This disease is also known as Iris Blight and Heterosporium Leaf Spot. It is most commonly associated with its infection of various species of iris flowering plants. Once thought to be a virus, this is actually a fungal infection called Didymellina poecilospora.
Most broad-leafed plants in which the leaves have no stems or “trunk” such as iris are susceptible to this fungus. A very few and rare occasions have shown some trees to be infected as well, but this is usually a short-lived, semi-seasonal infestation and is often spread from nearby iris plants.
The infection will begin as brown spots on the tips and edges of the leaves, spreading down the center of the leaf, downward. The outer edges of the infected area will be moist and “watery” looking. As the foliage is destroyed, the leaves will curl as if dehydrated and the fungus will continue spreading as the leaf deteriorates.
The first symptoms likely to be noted will be tiny brown spots towards the top and center of the leaves. The “splotched” appearance will spread before serious damage begins.
The Didymellina fungus lives as most fungi do. Its spore will overwinter in the ground on mulch and fallen leaves and then reappear when the conditions are ripe. It devours the leaves it infests and is most often spread by splashing rain and sprinkler water or through infected plant material coming in contact with other plants.
How This Disease Impacts the Plant
During the season, the leaf spots will appear usually before the iris begins to bud and flower. It will remain light and nearly unnoticed until flowering begins, at which time (in most cases) it will begin spreading rapidly. As the plant’s chemistry changes due to the flowering and reproductive process, the Didymellina will begin a rapid spread.
Leaves will be destroyed, flowering will likely be stunted and short-lived, and the plant will nearly always be weakened and unable to sprout the following spring.
How to Prevent Leaf Fungus
As with most fungi, infections can be prevented with proper care. Proper aeration through good landscaping and spacing is a good start. Taking care not to introduce infected items such as transplants, bulbs, or mulches is important. When plants shed leaves or in the fall after cutting, the dead leaves should be removed immediately. Infected debris left until spring can re-spawn.
Leaf Spot Control
Treatment options are limited, but there are several commercial fungicides available. Some natural options are also on the market. Most will have good success if used properly. Some non-commercial, DIY remedies are also possible and have had limited success.