Symptoms and Signs
Infections begin as irregular yellow patches on leaves; these chlorotic lesions later turn tan to light brown. If conditions are favorable, white fluffy growth of the fungus develops on the undersides of leaves. If disease development is extensive, leaves may take on a blighted effect as a result of numerous infection sites. Systemic infections can cause internal black streaks and patches to form in stems and floret branches of broccoli and cauliflower. Early symptoms on transplants may resemble bacterial leaf spot symptoms. Severely diseased seedlings may be stunted or die.
Comments on the Disease
Peronospora parasitica requires cool, moist weather for infection and disease development to take place. The pathogen survives between crops on weed hosts or as resilient oospores in crop residue. Spores are airborne. This disease is most serious on young seedlings; if cotyledons and the first true leaves are severely infected, the young plant may die.
Management
A few broccoli varieties are available that are resistant to downy mildew. Fungicide treatment of susceptible varieties is needed when the disease occurs on transplants or early in crop development in the field; repeated applications may be required, depending on weather. Treatment during early flowering is required on seed crops.
Organically Acceptable Methods
Resistant varieties and some copper sprays are suitable for organically grown crops.
| 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. | CHLOROTHALONIL | |||
| (various products) | Label rates | 12 | 7 | |
| MODE-OF-ACTION GROUP NAME (NUMBER1): Multi-site contact (M5) | ||||
| COMMENTS: Use at 7- to 10-day intervals, if necessary. | ||||
| B. | MEFENOXAM/CHLOROTHALONIL | |||
| (Ridomil Gold/Bravo 76.5) | 1.5 lb | 48 | 7 | |
| MODE-OF-ACTION GROUP NAME (NUMBER1): Multi-site contact (M5) and phenylamide (4) | ||||
| COMMENTS: Use at 14-day intervals, if necessary. | ||||
| C. | FOSETYL-ALUMINUM | |||
| (Aliette) | Label rates | 12 | 3 | |
| MODE-OF-ACTION GROUP NAME (NUMBER1): Phosphonate (33) | ||||
| COMMENTS: Use on a 7- to 21-day intervals as necessary. Do not tank mix with copper compounds. | ||||
| D. | COPPER# | |||
| (various products) | Label rates | See label | 0 | |
| MODE-OF-ACTION GROUP NAME (NUMBER1): Multi-site contact (M1) | ||||
| COMMENTS: Not all copper compounds are approved for use in organic production; be sure to check individual products. | ||||
| ‡ | 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 the two intervals is the minimum time that must elapse before harvest. |
| # | Acceptable for 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. For fungicides with mode-of-action group numbers 1, 4, 9, 11, or 17, make no more than one application 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. |