There are several leaf spotting diseases of English ivy, Hedera helix. Two are caused by fungal pathogens, and one is the result of a bacterial agent. All can cause leaf spots, stem cankers, and thinning of plant canopy.
The first disease we saw on English ivy at the Plant Clinic this year was anthracnose. Many different anthracnose fungi occur on many different plant species. The one on Hedera helix is a Colletotrichum species. We usually see this fungal disease following injury. In fact, the late frost in early May served as a precursor to anthracnose on English ivy this year. With proper sanitation, good air movement, and good growing conditions, that disease probably won’t persist.
Bacterial leaf spot and stem canker is the more common disease in Illinois. Unfortunately, it is the most difficult to manage. It thrives in warm, wet weather, such as we have experienced in much of the state lately. Bacterial leaf spot first appears as small, circular, dark green, water-soaked (oily) lesions on the leaves. As these enlarge, they have reddish brown to black centers with a water-soaked margin and (sometimes) a yellow halo. The spots also crack with age. The bacterium may cause black cankers on the stems and petioles; and those stems die, often with black tips. The bacterial pathogen is easily spread from plant to plant by splashing water.
The fungal leaf spots are caused by a variety of fungal species. Technically, anthracnose is included here as well. I have listed it separately because it usually follows an injury. The fungal diseases cause round to irregular spots in a variety of colors, much as with bacterial spot. One distinguishing characteristic is that often a series of concentric rings can be seen in the fungal spots (like a target). In addition, the fungal spots form fruiting structures within the spots. Look closely on the spots for small, black specks (pinhead-sized), which are fruiting structures containing spores of fungi. These fruiting bodies may be embedded in the spots (pycnidia), or they may be on the surface and have black, hairlike structures called setae (anthracnose). Bacterial spots do not contain fruiting structures. The diseases are easily distinguished in a lab but can be confusing in the field.
Inspect new plant purchases carefully. Remove any questionable leaves or stems from transplants. It is also a good idea to remove old leaves and debris from the beds each spring before new growth starts. Always work with the plants when they are dry to avoid spread of the pathogen. Infection occurs through moisture on the foliage, so water the soil rather than the foliage where possible. Consider using soaker hoses in ivy beds. It is a good practice to water early in the day.
If leaf spots have been severe, apply fungicides starting when new leaf growth begins in the spring. Registered chemicals are listed in the Illinois Commercial Landscape and Turfgrass Pest Management Handbook and the Home Yard and Garden Pest Guide. Because the chemicals are protectants, they usually specify repeats at 7- to 10-day intervals as long as wet weather persists in the spring and early summer. If you choose a systemic product, you do not have to spray as often. Chemical mobility is listed at the end of the disease chapters in the pest guides listed above. Many chemical options listed for both diseases. If you cannot determine which disease is present, use a product listed for both. Try to improve air movement in the area by thinning the stand and pruning surrounding plants. For more information about these diseases, consult Report on Plant Disease (RPD), no. 652, “Leaf Spot Diseases of English Ivy.” RPDs are available in Extension offices or on the Internet at http://www.ag.uiuc.edu/%7Evista/horticul.htm.