Silicon Sources on Biochemical Responses and Corynespora cassiicola Control in Cucumber Plants

Target spot, caused by Corynespora cassiicola, is an important leaf disease affecting cucumber plants, and alternative management studies are essential for the economic viability of this crop. Silicates were evaluated for the control of target spot and its pathogen, and the biochemical responses of plants were characterized. Preventive spraying was performed three times, at weekly intervals, with K2SiO3 + Cu and K2SiO3, at the concentrations 0, 28, 56, 84 and 112 mg L−1 silicon (Si). Assessments included mycelial growth and germination in vitro; disease severity; number and diameter of lesions; pathogen sporulation on leaves; activities of the enzymes peroxidase, polyphenol oxidase and β‑1,3‑glucanase, and leaf levels of phenolic compounds. The application of Si-based sources had no effect in vitro, nor did it affect lesion diameter and sporulation. Disease severity was lowest after five days of inoculation for treatments with 56 and 84 mg L−1 Si, but there was no disease control after three and seven days. The number of lesions was smallest from 28 to 112 mg L−1 Si, after five days of inoculation, and at 56 mg L−1 Si after seven days of inoculation for the treatment K2SiO3 + Cu, as well as at 56 and 112 mg L−1 Si after five days and at 84 and 112 mg L−1 Si after seven days of inoculation for K2SiO3. In general, there were no differences between the tested silicates. Enzyme activities and phenolic compound levels were not influenced by Si. The reduction in the disease severity and in the number of lesions, even if dependent on Si concentration or day of evaluation, demonstrated the possible viability of Si in controlling cucumber target spot, especially after further studies.