Symptoms and Signs
Caneberry leaves infected with powdery mildew initially may develop light green (chlorotic) patches on leaves. Leaves and shoots later are covered with white to gray mycelial growth. Affected leaves may be curved, twisted, or otherwise distorted. Severely infected shoots become long and spindly with dwarf leaves that curl upward. Severely diseased plants may be stunted.
Comments on the Disease
In California, the powdery mildew pathogen overwinters as mycelium in buds or on the surface of first-year canes (primocanes). Later, conidia are produced abundantly on the surface of infected tissue and serve as secondary inoculum for repeated cycles of infection throughout the growing season. The spores are airborne and the development of disease is favored by warm, dry weather.
Management
Blackberries and varieties of red, black, and purple raspberries vary in their susceptibility to powdery mildew. Where powdery mildew is a serious problem, avoid planting highly susceptible cultivars. The blackberries from the Arkansas breeding program such as Navaho, Apache, and Arapaho tend to have very low susceptibility to powdery mildew as well as other diseases.
Practices that allow good air circulation and direct light penetration have been reported helpful in limiting the spread of the disease. These include proper plant spacing, cane thinning, and maintaining narrow rows. Removing late-forming suckers with powdery mildew symptoms and cutting back of floricanes (second-year canes) to a horticulturally desired height may reduce primary sources of inoculum.
Incidence and severity of powdery mildew tends to be higher in macrotunnels because the warm, dry conditions found inside the tunnels favor disease development. Free water is not necessary for the disease to develop, but higher humidity favors the disease.
Organically Acceptable Methods
The use of resistant varieties, maintaining good air circulation in the planting, and sprays of sulfur or potassium bicarbonate are acceptable for use in organically certified crops.
Treatment Decisions
Treat when disease is first evident.
| 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. | MICRONIZED SULFUR | ||||
| (Microthiol Disperss)# | Label rates | 24 | 0 | ||
| (Sulfur 90W) | Label rates | 24 | 0 | ||
| MODE-OF-ACTION GROUP NAME (NUMBER1): Multi-site contact (M2) | |||||
| COMMENTS: Sulfur is phytotoxic to some varieties, especially when temperatures exceed 90°F. Repeat treatment at 10-day intervals. | |||||
| B. | PYRACLOSTROBIN+BOSCALID | ||||
| (Pristine) | 18.5–23 oz | 12 | 0 | ||
| MODE-OF-ACTION GROUP NAME (NUMBER1): Quinone outside inhibitor (11) and Succinate dehydrogenase inhibitor (7) | |||||
| C. | MYCLOBUTANIL | ||||
| (Rally 40WSP) | 1.25–1.5 oz | 24 | 0 | ||
| MODE-OF-ACTION GROUP NAME (NUMBER1): Demethylation inhibitor (3) | |||||
| COMMENTS: Initiate applications as early as budbreak and continue at 10- to 14-day intervals; shorter intervals may be used under heavy disease pressure. Do not apply more than 10 oz/acre per season. | |||||
| D. | POTASSIUM BICARBONATE | ||||
| (Kaligreen)# | Label rates | 4 | 1 | ||
| MODE OF ACTION: An inorganic salt. (NC) | |||||
| COMMENTS: Apply with sufficient water (25 gal/acre) to ensure complete and thorough coverage of the foliage and crop. Alternate application with other effective fungicides for resistance management on a 7- to 10-day interval. Can burn some varieties more easily than others; always test first when applying to a variety for the first time. | |||||
| E. | HORTICULTURAL OIL# | ||||
| (Organic JMS Stylet Oil) | 3–6 quarts/100 gal water | 4 | 0 | ||
| MODE OF ACTION: Contact including smothering and barrier effects. | |||||
| COMMENTS: Amount is for 100 gal/acre; may use up to 150 gal/acre water carrier. Do not apply more than 6 qt (1.5 gal) oil/acre. Spray with ground equipment for optimum coverage of leaf surfaces. Oil sprays need to be applied frequently to achieve acceptable control, however, frequent applications of oils can damage the plant and compromise fruit yield. Heed label warnings about compatibility with other materials. | |||||
| # | Acceptable for use on organically grown produce. |
| ‡ | 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. |
| 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. |