While tidying up my iris garden I noticed that many of the leaves of the German iris had a spongy look and bore light green, yellow or brown watery spots. Some leaves had large brown-black watery areas and other leaves were dead. Unfortunately I have an Iris Leaf Spot problem this year and other people may too. We have had a great deal of rain lately with temperatures on the cool side making a perfect environment for the growth of the fungus (Mycosphaerella macrospora) that causes this disease. Severe infection gradually reduces bloom and weakens the plants by destroying the leaves that produce food for growth. In addition, infected plants are more likely to be infected by other pests and diseases. Irises are not alone in being infected by this fungus; daylily, gladiolus, freesia and narcissus can also be infected so control at this point is important for more than just my irises.
Knowing something about the life cycle of this fungus makes it easier to understand how to control it. The fungus, Mycosphaerella, exists as a minute thread-like mass (called a mycelium) when inactive during the winter or periods of unfavorable growth but produces two different kinds of spores that are spread by air currents and splashing rain when conditions are right; dampness with rain and temperatures between 50F and 77F (68F optimum).
Controlling the infection involves doing all you can to reduce the production and spread of the spores. You can’t control the weather but you can do some things to make the effect of the weather less favorable for the fungus and this may alleviate the problem enough so that you don’t have to take more drastic steps like spraying with a fungicide.
Use resistant varieties or other kinds of iris, like Siberian iris which are less prone to leaf spot.
Plant iris in full sun, in well drained soil, with good air drainage and divide the clumps often to keep them open. If soil is acid, add lime.
Remove all dead foliage and flower stalks during the year so that air can freely circulate and keep the leaves dry being especially diligent in the Fall. Remember, the fungus over winters on the dead foliage. Never put iris foliage on the compost pile because the fungus will remain in the debris ready to infect plants. Either burn it or put it out in your refuse bin.
Ditto for infected leaves or parts of leaves during the growing season.
Avoid working with the iris when wet and infected as you will spread those nasty spores.
If the infection is severe, spray the iris with fungicide. Start when the fans are 4-6” tall and repeat every 5 to 10 days. Use a contact fungicide (Daconil 2787 or Dithane both containing chlorothalonil) to knock down the infection that is already present. Use a systemic fungicide (containing propiconazole) to help make the plants more resistant to new infection. Combine the two fungicides and add a wetting agent (1/4-1 teaspoon per gallon) to overcome the waxiness of the leaves and really soak them. The contact fungicide is removed by rain but the systemic is not and remains active for 7-10 days. Adjust your spraying routine to the weather. Ask at your local feed store or garden center for specific brands as availability varies with states.