Leaf Spot

Mole Gopher Slug Soft Rot Leaf Spot Borer Other

Leaf spotting may be caused by a bacterium or a fungus. Both produce rather similar symptoms, but with practice they can be told apart. The prevalence of leaf spotting was probably behind the old custom of cutting down the foliage of bearded irises after the blooming season. This practice may actually have been helpful, at least with bacterial leaf spot.

Fungal Leaf Spot:

Fungal Leaf Spot is common. As many as eight different leaf-spot fungi attack different sorts of irises, but the symptoms are all the same. Small spots appear anywhere on a leaf (but more usually toward the tips) and grow to about 1/4 inch in diameter, with a yellowish center and a distinct brown border. The spots do not get soft and run together as in bacterial leaf spot, but they can become so abundant that the whole leaf is destroyed. Heavily infected plants are seriosly weakened and will fail to thrive. Like bacterial leaf spot, fungal leaf spot occurs most often during wet weather.

In areas with dry summers, fungal leaf spot comes on with the autumn rains. Fortunately, systemic fungicides work quickly to limit thte spread of the disease. For the sake of appearances, badly spotted leaves can be removed. Even more so than with other diseases, fungal leaf spot can be prevented by avoiding crowding (the disease spreads from plant to plant) and by cleaning up dead foliage thoroughly in the fall and burning it. Some authorities say that if you can keep this disease out for two years, it willl not recur unless brought in on new plants. Some gardeners recommend soaking rhizomes in a solution containing benomyl before planting as a preventive or spraying with this solution if the disease is found on the leaves.

leaf spot 01 leaf spot
The photo on the left is extensive damage to the iris leaf. This plant needs attention immediately. The plant to the right seems less damaged, but still has a lot of leaf spot.

Bacterial Leaf Spot:

Bacteria leaf spot begins at the margins or tips of the leaves and produces soft, watery areas that spread rapidly and run together, working downward through the leaf. Eventually the whole leaf may die. This form of leaf spot is relatively uncommon and occurs during long spells of wet weather. It is found mostly in northeastern North America.

The only cure is to cut off any infected leaves well below the spotted region, using tools that are dipped in a bleach solution between cuts to sterilize them. Always put cut leaves into a container to prevent walking on the infected leaves and spreading the disease in this manner. Also, don't allow irises to become so crowded that air cannot easily circulate amongst them.

Since the spores that cause leaf spot can live over winter on garden debris, it is important to keep a clean garden. Be sure to remove old dead iris leaves. Keep an eye out for the onset of leaf spot during wet weather -- starting as small yellow and brown spots appearing near the tops of the leaves first. The spread of the disease slows or stops when the weather dries off.

Starting in the spring when conditions for leaf spot are favorable, some gardeners recommend a regular spraying program. Start spraying about six weeks before bloom. Spray again if rain should wash it off. Fungicide such as Captan, Fore, Bravo, Daconil, Kocide, and Bayleton are all effective in the control of leaf spot. Using two of these fungicides alternately is better than using one of them exclusively, since resistance to a given fungicide can build up with repeated use. The availability of any particular chemical depends on state and federal regulations. Consult your local Extension Agent for possible sources of these chemicals.