Symptoms and Signs
Botrytis leaf blight, sometimes also referred to as Botrytis leaf spot, occurs on onions. White, sunken spots on leaves are usually the first sign of infection. Spots are small, oval-shaped, and range from 0.06 to 0.25 inch (0.5–6 mm) long. They sometimes have a light-green halo and may appear water-soaked. The epidermis around the spots may be silvery. When numerous spots are present, leaf tips die back, and whole leaves may eventually die. In severely affected fields, plants appear blighted, with many dead and dried out leaves. When leaves are killed prematurely, bulb size is reduced, resulting in yield losses.
Comments on the Disease
Botrytis squamosa spores are airborne, and land on leaf surfaces. In the presence of moisture, the spores germinate and produce enzymes that kill leaf tissue. The fungus damages the leaf by causing leaf spotting.
Leaf surfaces must be wet from dew or rain for long periods (20 or more hours) for leaf spots to develop. Optimal temperature for spore germination is 59°F, and optimal temperature for mycelial growth is in the mid-70s (°F). The fungus forms survival structures (sclerotia) on infected tissue, which can persist in soil for months to years.
Poor air circulation in the onion canopy also favors the disease. This may occur when onions are planted too closely together, or next to trees or other structures that block air movement.
Management
Cultural Control
Culturally manage Botrytis leaf blight with the following methods:
- Isolate seed fields from fresh-market or processing onion fields, since bulbs are a major source of airborne Botrytis spores.
- To reduce the amount of sclerotia in the soil, rotate away from Allium crops for three years.
- Destroy volunteer and cull onions during this period.
- Plant with single row spacing at least 12 inches apart to speed up leaf drying and reduce severity of the disease.
- Time sprinkler irrigation to prevent extended leaf wetness and maximize leaf drying between irrigation events.
Chemical Control
Monitor fields and apply a fungicide at the first evidence of leaf spotting, when plants have at least five true leaves. Since Botrytis-specific fungicides are highly prone to resistance development, it is especially important to rotate fungicides with different modes of action. Fungicide efficacy can also be improved with the application of an adjuvant. The waxy surface of onion leaves helps to prevent crop injury from the adjuvant.
| 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. | BOSCALID/PYRACLOSTROBIN | |||
| (Pristine) | 14.5–18.5 oz | 12 | 7 | |
| MODE-OF-ACTION GROUP NAME (NUMBER1): Succinate-dehydrogenase inhibitor (7)/quinone outside inhibitor (11) | ||||
| COMMENTS: To limit the development of resistance, do not make more than two applications of Group 7 and Group 11 fungicides without alternating to a fungicide with a different mode of action. | ||||
| B. | FLUAZINAM | |||
| (Omega 500F) | 1.0 pt | Hand weeding: 24 Other activities: 12 |
7 | |
| MODE-OF-ACTION GROUP NAME (NUMBER1): 2,6-dinitro anilines (29) | ||||
| COMMENTS: Not registered for use on green onions. | ||||
| C. | PYRACLOSTROBIN/FLUXAPYROXAD | |||
| (Merivon Xemium) | 8–11 fl oz | 12 | 7 | |
| MODE-OF-ACTION GROUP NAME (NUMBER1): Quinone outside inhibitor (11)/succinate-dehydrogenase inhibitor (7) | ||||
| COMMENTS: To limit the potential for resistance development, do not make more than 3 applications per season or apply more than 33 fl oz product per acre per season. Do not make more than two applications of Group 7 and Group 11 fungicides without alternating to a fungicide with a different mode of action. | ||||
| D. | CHLOROTHALONIL | |||
| (Bravo Weather Stik, Echo 720) | Label rates | 12 | See label | |
| MODE-OF-ACTION GROUP NAME (NUMBER1): Multi-site contact (M5) | ||||
| E. | PENTHIOPYRAD | |||
| (Fontelis) | 16–24 fl oz | 12 | 3 | |
| MODE-OF-ACTION GROUP NAME (NUMBER1): Succinate-dehydrogenase inhibitors (7) | ||||
| 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, 7, 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 group number. |
| ‡ | 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 until the harvest may take place. In some cases, the REI exceeds the PHI. The longer of these two intervals is the minimum time that must elapse before harvest. |