elocation-id: e3836
In the northern region of Sinaloa, the fungus Rhizoctonia spp. causes plants to dry and rot in beans and corn, affecting germination, growth, and roots. This weakens and causes death in plants. Farmers often seek to mitigate the disease by applying chemical fungicides; however, the effectiveness of these treatments is often limited. This is due to inappropriate selection of fungicides, incorrect dosing during application, and pathogens’ potential acquired resistance to these compounds, among other factors. The purpose of this study was to evaluate the in vitro sensitivity of Rhizoctonia isolates from beans and corn to four synthetic fungicides (Azoxystrobin, Benomyl, Methyl thiophanate and Tebuconazole). A completely randomized design was performed, using the commercial dose of each fungicide and a control, with three replications per treatment. The conventional fungicide tebuconazole proved to be the most effective against all Rhizoctonia species evaluated, inhibiting growth by 100%. Effectiveness tests on the sensitivity of Rhizoctonia isolates to fungicides allow us to know the variability of behavior and facilitate the monitoring of isolates that present resistance to fungicides in the pathogen population. This is essential for the development of effective control strategies for Rhizoctoniasis.
fungicides, resistance, rizoctoniasis, tebuconazole.
The fungus Rhizoctonia is an economically important pathogen that reduces the productivity of crops, such as beans (Phaseolus vulgaris) and corn (Zea mays), in the world and some regions of Mexico, such as the state of Sinaloa. Rhizoctonia is classified into multinucleate species, such as R. solani and R. zeae, and binucleate species, such as binucleate Rhizoctonia (Perdomo, 2007; González, 2013; Yang et al., 2015). In beans and corn, Rhizoctoniasis infection results in crusty lesions and dark cankers at the base of stems and roots, affecting both plant growth and yield (Rabago et al., 2024).
Bean and corn producers employ strategies such as crop rotation, biocontrol agents, and certified pathogen-free seeds to control Rhizoctoniasis. Nevertheless, the use of chemical fungicides remains the most common practice in Sinaloa and other producing regions (Hernández et al., 2018). The use of chemical fungicides is not a sustainable solution due to their environmental impact and the resistance that pathogens can develop, as reported in R. solani in rice paddies in Louisiana (USA) and Henan (China). In addition, the different anastomosis groups (AGs) of Rhizoctonia show variable sensitivity to fungicides according to studies conducted in several countries, such as the United States of America, France, and Mexico (Muzhinji et al., 2018).
The FRAC (2024) warns that the repeated use of fungicides with a single mode of action increases the risk of pathogen insensitivity and constant monitoring of sensitivity is key to early detection of cases of resistance. Therefore, this study evaluated the in vitro sensitivity of Rhizoctonia isolates from beans and corn to four synthetic fungicides (Azoxystrobin, Benomyl, Methyl thiophanate and Tebuconazole).
Rhizoctonia isolates were collected in the 2020-2021 autumn-winter cycle in northern Sinaloa, subjected to pathogenicity tests and morphological and molecular identification, and preserved at 25 °C in tubes with sterile soil (Rabago et al., 2024). The following six species from beans and corn were reactivated in PDA medium and studied: R. solani AG-4 HGI, R. solani AG-4 HGIII, R. solani AG-7, binucleate R. AG-A, binucleate R. AG-G, and R. zeae.
The effectiveness of the following four commercial fungicides was evaluated: azoxystrobin, benomyl, methyl thiophanate and tebuconazole, for the control of the species Rhizoctonia solani AG-4 HGI, Rhizoctonia solani AG-4 HGIII, Rhizoctonia solani AG-7, binucleate Rhizoctonia AG-A, binucleate Rhizoctonia AG-G and Rhizoctonia zeae. Commercial doses of each fungicide (0.05 L ha-1 azoxystrobin, methyl thiophanate, tebuconazole, and 0.5 kg ha-1 benomyl) were incorporated into freshly sterilized PDA medium at 45 °C, which was then poured into sterilized Petri dishes of 8 cm diameter. Once the medium solidified, a 0.8 cm slice of mycelium-agar of each species was placed in the center of each dish.
The control treatment consisted of PDA without fungicide. The experimental design was completely randomized, using Petri dishes inoculated with the fungus as the experimental unit. Four treatments and the control were applied, with three replications per treatment and an additional replication of the experiment. The effect of the treatments was evaluated by measuring the radius of the colony (cm) of the fungus at 24, 48 and 72 h. The data were transformed into homogenize variances (Little and Hills, 1989) and analyzed using Anova. The means were compared with the Kruskal-Wallis test (p < 0.05) using InfoStat and the susceptibility of the isolates was determined by the percentage of inhibition of fungal growth.
The biological effectiveness of the conventional fungicides evaluated, at 24, 48 and 72 h, showed significant differences between treatments. In the medium with Tebuconazole, the smallest radius of the colony (p < 0.05) was recorded in the isolates Rhizoctonia solani AG-4 HGI, AG-4 HGIII, AG-7, binucleate Rhizoctonia AG-A, AG-G and Rhizoctonia zeae, which were susceptible to this fungicide according to the results by Pérez et al. (2019), who reported that the genus Rhizoctonia is susceptible to this product.
Rhizoctonia solani AG-4 HGIII and Rhizoctonia solani AG-7 showed sensitivity to all fungicides evaluated. In contrast, Rhizoctonia zeae and Rhizoctonia solani AG-4 HGI demonstrated tolerance to Methyl thiophanate and Benomyl. Likewise, binucleate Rhizoctonia AG-A and AG-G were tolerant to Azoxystrobin. In cultures without fungicide, fungi completely occupied the dishes at 72 h. Fungicide-tolerant species showed a behavior similar to the control, without presenting significant differences (Table 1).
These results coincide with the findings by Alburqueque and Gusqui (2018), who documented the tolerance of Rhizoctonia isolates to Azoxystrobin. Nonetheless, the work by Muzhinji et al. (2018) reported the susceptibility of Rhizoctonia isolates to this same fungicide under in vitro conditions, which underscores the genetic variability of the pathogen.
At 92 h, Tebuconazole achieved 100% growth inhibition of all isolates, whereas the other treatments showed variable results (Figure 1). This coincides with what González (2013) pointed out about the heterogeneity of Rhizoctonia, which allowed it to adapt to different conditions.
The study highlights the variability in response to the sensitivity of bean and corn Rhizoctonia species to different fungicides. The isolates tested showed sensitivity to Tebuconazole, which completely inhibited growth in 92 h. R. solani AG-4 HGIII, R. zeae, binucleate R. AG-A and G showed insensitivity to fungicides such as Methyl thiophanate, Benomyl and Azoxystrobin. This shows the genetic diversity of the isolates, which can affect their adaptation and response to treatments. The need for continuous surveillance and integrated strategies to manage this disease and the appropriate selection of fungicides according to the species and concentration to optimize its control in agricultural crops is emphasized.
Pérez-Rodríguez, L. R.; Pérez-Moreno, L.; Guzmán-Mendoza, R.; Sanzón-Gómez, D. y Belmonte-Vargas, J. R. 2019. Sensibilidad in vitro de hongos fitopatógenos causantes de enfermedades en fresa a controladores biológicos y fungicidas, en el estado de Guanajuato, México. Acta Universitaria. 29:1-11. https://doi.org/10.15174/au.2019.2339.
Rabago-zavala, K.; Valenzuela-Escoboza, B. E.; Lizarraga-Sánchez, G. J. and Valenzuela-Escoboza, F. V. 2024. Morphological, molecular and pathogenic caracterization of Rhizoctonia solani isolate associated whit been drying in Northern Sinaloa, Mexico. AgroProductividad. 17(11):253-261. https://doi.org/10.32854/agrop.v17i11.3143.