Symptoms and Signs
Leaf spot can cause symptoms on leaves and canes of blackberries. Leaf lesions are somewhat circular in shape, often having a brown or purple margin, and typically have a whitish center. Leaf spots are about 0.12 to 0.16 inch (3–4 mm) in diameter. Lesions on canes and petioles are similar to those on leaves but more elongate. Small, black pycnidia (fungus fruiting bodies of the Septoria stage) can be seen in the whitened area of the mature lesions. Symptoms appear late in the season although infection can occur multiple times throughout the season.
Comments on the Disease
Leaf spot is found on erect and trailing blackberry, dewberry, olallieberry, and boysenberry. Conidia are disseminated by wind and water splash throughout the growing season, and infection increases with increased rainfall. For many years this pathogen was thought to be the same as the one that causes raspberry leaf spot on raspberry. It is now clear that the causal agent for leaf spot, Mycosphaerella rubi (anamorph Septoria rubi), does not infect raspberry. Raspberry leaf spot is caused by the pathogen Sphaerulina rubi (anamorph Cylindrosporium rubi) and is not common in California. The methods of control for the two diseases are very similar, however.
Management
The key to managing leaf spot during the growing season is to increase air circulation within the planting and the row. This includes such practices as proper spacing, thinning to maintain proper cane density, and maintaining narrow rows. These practices lower canopy humidity and allow faster drying of foliage and canes, resulting in less infection. The most important management tool is dormant lime sulfur applications that serve to burn out old lesions and thus reduce sporulation.
The use of macrotunnels is very limiting to leaf spot infestations because of the near total lack of free moisture on the leaves and flowers of the plants.
The pathogen overwinters as mycelium and pycnidia in dead leaves and stems. To reduce sources of inoculum, it is important to prune out and remove from the site old fruiting canes, and to remove dead and damaged canes and leaf debris after harvest. Clean nursery stock limits introduction of the disease into the field.
Organically Acceptable Methods
Practices that increase air circulation within the planting, pruning out infected canes, and treatments with a copper sulfate either alone or in combination with lime sulfur are acceptable for use in an organically certified crop. Dormant applications of liquid lime sulfur eradicate a high percentage of overwintering pycnidia and should be done after putting canes on the wires. The use of macrotunnels is also acceptable.
Treatment Decisions
No fungicides are currently registered for use specifically against leaf spot; however, fungicides used to control anthracnose and Botrytis gray mold help control leaf spot. In addition, sprays of copper and lime sulfur offer some control.
| 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. | COPPER SULFATE# | Label rates | 24 | 0 |
| MODE OF ACTION GROUP NAME (NUMBER1): Multi-site contact (M1) | ||||
| COMMENTS: Apply in late August or September. | ||||
| . . . and . . . (optional) | ||||
| LIME SULFUR# | Label rates | 48 | 0 | |
| MODE OF ACTION GROUP NAME (NUMBER1): Multi-site contact (M2) | ||||
| COMMENTS: Apply in late August or September or during the dormant season. | ||||
| B. | PYRACLOSTROBIN+BOSCALID | |||
| (Pristine) | 18.5–23 oz | 12 | 0 | |
| MODE OF ACTION GROUP NAME (NUMBER1): Quinone outside inhibitor (11) and Carboxamide (7) | ||||
| ‡ | 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 two intervals is the minimum time that must elapse before harvest. |
| # | Acceptable for use on organically grown produce. |
| 1 | Group numbers are assigned by the Fungicide Resistance Action Committee (FRAC) according to different modes of actions. Fungicides with a different group number are suitable to alternate in a resistance management program. In California, make no more than one application of fungicides with mode of action Group numbers 1, 4, 9, 11, or 17 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. |