Cause A bacterium, Pseudomonas syringae, causes bacterial leaf spot or blight of clover. The bacterium overwinters on diseased crop residue. Infections in the field can be spread by splashing rain or irrigation water, and on contaminated tools or equipment. Cool, moist conditions favor disease, including rainy weather, excessive irrigation, or extended periods of dews. The bacteria enter plant tissue through natural openings such as stomata and hydathodes (latter are small openings where the leaf veins reach the margin of the leaf), or wounds from hail, wind-storms, insect feeding injury, and mechanical damage.
Symptoms Dark brown-to-black blotches that may have an angular appearance develop on leaves, petioles, and stems.
Cultural control
- Minimize plant injury.
- Avoid working diseased fields when wet.
- Power-wash equipment after working in diseased fields.
Chemical control
- Badge SC (Group M1) at 0.5 to 1.8 pints/A on 7- to 14-day intervals. 48-hr reentry.