During a visit to a greenhouse, dark angular leaf spots were observed on two coleus cultivars.
Necrotic spotting was more pronounced on the lower foliage than the upper foliage. (The necrotic
spots were confined between the secondary veins, which gave them a blocky appearance, but in the
case of these Hosta, were confined between the long parallel veins.) The pattern of necrosis where
damage is confined between veins can also occur with bacterial diseases and some stages of downy
mildew. Foliar nematodes (Aphelenchoides spp.) infestations start as a blotchy yellow discoloration
before the tissue dies completely. These nematodes infest a wide number of bedding plants as well
The nematodes, microscopic worms, typically spread in splashed water droplets and enter through
the stomata (pores on the undersides of the leaves). They require a thin film of water to move
through, so irrigation is an important factor is their spread. Dormant nematodes can survive in dried
plant materials for years: another reason good sanitation is important.
If you suspect foliar nematodes, a lab can help you diagnose their presence. Control: reduce
overhead irrigation, remove infected leaves, and sanitize tools and surfaces.
For more information, and a list of plant species affected, see this fact sheet: