BACKGROUND Early blight caused by Alternaria spp. is amongst the most important diseases in potato. Demethylation inhibitor (DMI) fungicides are widely used to control the disease but long‐term use may decrease its control efficacy due to fungicide resistance. This study investigated the occurrence of difenoconazole resistance in Alternaria spp. and molecular resistant mechanisms. RESULTS EC50 values of 160 isolates to difenoconazole ranged from 0.026 μg mL−1 to 15.506 μg mL−1 and the frequency of difenoconazole sensitivity formed a non‐normal distribution curve with a major and a minor peak. Isolates with EC50 values of 4.121 and 5.461 μg mL−1 were not controlled effectively at fungicide doses of 50 and 100 μg mL−1. Cross‐resistance was observed between DMI fungicides difenoconazole and propiconazole, but not between difenoconazole and other fungicide groups, including boscalid, iprodione, or carbendazim. The CYP51gene was 1673 bp encoding 525 amino acids in length and contained two introns. All sensitive and resistant isolates had the identical amino acid sequence of CYP51, with the exception of one resistant isolate carrying a mutation of R511W. A 6 bp insertion in the upstream region was observed in half of the resistant isolates. In the absence of propiconazole, the relative expression of CYP51 was not significantly different in sensitive and resistant isolates. In the presence of difenoconazole, expression of CYP51 gene was induced significantly in the DMI‐resistant isolates but not in the sensitive ones. CONCLUSION Induced expression of CYP51 in resistant isolates exposed to difenoconazole is an important determinant for DMI resistance in potato pathogens Alternaria sect. © 2019 Society of Chemical Industry

Rice blast caused by Magnaporthe oryzae in cultivated rice is a devastating fungal disease. The crop lost to rice blast is enough to feed more than 60 million people every year. At present, chemical control is still the primary measure to control rice blast. Pyraclostrobin is a methoxy acrylate fungicide, which has the characteristics of broad‐spectrum, high efficiency, environmental protection, and friendliness to non‐target organisms. Pyraclostrobin has been a good option for disease control in China. In this study, M. oryzae resistance to pyraclostrobin was evaluated. The pyraclostrobin‐resistant M. oryzae mutants were screened and the primary fitness parameters of resistant and sensitive strains were investigated. A total of nine pyraclostrobin‐resistant mutants were generated, which exhibited a mutation frequency of 3.46 × 10−4. The EC50 values of the mutants ranged from 2.2185 to 4.8946 μg/ml and resistant factors ranging from 467 to 4504. Biological character evaluations showed that all resistant mutants exhibited decreased fitness in mycelial growth rate, sporulation, and pathogenicity. Positive cross‐resistance was observed between pyraclostrobin and azoxystrobin or picoxystrobin, but not between pyraclostrobin and propiconazole or carbendazim. Sequence analysis of the partial fragment in cytb gene showed that there were two different point mutations in the nine resistant mutants. Seven mutants had a point mutation at position 427 (G to A transition), resulting in the conversion of glycine to serine (G143S). Two other mutants had a point mutation at position 428 (G to C transversion), resulting in the conversion of glycine to alanine (G143A). This mutation site is the main mechanism that has been reported to cause high resistance to QoI fungicides. Therefore, the G143A and G143S mutations found in this study could be the main reason for the mutant resistant to pyraclostrobin.

The contributions of the different leaf layers to maize yields identified as middle leaf > lower leaf > upper leaf, where the vertical photosynthetically active radiation (PAR) in the canopy gradually decreases. We hypothesized that the allocation of more PAR and nitrogen (N) to the highest contributing leaves will would be beneficial for higher yields and N use efficiencies. The N application rate and plant density effectively regulated the canopy light and N distribution. We evaluated the interactive effects of N rate and plant density on the agronomic and ecophysiological characteristics of leaves at different orientations in a 2019/2020 field experiment. In this study, an N application rate of 180 kg ha–1 coupled with a plant density of 82,500 plants ha–1 achieved the highest yield and N recovery efficiency (NRE). In contrast to the traditional farming practices in northern China, the density was increased and N rate was reduced. Densification from 52,500 to 82,500 plants ha–1 increased the population leaf area index (LAI) by 37.1% and total photosynthetically active radiation (TPAR) by 29.2%; however, excessive density (from 82,500 to 97,500 plants ha–1) drastically reduced the proportion of TPAR by 28.0% in the lower leaves. With increased density, the leaf areas and angles of the upper leaves decreased much more than those of the other leaves, which allowed the middle and lower leaves to access more light, which manifested a smaller extinction coefficient for light (KL). A high yield (>1,000 kg ha–1) of maize could be achieved simultaneously with higher NRE; however, it was negatively correlated with internal N use efficiency (IEN). Higher N concentrations and lower total performance index (PItotal) in the lower leaves may be an important rationale for the reduction of IEN in high-yielding maize. Additionally, decreased N rate without yield reduction under higher densities was primarily attributed to the more uniform vertical N distribution [a smaller extinction coefficient for N (KN)]. These results suggest that the N fertilizer rate can be moderately reduced without a reduction in maize yield under high plant densities in northern China.