Brussels sprout problem
It is difficult to tell for sure without seeing a sample of your plants. It may also be helpful for you to get a soil test done for nutrients, pH, and salts. It looks like some insects such as caterpillars may have been chewing on the foliage. The general off color yellowing, though, may be a nutrient deficiency. Brussels sprouts actually thrive in cooler weather, so it may have merely gotten too warm too early for these plants. You don't want to apply too much fertilizer or they will not flower and produce sprouts.
You can take a plant sample in to the Cooperative Extension office at the Carvel Center in Georgetown on Route 9, and can also take about 8 ounces of soil from a depth of 6 to 10 inches in for a soil test (for a fee). These tests will give you some information going forward.
Thank you for contacting Delaware Cooperative Extension,