicity tests were performed twice, with five strains of C. cliviae in potted soybean plants, cultivar 'BMX Potência' or 'AMS Tibagi' and confirmed the occurrence of C. cliviae causing anthracnose in soybeans in Brazil (Barbieri, 2017).

There are also several works in the literature that consider Colletotrichum truncatum as the main causal agent of soybean anthracnose. However, for Yang et al., 2012, 2014 there are several other species of Colletotrichum involved in different geographic regions. The works of Dias, 2014 and Barbieri, 2017 are the first report of C. cliviae involved with soybean anthracnose in Brazil. The study carried out at the University of Brasília showed that the isolate I5H, from the state of Tocantins, presented more severe lesions in soybean plants, comparatively, where the authors classified it as one of the most aggressive to soybean, causing lesions in both cotyledons and stems. The authors also pointed out that the prevalence of this anthracnose is geographically separated in the states of Tocantins, Maranhão and the Federal District, precisely the places where the most aggressive and recurrent cases of damage by this disease have been reported (Dias, 2014).

In light of these more recent studies, it can be seen that the soybean anthracnose pathosystem is complex and its etiology is confusing. The lack of information about the pathogen in the various soybean-producing regions can make control strategies difficult, whether through the use of resistant cultivars or even fungicides. If multiple species of soybean occur in Brazil, Colletotrichum causing anthracnose in soybeans, the varietal reaction of susceptibility to the disease itself may be differential between regions.

Furthermore, the most “aggressive” species may be being spread through contaminated seeds without due care, making it more difficult to control in regions where anthracnose was not previously such an important disease.

The disease is spreading, especially in the Brazilian savannahs, in a cunning way, causing hidden damage through neglect of seed pathology. The most commonly reported and observed symptoms of anthracnose in these conditions are dark, irregular lesions, forming depressions on the stems and petioles. Leaf symptoms are not commonly observed and the fungus has a preferential saprophytic behavior, colonizing structures (petioles) that are often already discarded by the plant. In legumes, when infected in the initial stage of formation, they acquire a dark brown to black coloration and become twisted, covered by the fungus' acervuli (use the sequence in a montage). In legumes in the process of grain formation, the black spots are initially concentrated in the grain region, and can reach the entire legume, and also present abundant production of acervuli.

Many authors explain that this type of infection causes the abortion of vegetables, which justifies the high potential for damage caused by this disease. However, when the infection is late, seeds are formed, but of low quality, which serve as a source of inoculum, since the disease has a high rate of transmission via seeds.

Some studies also report that at high severities the disease can induce leaf retention and green stem, due to the accentuated loss of vegetables (Embrapa, 2008; Recommendations, 2007; Balardin, 2002; Dias et al, 2012).

Despite the growing importance of the disease in Brazil (Dias et al, 2012; Barros, 2008), there is little information on appropriate control methods, including the availability of genetic resistance in soybeans. Official recommendations include cultural control methods such as crop rotation, soil management, potassium fertilization, greater spacing and lower planting density (Embrapa, 2008). However, Dias et al (2012) and other authors emphasize the use of resistant varieties as a key point in the integrated management of anthracnose in soybean crops.

Klingelfuss and Yorinori (2001) observed C. truncatum present in soybean leaflets, even in the absence of symptoms in the field. For these, anthracnose has a long latent period, where the pathogen infects the plant early, but symptoms are only observed in the legume formation phase.

Observations carried out in commercial crops in the states of Tocantins, Bahia and Mato Grosso, in the 2017 harvest, found a differential behavior regarding anthracnose symptoms, not only in different cultivars, but also related to the rainfall regime, especially during flowering. Some cultivars presented more symptoms in stems and petioles, while others showed more grainless legumes. Crops in which the sowing season coincided with high rainfall during flowering had 20% more null legumes, compared to the same cultivar in another sowing season, in the state of Bahia.

A proper understanding of the diversity of the pathogen, especially the differences in aggressiveness and epidemic behavior, is essential to understanding the differences observed in the efficiency of control measures, especially chemical control. In addition to the evaluation, by the producer himself, of the susceptibility of the cultivars in his location (and in his species of Colletotrichum) of cultivation, the first measure for controlling the disease is the use of pathogen-free seeds, produced in disease-free areas. These seeds must be treated with fungicides capable of reducing the risk of introducing the pathogen into the field. Results demonstrated last year that the use of mixtures of more than one active ingredient in the treatment of soybean seeds significantly reduced the transmissibility of C. truncatum for soybean seedlings.

However, the challenge for chemical control of anthracnose in the aerial part of soybeans is to reach the target. Considering the difficulty of reaching the leaves of the lower third of the crop in advanced stages, such as the formation of legumes, reaching legumes and stems in this phenological phase is certainly much more difficult. Therefore, it has been observed, not only in experiments, but also in commercial crops, that fungicide programs with active ingredients that are more effective on Colletotrichum in the first applications (vegetative and first flowers) they have shown good control results.

Data numerically demonstrated field observations and anthracnose damage. In addition to the known symptoms in legumes and soybean stems, Blood, at the Federal University of Paraná (UFPR), in 2015 observed that Colletotrichum horii is capable of infecting flowers, remaining latent and causing anthracnose symptoms in persimmon fruits. This has also been reported in citrus, olive and strawberry trees (Zulfiqar, 1996; Iannota, 1999; Wilson, 1990). In citrus, the symptoms are characterized by the presence of brown or orange necrotic lesions on the petals and the fall of newly formed fruits. An appropriate methodology may be necessary for the proper quantification and, more importantly, proof of floral infection in soybeans, since in several plant species where the fungus Colletotrichum It is pathogenic, the flower is an important part of the epidemic process.

Thus, it is speculated that floral infection by the fungus may occur, which would consequently result in a lower set of legumes in soybeans, explaining the high potential for damage caused by the disease. Therefore, the number of legumes per plant is being evaluated and cataloged in the most diverse regions, with the objective of measuring the efficiency of chemical control of anthracnose.

The results of the 2017 harvest, in experiments conducted in seven different municipalities in the state of Rio Grande do Sul, showed that the greater the retention of vegetables per plant, the lower the number of those with anthracnose symptoms. Finally, the impact on productivity is high, considering a less aggressive condition, compared to that observed in the states of Bahia and Tocantins in this same harvest.

* By Lucas Navarini (IFRS)

Article published in issue 218 of Cultivar Grandes Culturas Magazine