The lilies are probably suffering from Botrytis
or some other fungal leaf disease. Although wet weather is conducive to the development of fungal diseases, many factors affect the extent and severity of infection from year to year. Good air circulation helps limit infection, so allowing plenty of space between plants is recommended.
There are fungicides that can be used to control this fungal disease. They should be applied before the disease is advanced, and frequent applications will be needed during rainy weather. Thorough cleanup and disposal of infected stalks are also advised. Contact your local Cooperative Extension office
for fungicide recommendations.