This fungal leaf spot pathogen has gone through some name changes over the years. Probably the most common name for this pathogen is Mycosphaerella. To make things even more confusing, the disease may infect iris, daylily (Hemerocallis), freesia, gladiolus, and narcissus.
Iris leaf spot occurs primarily on the leaves, but may also infect stems and flower buds. Small, water-soaked lesions develop rather rapidly into 1/2-inch- long spots with brown centers and yellow margins. These eventually merge to cause streaks of necrotic tissue with yellow to brown borders. The presence of olive-brown spore masses in the center of the spots help confirm this fungal disease.
Most leaf spot diseases do not kill the host, but they weaken the plant, cause aesthetic loss, contribute to general decline, and--in the case of iris--cause premature leaf death, which weakens the rhizomes or bulbs.
Since the fungus overwinters on plant debris, it is important to remove leaf and flower stalk debris in the fall. The fungus spreads during the summer by splashing or blowing from diseased to healthy leaves. For this reason, infected leaf tissue should be removed as it occurs. Also, space plants to promote good air circulation, avoid overhead watering of foliage, work with plants only in dry weather, and use more resistant species. The many fungicides registered for use against this disease are all preventive sprays. It is too late to use them this year, but you may want to consider them for 1999. Consult the Illinois Homeowners' Guide to Pest Management if fungicides are to be used. For more about iris leaf spot, see Report on Plant Diseases No. 628.