Symptoms and Signs
Initial symptoms of downy mildew consist of dull to bright yellow spots that form on cotyledons and leaves of all ages. With time, these spots can enlarge and become tan and dry. Close inspection of the underside of the leaf often reveals the purple growth of the fungus (sporangia and sporangiospores). If disease development is extensive, leaves appear curled and distorted and may take on a blighted effect as a result of numerous infection sites.
Comments on the Disease
Downy mildew is clearly the most widespread and destructive spinach disease in California. Like all downy mildews, this pathogen requires cool, wet conditions for infection and disease development. The heavy canopy of densely planted spinach retains much moisture and creates ideal conditions for infection and disease development. Spores (called sporangia) are dispersed in the air from plant to plant and field to field by winds and splashing water. Peronospora farinosa f. sp. spinaciae infects only spinach, but may possibly infect a few Chenopodium weed species. The pathogen exists as distinct genetic races and shows an ability to adapt to new spinach cultivars; from 1989 to 2011, races 4, 5, 6, 10, 11, and 12 have occurred and caused significant damage to spinach in California.
In the field the pathogen can grow and spread rapidly, resulting in widespread crop damage if environmental conditions are favorable. In addition to loss of quality due to spots, the downy mildew infections can also break down and rot if packed in bags and cartons.
Management
The use of resistant cultivars is the most effective means of controlling spinach downy mildew. During the past 50 years in California, each outbreak of a new downy mildew race was later matched by the development of resistant spinach lines. Researchers and plant breeders are currently developing cultivars resistant to the most recently occurring races. All foliar fungicide materials are protectants and for best results must be applied before infection occurs and before symptom development.
Organically Acceptable Methods
Cultural controls are acceptable for use on organically grown produce.
| 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 product being used. | |||||
| A. | MEFENOXAM | up to 1 lb a.i./acre/season for all formulations | 48 | 21 | |
| MODE OF ACTION GROUP NAME (NUMBER1): Phenylamide (4) | |||||
| COMMENTS: A systemic fungicide applied to soil or plant foliage. There are several use recommendations that apply to all Ridomil formulations labeled for spinach. Do not exceed a total of 1.4 lb a.i./acre/growing season when using a combination of Ridomil Gold EC, Ridomil Gold GR, and Ridomil Gold/Copper. Plantback restrictions apply to some cereal grains and other food and feed crops. None of these products are registered for use in a greenhouse. | |||||
| (Ridomil Gold EC) | Label rates | 48 | 21 | ||
| COMMENTS: This liquid formulation is an emulsifiable concentrate and is registered for use in spinach as a soil application only. Applications may be made banded over the row, preplant incorporated, or injected with liquid fertilizer. Consult the label for specific application guidelines | |||||
| . . . or . . . | |||||
| (Ridomil Gold GR) | Label rates | 48 | 21 | ||
| COMMENTS: Granular formulation registered for use in spinach as a soil application only. Applications may be preplant incorporated or preemergence. Consult the label for specific application guidelines. | |||||
| . . . or . . . | |||||
| (Ridomil Gold/Copper WP) | Label rates | 48 | 21 | ||
| COMMENTS: This combination is for foliar application to spinach. Up to 2 applications are allowed per crop but curative applications greatly increase the risk of the fungus developing insensitivity to this active ingredient. | |||||
| B. | FOSETYL-AL | ||||
| (Aliette 80 WDG) | 2–5 lb | 12 | 3 | ||
| MODE OF ACTION GROUP NAME (NUMBER1): Phosphonate (33) | |||||
| COMMENTS: A systemic fungicide applied to the foliage. Apply when conditions favor disease development and continue on a 7–21 day interval. Use sufficient water volume for good coverage. Do not exceed 7 applications per season. Do not apply in less than 10 GPA. Speckling on leaves sometimes can occur after application with this product. Follow label recommendations with respect to mixing compounds, buffers, surfactants, and other adjuvants. | |||||
| C. | ACIBENZOLAR-S-METHYL | ||||
| (Blockade 50WG) | 0.75 oz | 12 | 7 | ||
| MODE OF ACTION GROUP NAME (NUMBER1): Benzo-thiadiazole (P1) | |||||
| COMMENTS: May cause leaf twisting or other growth responses. Only for use in Santa Clara, Monterey, San Luis Obispo, Santa Barbara, Ventura, Los Angeles, San Diego, Santa Cruz, San Benito, and San Mateo counties. See label for plantback restrictions. | |||||
| D. | FLUOPICOLIDE | ||||
| (Presidio) | 3–4 fl oz | 12 | 2 | ||
| MODE OF ACTION GROUP NAME (NUMBER1): Benzamide (43) | |||||
| E. | MANDIPROPAMID | ||||
| (Revus) | 8 fl oz | 4 | 1 | ||
| MODE OF ACTION GROUP NAME (NUMBER1): Carboxylic acid amide (40) | |||||
| F. | FENAMIDONE | ||||
| (Reason 500SC) | 5.5–8.2 fl oz | 12 | 2 | ||
| MODE OF ACTION GROUP NAME (NUMBER1): Quinone outside inhibitor (11) | |||||
| COMMENTS: Use allowed under a Supplemental Label. | |||||
| G. | PHOSPHOROUS ACID | ||||
| (Fosphite) | 1–2 qt | 4 | 0 | ||
| MODE OF ACTION GROUP NAME (NUMBER1): Phosphonate (33) | |||||
| COMMENTS: Do not apply with copper-based fungicides or fertilizers. Allow 10 days before applying a copper-based compound to a crop treated with this product or 20 days before applying this product to a copper-treated crop. | |||||
| H. | COPPER HYDROXIDE | Label rates | 24 | 0 | |
| MODE OF ACTION GROUP NAME (NUMBER1): Multi-site contact (M1) | |||||
| COMMENTS: Copper sprays are only marginally effective even when several applications are made. | |||||
| ‡ | 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 fungicide with a different mode of action Group number. |