Soybean anomaly or rot or necrosis of the stem and pod?

In the last three harvests, an increase in soybean breakage has been noticed, with consequent lodging and plant tipping over. The problem was initially attributed to stink bugs that attack the crop in the initial phase or to climatic and nutritional problems of the plant, such as potassium, nitrogen, boron, calcium, due to a different absorption rate due to water stress and others.

In the 2021/2022 harvest, more than 100 samples were collected in municipalities in the state of Mato Grosso, along BR 163 (Nova Mutum, Sorriso and Sinop), with the support of Aprosoja-MT, in cultivar fields (showcases) and or area of ​​producers with the problem. The Alta Floresta region and surrounding areas were also sampled. On that occasion, the presence of internal necrosis of the medulla was noticed most of the time, whether or not it was associated with stem breakage and bedding. From that moment on, given the similarity of symptoms, it was suspected that something similar to stem cancer (complex Phomopsis phaseoli, Diaporthe phaseolorum, and its various subspecies) that devastated Brazil in 1988 and 1989, being responsible for major losses at that time.

Something similar was also observed in Goiás, in the central pivots, in the region of Bom Jesus (GO), and Ibiá (MG), harvest (2017/2018), with proven pathogenicity in the cultivar BMX Desafio RR, one of the most cultivated in the region and in Brazil at that time.

the complex Diaporthe Nitschke, with more than 800 specific names, constitutes the teleomorphic state of Phomopsis (Sacc.) Bubák, an anamorphic genus with more than 900 species recorded worldwide. An important number of species within this group have been reported as destructive pathogens causing cankers, dieback, root rot, fruit rot, leaf spot, seed rot, and wilt on a wide range of plant hosts worldwide, including species of various cultures.

D. phaseolorum var. potatoes (Crall, 1950) is the causal agent of dry root in sweet potatoes (Ipomea potato L.). Hobbs and Phillips (1985) proposed differentiation between northern US stem cankers and southern stem cankers. Morgan-Jones (1989) decided to transform them into formae speciales, based on morphological characteristics and physiological differences, designating D. phaseolorum f. sp. meridionalis for the teleomorphic southern US isolates; and D. phaseolorum f. sp. caulivora for northern US isolates.

Fernández and Hanlin (1996), based on differences in the number and type of lesions shown by cultivated plants, adopted the concept of “variety”. Since then, the accepted name has been D. phaseolorum var. caulivora and D. phaseolorum var. meridionalis. Based on nucleotide sequence data, cultural, phytopathological, and morphological evidence, Rensburg et al. (2006) proposed that D. phaseolorum var. meridionalis must be treated at the species level together with the causal agent, Diaporthe aspalathi. As can be seen, the complexity of the species and its subspecies is very great, making its real and correct diagnosis difficult.

In the isolations carried out from samples from BR 163, the presence of the genera of the complex was confirmed Fusarium spp (80% of samples) and Phomopsis / Diaporthe (20%). Isolations in BDA (potato-dextrose-agar), directly from the sampled stems and removing small sections of the medulla and internal necrosis, raised the suspicion of double involvement, of the complex Fusarium spp and Diaporthe / Phomopsis, in the disease complex. The involvement of other fungi, such as Cadophora e Colletotrichum spp. From this initial note, around 50 isolates of the fungus were sent to the Agricultural Laboratory – Porto Alegre (RS) for molecular identification. Of the 50 culture samples obtained at the UFU Mycology and Plant Protection Laboratory (LAMIP-UFU), only 8 proved to be of the species Diaporthe phaseolorum, and its subspecies. Two confirmed to be D. miriciae / ueckerae. Regarding the complex Fusarium species from the complex were identified F. fujikuroi, F. solani e F. oxysporum.

Table 1 presents the fungal complex obtained, with their respective pathogenicities in soybean plants of the BMX Desafio cultivar at 100 days of cycle. Visualization was done by measuring the extent in centimeters (cm) of the internal necrosis of the stem through inoculations using a toothpick colonized by fungal mycelia, or by cutting the apex of plants with three trefoils, and then deposition of a disc of the mycelium. of the fungus. The experiment was repeated more than three times at different times during the 2021/22 and 2022/23 harvests.

Figure 1C shows the symptoms of the inoculated plants after 100 days.

Due to the proven pathogenicity presented by the complex Fusarium spp and some isolates from the complex Phomopsis / Diaporthe Some strategies and management are proposed to minimize the damage and losses that farmers are suffering in Mato Grosso and throughout Brazil, notably in the hottest regions where soybeans are grown in Brazil:

1 - Choice of cultivar: Give preference to cultivars that have not presented any reports of the problem in the field, and carry out a health analysis of the seeds to determine the incidence of the complex Phomopsis / Diaporthe e Fusarium spp. in seeds, since there are several pathogenic forms and/or species in soybeans. There is also seed-plant-seed transmission (since fungi can be systemic in the plant from infected or contaminated seeds).

2 - It is urgent to evaluate Brazilian soybean germplasm to analyze the resistance reaction to the complex using more aggressive isolates of the two or more species (complexes), with selection of the same (more aggressive) based on previous pathogenicity tests and not molecular analysis.

3 - Use seed treatments that involve formulations based on prothioconazole and thiran. The use of biological organisms as Trichoderma spp and Bacillus subtilis they may be useful because they present antibiosis for the second and hyperparasitism for the first, via the soil and via seeds or sowing furrows, if the soil is moist.

4 - Fertilization with potassium and calcium via soil and foliar use can help reduce damage due to increased resistance in the middle lamella and greater resistance of the stem and systemic infection in the medulla, if the levels of both nutrients are adequate in the soil and plants .

5 - As for the plant population in the canopy, there is no point in reducing it if the fungal complex is in the system or present in cultural remains. Lodging is another adaptive factor of the cultivar or genetic background to the soil-climate, especially when pathogens are absent in the system.

6- The aerial application of fungicides in combinations containing difeconazole, solatenol and prothioconazole in the initial stages of the crop, up to the pod formation phase and in combinations with multisites (mancozeb and chlorothalonil), can reduce the progression of the disease, but essential is the seed quality and treatment.

7 - Finally, good agronomic practices of integrated management must be used, to avoid problems at harvest time and farmers' frustration with machine breakdowns (cutting platforms break the stems when harvesting plants that have fallen close to the ground) and loss of their capital and investment.

Figures 2, 3 and 4 show the extent of the problem in areas of the states of Goiás and Minas Gerais (2017/2018) and Mato Grosso and Rondônia, in the 2021/2022 harvests.

Figure 5 shows symptoms of the fungi on the colonized stems, with their abundant sporulation (conidia) (figure 6), after incubation at a temperature of 25 ºC for 10 days and with a shift of 12 hours of light and 12 hours of dark (daylight lamps). day).

Figure 8 shows the molecular analysis performed for the different isolates of Phomopsis/Diaporthe involved in the complex.

Figure 9 shows the susceptibility reaction of the cultivar BMX Desafio RR (internal necrosis of the stem), against the inoculated pathogen, whether it belongs to the complex Fusarium spp or Diaporthe / Phomopsis, 100 days after inoculation, using the “straw test” method or tips with fungal mycelium.

All isolates that were pathogenic in the BMX challenge RR cultivar were reisolated on PDA (potato dextrose agar), re-inoculated in plants with three trefoils and symptoms of internal stem rot were again confirmed.

Figure 10 shows the presence of Phomopsis spp in the pods and seeds/grains, which directs the pathogen (A- pycnidia close to the tegument of the future seeds - dark (black) spots, for initial infection in newly emerged plants. B – Mycelial growth of Phomopsis spp and its subspecies or varieties ( symptoms after inoculation in the pods. C and D symptoms after inoculation in the pods.

In conclusion: what is the anomaly? The anomaly is a phytopathological problem with a fungal cause. It is associated with the susceptibility of the genotype or cultivar. Initial errors in diagnosis in the field, for three consecutive harvests, led to the understanding of other factors involved; and not a cause involving phytopathogenic microorganisms. In simple stem breakage without necrosis, we do not have pathogens involved by other factors.

When internal and external necrosis of the stem or main stem occurs at any stage of the plant and pod formation, at different times, we will have the causative or etiological agent. This explains the involvement of fungi such as Fusarium spp. (multiple species complex) and Diaporthe / Phomopsis (multiple species complex).

Therefore, it is important to use healthy and treated seeds, from tolerant or resistant cultivars (research target). And adopt cultural management with balanced nutrition, with adequate levels of potassium (K) and calcium (Ca) in the soil and plants.

The effectiveness of chemical and biological fungicides in seed treatment and spraying is linked to the active ingredient and its penetration into the plant tissues. The moment or stage of the plant and interval of applications will influence the final result and grain production. Hence the need to use robust and well-structured programs by experts or consultants.

Therefore, it is correct to characterize the problem as rot or internal necrosis of the stem and pod and not an anomaly of the soybean.

By Fernando Cezar Juliatti (Juliagro B,G & P Ltda), Iara Gonçalves Guerin, Gustavo Mendes Espíndola e Roberto Resende dos Santos (UFU)

Article published in issue 289 of Cultivar Grandes Culturas Magazine