Tar-spot infection delays fungal colonization and decomposition of maple leaves

Tar spot is a common leaf disease of Acer caused by Rhytisma spp. (Ascomycetes). Initial symptoms are small green-yellow spots. These infected areas turn into black fungal stromata, which resemble blobs of tar. Stromata with apothecia overwinter and release ascospores in the following spring. We measured rates of decomposition of leaf litter of infected and uninfected Norway maple (Acer platanoides). In addition, we assessed changes in leaf-litter N and P and rates of sporulation of aquatic hyphomycetes. We incubated leaf disks from areas covered with tar spot, disks touching but not overlapping with tar spot (adjacent disks), and disks from uninfected leaves in a stream. The exponential decay rate of uninfected disks was 50% greater than those for infected and adjacent disks. Initial N and P concentrations were highest in infected disks. N and P concentrations dropped in all treatments during the 1st wk in the stream and subsequently increased. Conidium release by aquatic hyphomycetes in the laboratory was lowest from infected disks and highest from uninfected disks. Cumulative sporulation over 10 wk of stream exposure increased from infected (109 conidia mg(-1) d(-1)) to adjacent (215 conidia mg(-1) d(-1)) to uninfected (521 conidia mg(-1) d(-1)) disks. Similarity of fungal communities was highest between adjacent and uninfected disks (71.5%) and lowest between infected and uninfected leaves (51.9%).