Each year, English ivy ground covers are attacked by leaf spots and cankers. There is a common fungal disease as well as a bacterial one, and you need to know the difference to attain adequate disease control.
Bacterial leaf spot and stem canker is the more common disease. It first appears as small, circular, dark green, water-soaked (oily) lesions on the leaves. As these lesions enlarge, they have reddish brown to black centers with a water-soaked margin and sometimes a yellow halo. The spots also 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 spots are caused by a variety of fungal species. They cause round to irregular spots in a variety of colors. Often, a series of concentric rings can be seen in the spots. Look closely on the spots for small, black specks, which are fruiting structures containing spores of fungi. Bacterial spots do not have fruiting structures because bacteria do not form spores.
If you establish a bed of ivy this year, look closely at new plants to be certain that you do not introduce diseased plants. If you receive gift plants from a friend, inspect the established bed for disease problems before taking them. 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.
Because these diseases require water on the foliage to infect the blades, water the soil rather than the foliage where possible. Water early in the day so wet foliage dries quickly.
If leaf spots have been severe in the past, apply fungicides, starting when new leaf growth begins in the spring. Registered chemicals are listed in the Illinois Commercial Landscape & Turfgrass Pest Management Handbook, 1998–1999 and the Illinois Homeowners’ Guide to Pest Management. Because the chemicals are protectants, they usually specify that they be repeated at 7- to 10-day intervals as long as wet weather persists in the spring and early summer. Few chemicals protect plants from bacterial leaf spot and stem canker. The copper compounds and Chipco Aliette may help, but control is not complete. 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 more information about these diseases, consult Report on Plant Disease No. 652, “Leaf Spot Diseases of English Ivy.”