Symptoms and Signs
Downy mildew initially causes a light green to yellow discoloration on the upper blackberry or olallieberry leaf surface that eventually progresses to red and purple. Mature lesions are often angular and restricted by veins. White to gray spore masses also appear on the opposite side of lesions on the leaf underside, but they may be sparse and difficult to see.
Primocanes systemically infected by downy mildew are often stunted and have red streaks on the side of the cane that faces the sun, with reddish colored terminal leaves. Downy mildew infected fruit (dry berry) is dull in luster, lacking in turgidity, and dries out rapidly. Early infection of green fruit induces premature reddening, shriveling, and hardening. Fruit infection later in the season causes shriveling, drying, and the fruit splitting into two parts. Downy mildew infected pedicels are dry and red.
Comments on the Disease
Downy mildew is most prevalent during wet weather at temperatures of 65°F (18°C). The pathogen overwinters as mycelium inside roots, crowns, and canes. Sporulation is usually found in dense foliage near the cane or at the base of the plant. Airborne spores are produced during cool, wet nights and are disseminated by wind. Symptoms develop within 10 to 11 days after infection.
Weed growth and dense canopies create humid environments that favor the development of the disease on suckers.
Management
Use pathogen-free planting stocks. If possible, avoid planting in sites with a history of this disease. The use of macrotunnels is very limiting to downy mildew infestation, because of the near total lack of free moisture on the leaves and flowers of the plants.
Destroy alternate hosts, such as rose or wild blackberries, that are in close proximity to a planting. Once the planting is established, remove suckers and weeds to reduce humidity at the base of the plant. Remove and destroy old fruiting canes after harvest. Reducing moisture in the hedgerow by pruning can be key in managing downy mildew.
Organically Acceptable Methods
The use of clean planting stock, careful site selection, proper pruning, maintenance of a dry hedgerow, and some copper sprays are acceptable for use on organically certified produce.
Treatment Decisions
Fungicide sprays may be applied in spring to protect blackberry foliage, flowers, and developing berries from infection.
| 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. | FOSETYL-AL | |||
| (Aliette WDG) | 5 lb | 12 | 60 | |
| MODE OF ACTION GROUP NAME (NUMBER1): Phosphonate (33) | ||||
| COMMENTS: Do not tank mix with copper compounds, surfactants, or foliar fertilizers. | ||||
| B. | PHOSPHOROUS ACID | |||
| (Fosphite) | Label rates | 4 | 0 | |
| MODE-OF-ACTION GROUP NAME (NUMBER1): Phosphonate (33) | ||||
| COMMENTS: Do not tank mix with copper compounds, surfactants, or foliar fertilizers. Allow 20 days before applying to a copper-treated crop. | ||||
| C. | COPPER# | Label rates | 24 | 0 |
| MODE-OF-ACTION GROUP NAME (NUMBER1): Multi-site contact (M1) | ||||
| COMMENTS: For organically certified produce, check with your certifier for acceptable copper formulations. | ||||
| ‡ | 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. |