Symptoms and Signs
The first signs of powdery mildew are small, white powdery spots that appear usually on the under surface of older leaves when sugarbeet plants are 2 to 6 months old. Under suitable conditions, the fungus spreads rapidly over the entire surface of the leaf, and eventually to all leaves on affected plants. Older leaves may yellow and eventually become necrotic and die.
Comments on the Disease
Powdery mildew is an annual problem on sugarbeet in California. The fungus overwinters on sugarbeet and other Beta species such as swiss chard, table beets and wild Beta species that grow throughout the winter. Ideal conditions for disease development are warm, dry weather; optimum temperatures for growth of the fungus are between 60° and 86°F (15° and 30°C). Very high daily temperatures of 100°F (38°C) or higher tend to arrest disease development. Following initial infection, the fungus grows over the surface of the leaf and produces asexual spores (conidia), which give the leaf a powdery appearance. The conidia are airborne and can be carried considerable distances to start new infections. If the disease is not controlled, 20 to 35% loss in sugar yield can occur.
Management
Currently, varieties with moderate resistance are available. Use these varieties in combination with chemical control measures. Apply a fungicide before, if possible, or when the first small, white powdery spots appear on the undersurface of leaves. Repeated applications are necessary at 3- to 6-week intervals if the disease reappears. Good coverage of the beet leaf surfaces is essential.
| Common name | Amount per acre | REI‡ | PHI‡ | |
|---|---|---|---|---|
| (Example trade name) | (hours) | (days) | ||
| Not all registered pesticides are listed. The following are listed 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 the pesticide’s properties and, application timing, honey bees, and environmental impact. Always read the label of the product being used. | ||||
| A. | SULFUR# | |||
| (Dust) | 30 lb | 24 | 0 | |
| (Micronized wettable) | 10 lb | 24 | 0 | |
| MODE-OF-ACTION GROUP NAME (NUMBER1): Multi-site contact (M2) | ||||
| COMMENTS: Other types of sulfurs may be used. | ||||
| B. | PYRACLOSTROBIN | |||
| (Headline) | 9–12 fl oz | 12 | 7 | |
| MODE-OF-ACTION GROUP NAME (NUMBER1): Quinone outside inhibitor (11) | ||||
| COMMENTS: Maximum use per year is 48 oz/acre, but do not reapply. Alternate use with other mode-of-action fungicides to avoid the rapid development of resistance by the disease organism. | ||||
| C. | AZOXYSTROBIN | |||
| (Quadris) | 9.2–15.5 fl oz | 4 | 0 | |
| MODE-OF-ACTION GROUP NAME (NUMBER1): Quinone outside inhibitor (11) | ||||
| ‡ | 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 action. 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 fungicide with a different mode-of-action group number. |