FUNGUS AMONG US … or Leaf Spot

by JoAnn Mukherjee
August 2007
DVIS Newsletter

 

Help!  I see spots before my eyes!

Leaf spot is a very common disease that affects irises in our area.  It can affect bulbous and rhizomatatous types of irises, but is most often a problem on bearded irises.  Leaf spot is caused by the fungus Mycosphaerella macrospora, which was formerly known as Didymellina macrospora.  In this instance, it is important to also know the old name of the disease because some of the available fungicides still have Didymellina leaf spot listed on the label as the disease to be treated rather than the revised name.

Leaf spot first appears in rainy or humid weather as the plants are growing.  They look like small brown spots with a water-soaked margin surrounded by a yellow halo usually on the upper halves of the iris leaves.   The fungus grows more slowly early in the season, but after the irises bloom as the summer temperatures rise, the fungal spots enlarge.  Eventually the spots will develop a grayish center, where fungal spore production occurs, with a reddish brown edge.  In severe infections the spots will merge together and the upper portions of the leaves can turn brown and die.  The lower portions of the leaves or even the bloom stalks can also be affected if the disease is severe enough.

Leaf spot is a challenge because if your plants have been affected by it, it may take a couple years to truly get rid of it. The Mycosphaerella overwinters in the garden as mycelium, fine fungal thread-like strands, on old infected plant debris.  In the mild wet weather of spring the mycelium produce conidia.  Conidia are essentially fungal spores that are genetically identical to the parent and can develop into new organisms when conditions are favorable.

These spores can get onto the plant leaves by being wind-blown or by water splashing them onto the leaves.  If moisture is present on the leaves in the form of dew or rain, the conidia germinate and penetrate into the leaf tissue and the infection begins.  Later in the season when the leaf spots have enlarged and developed the grayish centers, conidia also form in them, which can then cause the infection to spread to new leaves within a clump or to other clumps later in the season.

Leaf spot doesn’t overtly kill the plant right away.  However, severe or repeated infections will weaken the plant.  The growth and increase in the clump will be reduced, and flowering will be reduced because the food reserves become depleted.  Leaf spot also makes the weakened plant more susceptible to other pests, diseases and cold damage.

The good news is that there is a great deal you can do to reduce or eliminate the amount of leaf spot present in your garden both through natural or chemical means.  We will start with the natural approach.  As is the case with most problems affecting irises, the key is proper culture and good garden sanitation.

Mycosphaerella thrives in humid conditions where air circulation is poor.  Plant your irises in sunny areas with good airflow.  Don’t allow your clumps to get overcrowded where the inner portions of the clump don’t dry quickly or get adequate air circulation.

Try to avoid planting in low-lying areas that are subject to heavy dew or ground fog (near streams, lakes, etc.).  The worst infections tend to occur where old infected iris debris has been allowed to accumulate.  Make sure all the dead leaves are removed from your plants in the fall and spring and you will have a huge reduction in the number of spores available to affect your plant the next year.  It is thought that the practice of cutting the iris leaves back in the fall probably started to try to reduce leaf spot infections.  In this context, it is a good practice.

Late in the growing season, if you see leaves with leaf spot, you may want to cut off the badly affected portions of leaves to help limit secondary infections.  Also, in the fall, to try to reduce the severity of and the spread of leaf spot infection for the next season, you can cut back the affected portion of the diseased leaves.  If the newer leaves don’t have noticeable leaf spot on them, then don’t cut them back.  Only cut the leaves back as far as they are infected.  You want the plants to have all the energy they can have in the spring; cutting leaves off unnecessarily makes the plant have to work harder to get up-to-speed.  Dispose of all dead and diseased leaves and bloom stalks in the trash rather than composting them.

Traditional theory says that if you can keep leaf spot out of your garden for three years straight, then you should be free of it unless you bring more infected plants into your yard.

Another important point to note for people who have mixed flower borders rather than iris-only beds is that other plants, including daylilies, gladiolus, narcissus and freesias, act as hosts for the fungus.  Therefore, if you take care of the irises, but leave the dead foliage of these plants, the irises can be re-infected the next year.

Other important control measures to reduce leaf spot infection include:    

• dividing your iris clumps frequently so they do not become overcrowded,

• keep your iris beds weed-free to promote faster drying and better air circulation,

• and don’t work in or walk through you iris beds when they are wet so you don’t spread spores from one plant to another.

Some cultivars seem to contract leaf spot more regularly than others.  Where possible, try replacing them with similar-looking but more vigorous cultivars.  If you have a favorite iris that is susceptible that you can’t live without, try moving it to a very sunny, elevated area as a specimen plant so it is less likely to develop leaf spot and is also less likely to it on to its neighbors. 

A number of fungicides are available to homeowners.  See article about fungicides, FUNGUS AMONG US … Fungicides.