Anthracnose (Colletotrichum sp) is common on many ornamental plants including mandevilla. Disease symptoms are leaf spots and dieback as seen in these photos. Infected leaves develop tan to brown spots or lesions that are typically associated with leaf veins. In severe cases leaf drop may occur. Because anthracnose symptoms take on different forms and appearances it can easily be mistaken with other fungal diseases. It is therefore beneficial to submit samples to a diagnostic laboratory for proper identification.
This disease is favored by overhead irrigation, splashing water and prolonged foliar wetness.
To manage anthracnose, remove and discard fallen leaves and infected plants. Prune out dead or infected vine branches and treat plants with a fungicide. Some suggested fungicide options for managing anthracnose include: Pageant (pyraclostrobin plus boscalid, FRAC group 7 and 11), Daconil (chlorathalonil, FRAC group M5), Spectro 90WDG (chlorothalonil + thio-phanate methyl, FRAC group M1 and M5) and Protect (mancozeb 75%, FRAC group M3). Read and follow pesticide labels. See the most recent New England Greenhouse Floriculture Guide for current fungicides.