Plant Disease Clinic--Highlights

Bacterial Blight - Geranium

Rainy weather has favored the development of foliar diseases on flowering plants. Bacterial blight on geraniums is an example. The spread of this disease and its symptoms are usually most severe under rainy, hot weather.

The disease organism causes brown spots on leaves, often surrounded by a yellow halo. V-shaped, yellow-brown lesions may also be evident on the margins of leaves. The bacterium can invade the plants vascular system, causing plants to wilt and eventually die. The disease may spread quickly through a bed of geraniums by splashing water, plant handling, or insects.

Fixed copper fungicides can slow the spread of the disease. Because the disease organism can survive the winter on plant debris, it is important to remove diseased plants and turn under any remaining debris. This will help prevent a disease problem the following year. It is also important to space plants properly for good air circulation and to avoid wetting leaves when irrigating.

Black Spot - Rose

Why are the leaves falling off my roses? This is a question often asked by rose growers. In many cases the answer is black spot. Black spot is caused by the fungus Diplocarpon rosae. Circular, feathery black spots occur on the upper surface of leaves. These spots are frequently surrounded by a yellow halo. Infected leaves turn yellow and fall prematurely. Purple-red lesions may also develop on first-year canes.

Sanitation measures are important for controlling black spot. Begin with a thorough clean-up in the fall. Diseased leaves on the ground should be raked and destroyed. All diseased canes should be pruned off. These precautions reduce overwintering fungi.

If a preventative fungicide program is used, it should start before leaves become spotted. From then until frost, the leaves may require additional protective sprays. There are a number of rose sprays and dusts on the market. Consult the label for specific information on timing and rates.

It has been reported that a common household product- baking soda- and a horticultural spray oil (such as Sunspray) combine into a treatment that is effective against black spot and powdery mildew on rose. The spray consists of 1 Tbsp. of baking soda and 2-1/2 Tbsp. of horticultural spray oil per gallon of water. This combination product may be worth a try. Keep in mind that this spray has shown the best results in greenhouses rather than outdoor trials and that baking soda has not yet received a Federal label for disease control.

This article originally appeared in the July 28, 1993 issue, p. 131.


Links to this article are strongly encouraged, and this article may be republished without further permission if published as written and if credit is given to the author, Horticulture and Home Pest News, and Iowa State University Extension and Outreach. If this article is to be used in any other manner, permission from the author is required. This article was originally published on July 28, 1993. The information contained within may not be the most current and accurate depending on when it is accessed.