How to Manage Pests

UC Pest Management Guidelines

Strawberry

Powdery Mildew

Pathogen: Sphaerotheca macularis

(Reviewed 6/08, updated 6/08)

In this Guideline:


SYMPTOMS AND SIGNS

Leaves infected with powdery mildew initially have small, white powdery colonies on the undersides of leaves. These colonies enlarge to cover the entire lower leaf surface, causing the edges of the leaves to roll up. Purple reddish blotches appear on the upper and lower surface of leaves. Infected flowers produce deformed fruit or no fruit at all. Severely infected flowers may be completely covered by mycelium and killed. Infected immature fruits become hardened and desiccated. Infected mature fruits become seedy in appearance and support spore-producing colonies that look powdery and white.

COMMENTS ON THE DISEASE

The disease overwinters as mycelium on leaves in California, so it is most likely introduced into the field through planting material or spores from neighboring fields. Spores are wind disseminated and short-lived. The pathogen also survives as mycelium and cleistothecia on plants coming from nurseries. Ideal conditions for infection are dry leaf surfaces, high relative humidity, and cool to warm air temperatures. Accordingly, the disease is mostly limited to the coastal growing regions and northern nurseries and causes very little damage in inland growing regions.

MANAGEMENT

To control powdery mildew, apply fungicides at the first sign of disease. This is especially important for protectants such as sulfur. During routine field surveys, watch for the leaf distortion and discoloration that are the first signs of powdery mildew, especially in fall and spring. Controlling powdery mildew in the fall reduces the amount of disease that develops the following spring, and controlling foliar disease helps prevent fruit infections. The standard practice of removing leaves from transplants during harvest and packing helps minimize introduction of the disease, although inoculum may still be present on crowns. Cultural practices are important in helping to prevent disease buildup.

Cultural Control
Avoid overhead irrigation and excess use of nitrogen and use resistant cultivars where practical.

Organically Acceptable Methods
Applications of mined sulfur or insecticidal soap are acceptable on organically certified strawberries. Use resistant cultivars where practical. Select field sites where environmental conditions are not conducive to disease development.

Treatment Decisions
Apply fungicides about 1 month after planting and again 3 to 4 weeks later. Make additional treatments when plants begin to bloom.

Common name Amount/Acre** R.E.I.+ P.H.I.+
(trade name)   (hours) (days)

  Calculate impact of pesticide on air quality
The following materials are listed in order of usefulness in an IPM Program, taking into account efficacy. Also consider the general properties of the fungicide as well as information relating to environmental impact. Not all registered pesticides are listed. Always read label of product being used.
 
A. QUINOXYFEN
  (Quintec) 4–6 fl oz 12 1
  MODE OF ACTION GROUP NAME (NUMBER1): Quinoline (13)
 
B. MYCLOBUTANIL
  (Rally 40W) 2.5–5.0 oz 24 0
  MODE OF ACTION GROUP NAME (NUMBER1): Demethylation inhibitor (3)
  COMMENTS: Some resistance to this fungicide may be occurring. Apply in a minimum of 100 gal water/acre. Do not apply more than 30 oz/acre/year.
 
C. TRIFLUMAZOLE
  (Procure 50WS) 4–8 oz 12 1
  MODE OF ACTION GROUP NAME (NUMBER1): Demethylation inhibitor (3)
 
D. PYRACLOSTROBIN/BOSCALID
  (Pristine WG) 18.5–23 fl oz 12 0
  MODE OF ACTION GROUP NAME (NUMBER1): Quinone outside inhibitor (11) and Carboxamide (7)
  COMMENTS: Under warm, sunny conditions, fruit bronzing may occur. To limit the potential for development of resistance do not make more than 5 applications of strobilurin or anilide fungicides per season. Do not make more than 3 sequential applications of this fungicide before rotating to a fungicide with a different mode of action.
 
E. MICRONIZED SULFUR# 5–10 lb 24 0
  MODE OF ACTION GROUP NAME (NUMBER1): Multi-site contact (M2)
  COMMENTS: Sulfur application during high temperatures may burn foliage. Do not apply within 3 weeks of an oil application.
 
F. AZOXYSTROBIN
  (Abound) 6.2–15.4 fl oz 4 0
  MODE OF ACTION GROUP NAME (NUMBER1): Quinone outside inhibitor (11)
  COMMENTS. Do not apply more than 2 consecutive foliar applications before switching to alternative chemistry. Do not apply more than 1 lb a.i./acre/season.
 
G. INSECTICIDAL SOAP#
  (M-Pede) 2.5 oz/gal 12 0
  MODE OF ACTION GROUP NAME (NUMBER1): A contact fungicide with smothering and barrier effects.
  COMMENTS: The potential for phytotoxicity has not been fully evaluated. Growers are encouraged to test product or product mixes for phytotoxicity before field applications to determine safety margins. Do not use on new transplants, unrooted cuttings, or water-stressed plants. Avoid applying when leaf temperature exceeds 90°F. Thorough coverage is important. Avoid spraying when blossoms are present.
 
** Apply all materials in 200 gal water/acre to ensure adequate coverage.
+ Restricted entry interval (R.E.I.) is the number of hours (unless otherwise noted) from treatment until the treated area can be safely entered without protective clothing. Preharvest interval (P.H.I.) 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 actions (for more information, see http://www.frac.info/). 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.

[Precautions]

PUBLICATION

[UC Peer Reviewed]

UC IPM Pest Management Guidelines: Strawberry
UC ANR Publication 3468

Diseases

  • S. T. Koike, UC Cooperative Extension Monterey County
  • G. T. Browne, USDA Crops Pathology and Genetics, UC Davis
  • T. R. Gordon, Plant Pathology, UC Davis
Acknowledgment for contributions to Diseases:
  • W. D. Gubler, Plant Pathology, UC Davis

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