Species composition of Fusarium root rot of spring triticale and the influence of climate

V. A. Radivon

Introduction. Fungi of the genus Fusarium are pathogens of many diseases of grain crops, including root rot spring triticale. The disease manifests as browning of the roots, sub-crown internode, tillering node, and stem base, which leads to seedling death, stunted growth, death of productive stems, whiteheads, and grain shriveling. Root rot is among the most widespread crop diseases and is noted annually in fields. Infection of the root system by fungi of the genus Fusarium leads to a significant decrease in the main yield indicators of spring triticale. Due to disease development, there is a reduction in the number of grains per ear by 5.0–35.7%, grain mass per ear by 25.0–40.0%, and 1000-grain weight by 4.3–9.7%.

Based on the nature of their relationship with grain crops, fungi of the genus Fusarium are considered facultative parasites; that is, they are capable of saprotrophic feeding in the soil and switching to parasitism when plants are weakened. Consequently, the development of Fusarium root rot is facilitated by both a deficiency and an excess of soil moisture, which lead to a deterioration in the general condition of the plants. This is confirmed by the polynomial dependence (R2=0.75; p<0.01) identified in our previous studies between the total precipitation during the growing season and the intensity of root rot development throughout the growing season.

The species composition of Fusarium root rot is represented by a complex of pathogenic fungi from the genus Fusarium, which varies depending on the grain crop, location, cultivar, and plant growth stage. The most widespread globally are F. graminearum Schwabe and F. culmorum (W.G. Sm.) Sacc., followed by F. avenaceum (Fr.) Sacc., F. oxysporum Schltdl., F. equiseti (Corda) Sacc., F. poae (Peck) Wollenw., F. sporotrichiodes Sherb., and F. solani (Mart.) Sacc. (according to modern taxonomy – Neocosmospora solani (Mart.) L. Lombard & Crous). In Belarus, in cereal crops at the end of the 1990s, the basis of the Fusarium root rot complex on winter grain crops consisted of F. oxysporum, F. culmorum, F. sporotrichiella, F. avenaceum, and F. sambucinum; in spring crops, the species F. oxysporum and F. culmorum dominated. In the 2000s, F. culmorum, F. oxysporum, and F. poae dominated on the root system of winter wheat, and a decade later – F. culmorum, F. avenaceum, and F. equiseti.

Climatic factors have a significant influence on the structure of the Fusarium fungal complex affecting the root system. In addition, interspecific relationships in the form of competition and antagonism are observed among Fusarium fungi, which causes variability in the structure of the Fusarium complex.

The purpose of the research was to determine the species composition of fungi of the genus Fusarium during the growing season of spring triticale, to refine the dominant species, and to identify the hydrothermal conditions influencing the structure of the pathogen complex.

Materials and Methods. Spring triticale cultivation during the research period was carried out at the experimental field of the RUE "Institute of Plant Protection". Sowing was carried out in the third ten-day period of April with an application rate of 5.0–5.5 million grains per hectare. Research in 2015–2021 was conducted on the 'Uzor' cultivar, and in 2022 on the 'Dublet' cultivar. The soil of the experimental plots is sod-podzolic light loamy; soil acidity varied from 5.0 to 6.5, and humus content ranged from 2.0 to 2.8%. Preceding crops were winter grains (2015, 2017, 2020, 2021), potato (2016, 2022), flax (2018), and maize (2019). Tillage and crop maintenance were carried out according to the standard technology for spring triticale cultivation in the central agro-climatic zone of the Republic of Belarus.

In order to study the species composition of fungi infecting the root system of spring triticale throughout the growing season, we collected samples at the 2-node stage (GS 32), mid-flowering (GS 65), and soft dough stage (GS 85). The 2-node stage occurred in the I–II ten-day period of June, GS 65 in the III ten-day period of June, and GS 85 in the III ten-day period of July.

To isolate the pathogens of spring triticale root rot, a sample (25 plants) was taken, and the roots were washed under running water for an hour. Then they were cut into fragments 1–1.5 cm long, immersed in 70% alcohol for 15–20 seconds for disinfection, dried between layers of sterile filter paper, and placed on potato dextrose agar (PDA) in Petri dishes. Previously, 1–2 drops of Triton X-100 detergent were added to the PDA to limit the linear growth of Fusarium fungi, and 6 ml of a 5% streptomycin solution was added to suppress bacterial growth. The dishes were incubated for 7–10 days at a temperature of 22–25 °C, after which the grown fungal colonies were transferred to test tubes with PDA medium. Species identification of the Fusarium fungi was carried out using the identification key compiled by W. Gerlach and H. Nirenberg. The frequency of occurrence (%) of the fungi was calculated as the ratio of the number of analyzed colonies to the total number of colonies.

Dynamics of Fusarium development and weather influence

Root system of spring triticale is susceptible to infection by fungi of the Fusarium genus throughout the entire growing season, with the intensity of the process increasing as the crop matures. At the beginning of stem elongation (GS 32), root infestation is relatively low, but by the time of flowering (GS 65) and dough ripeness (GS 85), it increases more than twofold. To plan plant protection measures, it is important for an agronomist to consider not only the current developmental stage of the plants but also the prevailing weather conditions, which determine pathogen activity.

• Average infestation at GS 32 — 11.4%
• Average infestation at GS 65 — 24.0%
• Average infestation at GS 85 — 28.0%
• Proportion of F. equiseti during maturation — 31.9%

The influence of meteorological conditions on the development of the pathogenic complex was assessed based on the results of long-term observations. Precipitation totals and average air temperatures for key periods of the growing season (where stage 09 is emergence) are provided in the table below. These data help to understand how weather changes alter the balance of power in the soil microflora.

As the growing season progresses, the proportion of Fusarium fungi in the overall structure of pathogens contaminating the root system increases significantly. At early stages, the proportion of Fusarium diseases ranges from 3.7% to 41.1%, and by the end of the season, they become the dominant entities on the roots. Complete data on the occurrence of pathogens are presented in the microflora dynamics table.

* Note: the "Other" group includes Bipolaris sorokiniana, Alternaria spp., and other fungi.

Species composition of pathogens and their interaction

In total, 11 species of Fusarium fungi were identified on spring triticale during the experiments: F. equiseti, F. solani, F. oxysporum, F. avenaceum, F. culmorum, F. graminearum, F. cerealis, F. sporotrichioides, F. poae, F. dimerum, and the Gibberella fujikuroi species complex. By the flowering stage (GS 65), the leading positions are occupied by F. equiseti (average 30.5%) and F. oxysporum (23.6%). The distribution of other species fluctuates significantly by year depending on external factors.

The fungus F. solani exhibits unstable activity: in 2017 and 2021, its share reached 64.4% and 45.5%, respectively, whereas in other seasons it was absent. F. avenaceum also belongs to rare species with a maximum frequency of occurrence up to 17.8%. The development of the remaining pathogens, including the dominant F. equiseti, proceeded independently of temperature and precipitation during the flowering phase.

By the dough ripeness stage (GS 85), F. equiseti remains the dominant species with an average share of 31.9%, while the occurrence of F. oxysporum decreases to 16.0%. The behavior of F. culmorum is noteworthy: in the dry years of 2016 and 2017, it was absent from roots during flowering, but by the dough ripeness stage, its share grew to 28.1% and 20.8%, respectively. This is directly related to precipitation in the second half of the growing season, the volume of which increased nearly twofold compared to the previous period. At the same time, no reliable correlations with weather conditions at the GS 85 stage were found for the studied fungi.

According to literature data, the fungus F. equiseti is widespread, occurs on many plants, and is frequently isolated from soil. It is usually considered a secondary pathogen, as the fungus is often isolated from plants already colonized by other species. Also, F. equiseti, F. oxysporum, and F. solani fungi persist significantly longer in soil samples than other species, which is attributed to their ability to abundantly form chlamydospores.

Conclusion. Species composition of root rot of spring triticale is represented by 11 fungal species, among which *F. equiseti* and *F. oxysporum* were dominant at growth stage 65, with an average frequency of occurrence over the study years of 30.5% and 31.9%, respectively. At growth stage 85, the fungus *F. equiseti* also dominated, with an average share in the pathogenic complex of 31.9%, while the frequency of occurrence of the fungus *F. oxysporum* decreased to 16.0%. Depending on the year of the study, a high frequency of occurrence was noted for the fungi *F. solani* – up to 64.4% at growth stage 65, *F. culmorum* – up to 50.0% at growth stage 65, and *F. avenaceum* – up to 42.9% at growth stage 85. Abundant precipitation from sowing to the mid-flowering stage contributes to an increase in the frequency of occurrence of the fungus *F. culmorum* (r = 0.72), whereas a deficit of precipitation causes an increase in the proportion of the fungus *F. avenaceum* (r = −0.82). No dependence on weather conditions was found for the fungi *F. equiseti*, *F. oxysporum*, and *F. solani*.

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V. A. Radivon RUE «Institute of Plant Protection», Priluki, Minsk region

SPECIES COMPOSITION OF FUSARIUM ROOT

ROT OF SPRING TRITICALE AND THE INFLUENCE

OF HYDROTHERMAL CONDITIONS ON THE

FREQUENCY OF OCCURRENCE OF FUNGI OF THE

Annotation. Studies on the species composition of Fusarium Root Rot of spring triticale that were carried out in 2015–2022 showed that the dominant species from the complex of fungi of the genus Fusarium is the fungus F. equiseti with a frequency of occurrence of 30,5 % in the growth stage Flowering halfway (GS 65) and 31,9 % in Soft dough (GS 85). The frequency of occurrence of the fungus F. oxysporum was 23,6 % (GS 65) and 16,0 % (GS 85), F. solani – 13,7 % (GS 65) and 11,5 % (GS 85), F. culmorum – 10,2 % (GS 65) and 11,3 % (GS 85), F. avenaceum – 5,1 % (GS 65) and 8,9 % (GS 85). For the fungus F. avenaceum in GS 65, there was a negative correlation (r = −0,82) between the frequency of occurrence of the fungus and the amount of precipitation that fell during the period from sowing to growth stage Flowering halfway, and for the fungus F. culmorum there was a positive correlation (r = 0,72) for the same period.

Key words: spring triticale, root rot, fungi of the genus Fusarium, frequency of occurrence.