Evaluation of the Potential of Pyrimidine Nucleoside Antibiotics Against Alternaria spp. Resistant to QoIs Fungicides: Insights for the Management of Ginseng Alternaria Leaf and Stem Blight Disease

To manage the developing resistance of Alternaria spp. [the causal fungi of ginseng Alternaria leaf and stem blight (GALSB)] to QoIs fungicides, the toxicity and biochemical activity of pyrimidine nucleoside antibiotics (PNA) against Alternaria spp., cross-resistance between PNA and eight other fungicides currently used to control GALSB disease, and the efficacy of PNA for controlling GALSB in vitro and in vivo were investigated. The distributions of EC50 values of PNA against the mycelial growth (115 isolates) and conidia germination (89 isolates) of A. alternata were unimodal, with mean EC50 values of 10.192 ± 4.961 μg/mL and 0.828 ± 0.101 μg/mL, respectively. There were no significant correlations between the sensitivity of A. alternata to PNA and eight other fungicides (p < 0.05). PNA caused morphological changes in A. alternata mycelia and germ tubes, increased cell membrane permeability, and reduced intracellular DNA and protein levels. On detached ginseng leaves, 300 μg/mL PNA achieved mean protective and curative effects of 87.93% and 94.77% against A. alternata 7 days post-inoculation, outperforming that of 300 μg/mL kresoxim-methyl. Field trial results showed that PNA (180 g a.i./hm2) achieved mean efficacies of 85.63%, 84.07%, and 72.55% at three sites 7, 15, and 30 days after the last spray, which were 5.28–37.74% higher than those of control fungicides pyraclostrobin, azoxystrobin, and kresoxim-methyl at corresponding time points. Overall, our findings indicate that PNA are effective agents for the management of Alternaria spp. resistance to QoIs fungicides.