Bacterial pathogens usually thrive in wet conditions. The 1998 growing season has provided almost ideal conditions for bacterial pathogens. The Plant Clinic recently diagnosed a sample of bacterial leaf spot on larkspur. Although larkspur is not a major crop or a host that we see often at the clinic, we do see a variety of annuals and perennials—many with similar bacterial problems.
The larkspur bacterial pathogen is a species of Pseudomonas, as are many of the bacterial pathogens of ornamentals. The bacterium causes the development of black, almost tarlike spots on the leaves and stems; the spots cannot be rubbed off. They are small (1/4 inch diameter) at first but quickly enlarge or merge to cause large blotches on the leaves. In very wet conditions, they turn entire leaves black. Most of our sources list this as a serious disease of seedlings, but it will affect lower leaves of large plants and continue to move up the plant as long as wet weather continues.
It is difficult to control bacterial diseases. Generally, control measures concentrate on sanitary measures such as removing affected leaves (only when dry) as they occur. Because the bacterium overwinters in the crowns, new growth in the spring should be monitored and affected leaves removed as soon as possible. Watering the soil (as opposed to the foliage) will help prevent splashing the bacteria and causing further infection.
Bactericides are not recommended for most bacterial diseases of ornamentals. You won’t find control measures listed for this disease in the Illinois commercial or homeowner pest handbooks, because in most years we do not see this disease. Copper fungicides are sometimes used as preventives of bacterial diseases with some success. In the case of larkspur bacterial leaf spot, copper fungicides can be effective if used as a drench over plant crowns in autumn, followed by a spray of the new leaves as they emerge in spring. Hi-Yield produces a bordeaux mixture for homeowner application. Many different copper fungicides are available in retail centers in package sizes appropriate for homeowner use. Choose one that lists your particular host on the label: it is illegal to use a pesticide in a manner not listed on the label.