Progression of Corn Tar Spot and Reproducibility Assessment of an Inoculation Protocol in Semicontrolled Environments

Tar spot, caused by Phyllachora maydis, is a threatening disease to maize production across the Americas. This study aimed to test an inoculation protocol for P. maydis in maize, addressing variability across geographic regions. We tested a whorl inoculation method under semicontrolled environments in Ecuador and the United States, focusing on three objectives: (i) to develop a noninvasive inoculation protocol; (ii) to assess the effects of inoculum source, cultivar susceptibility, and leaf position on disease progression over time; and (iii) to evaluate the reproducibility of the protocol. Inoculum preparations from fresh or stored leaves were tested across multiple maize cultivars. Results indicated that the use of a fresh inoculum source led to faster disease onset (15 to 17 days after inoculation) and higher stromata counts compared with inoculum prepared from stored leaves. In Ecuador, fisheye lesions were prominent, whereas they were absent in the United States; however, the factors driving those differences remain unclear. Reproducibility was high in Ecuador and less consistent in the United States, likely because of environmental variability and inoculum quality. Future research should focus on optimizing artificial inoculation methods by exploring the interaction between environmental conditions, inoculum quality, and host susceptibility. Overall, the findings contribute to refining inoculation protocols for tar spot in maize.