Comparative Evaluation of Inoculation Methods to Screen Aspergillus niger-Induced Crown Rot Disease Resistance in Peanut

Peanut crown rot, caused by Aspergillus niger (A. niger), can cause significant yield loss ranging from 10 to 50%, mainly as a result of stand loss due to seedling death. Symptoms include damping-off of seedlings; dark grey vascular tissue; and the development of a sooty, black spore mass on the crown region beneath the soil. Currently, there is no commercial peanut cultivar resistant to crown rot, and no inoculation methods have been tested to effectively screen for A. niger resistance. In this study, we tested toothpick poking, seed inoculation, and soil inoculation to establish an effective protocol for screening peanut plants for resistance to crown rot under greenhouse conditions. Results showed that toothpick inoculation with A. niger spores caused 100% seedling mortality, whereas non-inoculated poked seedlings and controls showed 100% survival. For the seed inoculation method, the seedling stand percentage was significantly higher in the control (>90%) than in spore treatments across eight weeks (p < 0.001). Spore concentrations (106, 107, and 108 spores mL−1) did not differ significantly, although 108 spores mL−1 was most effective, reducing the seedling stand percentage to 1% by week 8. Soil inoculation caused seedling mortality from 3 weeks after sowing and significantly reduced the seedling stand percentage from weeks 2 to 8 (p < 0.01 to p < 0.001). The 108 spores per gram of soil treatment was the most effective, reducing the seedling survival rate to 20% by week 8. To conclude, all three tested inoculation methods were effective. However, toothpick inoculation ensured 100% infection by bypassing all the natural barriers to infect the plant. Seed inoculation was more effective than soil inoculation, which takes longer to cause crown rot infection. These results will enable plant breeders to select the appropriate inoculation method to screen breeding materials and germplasm for potential A. niger resistance and, ultimately, to develop crown-rot-resistant cultivars.