As the Mid-Atlantic enters fall after a hot and humid summer, gardeners face many challenges. Among these are controlling pests and battling disease. Pest and disease infestations tend to be a direct result of a stressful growing season. When the summer weather is steamy, fungal diseases rear their ugly head much more often. One disease you might be dealing with after a hot summer is iris leaf spot.

Iris leaf spot is caused by the fungus Cladosporium iridis and affects irises (Iris spp. and cvs., Zones 3–10). Fortunately, it can be controlled relatively quickly once it is identified. Typically, the fungus takes a few seasons to kill affected plants, but if the proper measures are put in place, it can be easily remedied.

What does iris leaf spot look like?

First, you must identify the fungus. Of the many irises grown in the Mid-Atlantic region, the bearded or German iris (I. × germanica, Zones 3–10) is the most susceptible to iris leaf spot. The most common symptom appears on the leaf blades, starting as small, yellowish lesions that turn brown soon after appearing. The lesions often spread through the leaves after the irises bloom. The leaf blade may eventually become completely necrotic and die back.

Take proactive steps to prevent iris leaf spot infestation

Since the fungus overwinters as a network of fungal threads (or mycelium) on the dead foliage and stems, spores can accumulate in the spring and lead to new infections throughout the growing season. In order to control this cycle of infections, the following measures can be easily taken to slow and ultimately eliminate the fungus.

1. Remove all dead debris in the fall.
2. Remove any foliage with lesions as soon as you see it.
3. Increase air circulation by dividing irises if they are too crowded and removing other plants if they are located too closely.
4. Avoid getting water on the foliage when watering, and do not work with the plants when the foliage is wet. This will prevent the spores from spreading.
5. Plant irises in full sun and well-drained soil.
6. Space plants apart to promote adequate air circulation.
7. Maintain a neutral soil pH, and add lime if the pH is below 6.
8. Plant more leaf spot–resistant iris species, such as Siberian iris (Sibirica, Zones 3–8).

Typically, the cultural practices mentioned above can be utilized to reduce and eliminate iris leaf spot. Spraying a chemical fungicide is often not necessary if the fungus is properly monitored and treated promptly. As fall approaches, be sure to remove any visibly infected foliage as it appears, and the disease will start to subside in subsequent seasons.

—Michele Christiano has worked in public and private gardens for most of her career. She lives in southern Pennsylvania.

Learn more

More Mid-Atlantic Regional Reports

More from Michele Christiano

More on Irises

Fine Gardening Recommended Products

A.M. Leonard Deluxe Soil Knife & Leather Sheath Combo

Fine Gardening receives a commission for items purchased through links on this site, including Amazon Associates and other affiliate advertising programs.

MULTITASKING DUAL EDGES: a deep serrated edge and a tapered slicing edge ideal for tough or delicate cuts. DURABLE 6-inch stainless steel blade withstands 300 lbs of pressure. TWINE CUTTING NOTCH, DEPTH GAUGE MARKINGS & spear point - no need to switch tools when using this garden knife. LEATHER SHEATH: heavy duty, protective, clip on sheath to keep your knife convenient and secure. LIFETIME WARRANTY.

Buy at Amazon

Worx WG323 20V Power Share 10" Cordless Pole/Chain Saw with Auto-Tension (Battery & Charger Included)

Fine Gardening receives a commission for items purchased through links on this site, including Amazon Associates and other affiliate advertising programs.

[12’ EXTRA REACH] It takes just a few minutes and no tools to attach the pole. And with a reach of up to 12’ and only 10 lbs., you’ll be able to get to branches and limbs before they become a problem. Please refuel before use, but oil is sold separately. [FAST, CLEAN CUTS] This cordless 20V PowerShare chain saw is as fast and powerful as anything in its class. And with no gas, and no cord, there’s much less muss and fuss. [SAME BATTERY, EXPANDABLE POWER] The same battery powers over 75plus 20V, 40V, and 80V lifestyle, garden, and power tools in the Power Share family. [AUTO-CHAIN TENSION] The automatic, tool-free chain tension system ensures you’ll always have the correct tension for the job at hand. [AUTO-CHAIN LUBRICATION] The automatic chain lubrication system not only makes for smoother, more efficient cuts, but it extends the life of the tool. And the oil level indicator lets you know exactly where you stand. [COMPACT, LIGHTWEIGHT] The saw itself weighs only 10 lbs. You’ll work longer, with less strain on your hands, arms, back, and legs. [PLENTY OF JUICE] These are the same batteries we use on our Jawsaw and in our lawnmowers. They’re powerful enough for those, and powerful enough for a chainsaw.

Buy at Amazon

Corona AG 4930 Long Straight Snip, Tempered Steel

Fine Gardening receives a commission for items purchased through links on this site, including Amazon Associates and other affiliate advertising programs.

Long, straight, pointed blade for a wide range of applications. Fully heat-treated steel blades for strength. Blades, spring, and handle coated to prevent rust. Unobtrusive leather strap lock.

Buy at Amazon