This popular ground cover has two major disease problems—one caused by various fungi and the other by a bacterium. Accurate identification is important because control measures differ for the two diseases. In Illinois bacterial leaf spot is more common than fungal leaf spot.
Bacterial leaf spot and stem canker appears as small, circular, dark green, watersoaked (almost oily looking) lesions on the leaves. As these enlarge, they develop reddish brown to black centers with a water-soaked margin and often a yellow halo. Hold the leaf up to the light to see the halo. The spots crack with age. In warm, wet weather the bacterium causes black cankers on the stems and petioles; stems die, often with black tips. The fungal leaf spot diseases do not cause stem cankers.
Fungal leaf spots are caused by a variety of fungal species that cause round to irregular spots in a variety of colors. Often a series of concentric rings can be seen in the spots, giving them a target effect. Look closely at the spots for small black specks that do not rub off; these are the fruiting structures of the fungi. Bacteria do not form such fruiting bodies.
When establishing a new bed of ivy, inspect plants to be certain you are not introducing these diseases. Remove any questionable leaves or stems and remove old leaves and debris from the beds. Because these diseases require water on the foliage for infection to occur, water the soil rather than the foliage--though that may be hard to do in a wet year such as this. Water early in the day to promote quicker drying.
If leaf spots have been severe in the past, determine whether the fungal or bacterial pathogens are to blame. Few chemicals protect plants from the bacterial leaf spot and stem canker. Try to improve air movement in the area by thinning the stand and pruning surrounding plants. Never work with the plants when they are wet.
For fungal infections, consider a chemical application program as soon as possible. Registered chemicals are listed on page 126 of the Illinois Commercial Landscape and Turfgrass Pest Management Handbook, 1998–1999 or page 18 of the Homeowners’ Guide to Pest Management. New growth can be protected from infection using fungicides at seven- to ten-day intervals as long as wet weather persists in the spring and early summer.
For more information about these diseases, consultReport on Plant Diseases No. 652.