Alternaria Leaf Spot -- Alternaria alternata, Alternaria spp.
Host: Calathea, impatiens, marigold, pittosporum
Symptoms: Alternaria leaf spot first appears as water-soaked spots that are less than 1 mm in diameter. Eventually, the spots turn reddish-brown to black in color and become somewhat circular in shape. While the spots can enlarge up to 2 mm in diameter, they usually do not coalesce.
Conditions Favoring Disease: This disease often occurs where overhead irrigation is used and with exposure to rainfall.
How Pathogen Survives/Disperses: Conidia are dispersed to new leaves by irrigation or rain splash. Seed-borne disease is also common.
Photo and write up provided by Syngenta Professional Products