Tunisian Rhizoctonia solani AG3 strains affect potato shoot macronutrients content, infect faba bean plants and show in vitro resistance to azoxystrobin

Abstract

Rhizoctonia solani reduces quality and yield of potato crops in the south of Tunisia and has become an impediment for potatoes export. The lack of studies on the pathogen prompted us to characterize the growth capacity at different temperatures, the aggressiveness, the host specificity and the sensitivity to the fungicides fludioxonil, pencycuron and azoxystrobin of local R. solani strains. Morphological characterization and sequencing of the rDNA ITS gene of the fungal strains showed that they belong to R. solani anastomosis group 3 (AG3). All strains showed a maximum growth at 25 °C with a preference to cold (15–20 °C) in comparison to high temperatures (30–36 °C). All R. solani strains decreased potato shoot dry weight in infected versus control plants. The nitrogen and potassium levels were higher in the shoot of infected potato plants in comparison to the control; however the opposite behaviour was noticed for phosphorus concentration, indicating an imbalance in the nutritional statue of infected potato plants. The effect of R. solani on potato macronutrients uptake is discussed. All R. solani strains were able to infect faba bean roots with the strain RS5.2 reduced significantly plant growth and nodulation. Therefore, faba bean may provide a suitable alternative host for R. solani AG3. The local R. solani strains were highly sensitive to fludioxonil (EC50: 0.007 μg a.i. ml−1) and pencycuron (EC50: 0.006 μg a.i. ml−1); but resistant to azoxystrobin with an EC50 exceeding 19 μg a.i. ml−1. This highlights the risk of resistance to quinone outside inhibitor fungicides (QoI) in R. solani populations, although up to now no decrease in field performance was noticed. It is recommended to delay build up of QoI resistance by an integrated approach, combining optimised fungicide use with the choice of R. solani resistant cultivars and the application of farming practices.