How to Manage Pests

UC Pest Management Guidelines

Initial symptoms of downy mildew consist of 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.

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 and a few Chenopodium weed species. The pathogen exists as distinct genetic races.

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.

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. A recent example of this pathogen/cultivar cycle took place in early 1998 when race 4-resistant cultivars were infected by races 5 and 6. Researchers and plant breeders are currently developing cultivars resistant to the new 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/Acre	P.H.I.+
(trade name)			(days)
A.	MEFENOXAM	up to 1 lb a.i./acre/season for all formulations	21
	MODE OF ACTION: A PA(Group 4)1 acylalanine fungicide.
	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
	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
	COMMENTS: This is a 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
	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	3
	MODE OF ACTION: A phosphonate (Group 33)1 fungicide.
	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
	(Actigard) 50WG	0.75 oz	7
	(Blockade) 50WG	0.75 oz	7
	MODE OF ACTION: A benzo-thiadiazole (Group P)1 fungicide.
	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.	PHOSPHOROUS ACID
	(Fosphite)	1–2 qt
	MODE OF ACTION: A phosphonate (Group 33)1 fungicide.
	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.
E.	COPPER HYDROXIDE	Label rates
	MODE OF ACTION: A multi-site contact (Group M)1 inorganic fungicide.
	COMMENTS: Copper sprays are only marginally effective even when several applications are made.
+ Preharvest interval. Do not apply within this many days of harvest. 1 Group designations are assigned by the Fungicide Resistance Action Committee (FRAC) according to different modes of actions. Fungicides with a different group designation are suitable to alternate in a resistance management program. For more information, see www.frac.info/.

PUBLICATION

UC IPM Pest Management Guidelines: Spinach
UC ANR Publication 3467
Diseases
S. T. Koike, UC Cooperative Extension, Monterey Co.
M. LeStrange, UC Cooperative Extension, Tulare Co.
F. F. Laemmlen, UC Cooperative Extension, Santa Barbara Co.

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