We can't identify a specific disease, but senescing leaves (those shedding in autumn) often develop minor secondary infections by opportunistic pathogens. In those cases, they don't threaten the health of the plant, and leaves that are an eyesore can be clipped off and disposed of. Wet conditions - either from weather or due to sprinkler irrigation - promote conditions suitable for many pathogens to infect foliage. We've had periods of unusually-wet weather this growing season and this can lead to above-average levels of infection. Such patterns typically don't repeat to the same extent year-to-year, meaning that an extensive infection one year won't necessarily result in an equally prominent problem the next. Similarly, given how late in the year it is and the fact that plants are shedding foliage for the winter, fungicide use is not recommended. We expect the plants will resprout normally next spring without intervention.

Miri