Symptoms and Signs
The most serious symptoms occur on stems near the soil line where elongated, sunken, brown lesions form. These lesions may girdle the stem, resulting in stunting, wilting, and general poor growth of the plant. If the lesions enlarge, the stem may break, causing the plant to fall over. Lesions usually contain minute, spherical, dark structures that are the fruiting bodies of the pathogen. If conditions are right, pink masses of spores exude from these structures. If seedlings are infected early, they may die. Less important are the leaf spots that may develop on foliage. Leaf spots are circular, light tan, and contain the dark, spherical fruiting bodies of the pathogen. The disease damages the water-conducting tissue, and blackened streaks of xylem can be seen by cutting open the stem.
Comments on the Disease
Of particular importance is the ability of this pathogen to be carried in and on seed. This is how the fungus is introduced into greenhouse and field plantings. The pathogen can live in crop debris if such material is not fully decomposed. Cool, moist conditions enhance disease development. Spores are spread with splashing water. A second spore type may occur that can be blown long distances on wind currents.
Management
Black leg can be managed by using disease-indexed seed, by cultural practices, and with foliar sprays. Remove cruciferous weeds and volunteer plants that may harbor the pathogen. Plow under debris in diseased fields to allow for more rapid and thorough decomposition. Practice crop rotation; rotate infested fields out of cruciferous crops for 1 or 2 years.
| Common name | Amount per acre | REI‡ | PHI‡ | |
|---|---|---|---|---|
| (Example trade name) | (hours) | (days) | ||
| Not all registered pesticides are listed. The following are ranked with the pesticides having the greatest IPM value listed first—the most effective and least likely to cause resistance are at the top of the table. When choosing a pesticide, consider information relating to the pesticide's properties and application timing, honey bees, and environmental impact. Always read the label of the product being used. | ||||
| A. | IPRODIONE | |||
| (Rovral) | 2 pt | 24 | 0 | |
| MODE OF ACTION GROUP NAME (NUMBER1): Dicarboximide (2) | ||||
| COMMENTS: Registered for broccoli only. Use as a foliar treatment applied as a directed spray to the base of the plant. If conditions persist, a second application may be made. Do not make more than two applications per crop. Effective against the black leg pathogen in other Brassica crops outside of California, but no research has been done in California to test its effectiveness on broccoli. | ||||
| ‡ | Restricted entry interval (REI) is the number of hours (unless otherwise noted) from treatment until the treated area can be safely entered without protective clothing. Preharvest interval (PHI) is the number of days from treatment to harvest. In some cases the REI exceeds the PHI. The longer of the two intervals is the minimum time that must elapse before harvest. |
| 1 | Group numbers are assigned by the Fungicide Resistance Action Committee (FRAC) according to different modes of action. Fungicides with a different group number are suitable to alternate in a resistance management program. For fungicides with mode-of-action group numbers 1, 4, 9, 11, or 17, make no more than one application before rotating to a fungicide with a different mode-of-action group number; for fungicides with other group numbers, make no more than two consecutive applications before rotating to a fungicide with a different mode-of-action group number. |