Agriculture: Strawberry Pest Management Guidelines

Powdery Mildew

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 chasmothecia (closed spore-bearing structures) 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. Powdery mildew is particularly severe in greenhouse and plastic tunnel production systems.

Management

During routine field surveys, watch for the leaf distortion and discoloration that are the first signs of powdery mildew, especially in fall and spring. To control powdery mildew, apply fungicides when disease is first detected. This is especially important for protectants such as sulfur. 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 nursery 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. Use less-susceptible cultivars, such as Albion, San Andreas, and Fronteras, where practical.

Organically Acceptable Methods

Apply mined sulfur or insecticidal soap on organically certified strawberries. Use less-susceptible 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 fungicide applications when plants begin to bloom.

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. PYRACLOSTROBIN/FLUXAPYROXAD
  (Merivon) 4–7 fl oz 12 0
  MODE-OF-ACTION GROUP NAME (NUMBER1): Quinone outside inhibitor (11) and succinate dehydrogenase inhibitor (7)
 
B. QUINOXYFEN
  (Quintec) 4–6 fl oz 12 1
  MODE-OF-ACTION GROUP NAME (NUMBER1): Quinoline (13)
 
C. PENTHIOPYRAD
  (Fontelis) 16–24 fl oz 12 0
  MODE-OF-ACTION GROUP NAME (NUMBER1): Succinate dehydrogenase inhibitor (7)
 
D. MYCLOBUTANIL
  (Rally 40W) 2.5–5.0 oz 24 0
  MODE-OF-ACTION GROUP NAME (NUMBER1): Demethylation inhibitor (3)
  COMMENTS: There may be some resistance to this fungicide. Apply in a minimum of 100 gal water/acre. Do not apply more than 30 oz/acre per year.
 
E. TRIFLUMIZOLE
  (Procure 480SC) 4–8 oz 12 1
  MODE-OF-ACTION GROUP NAME (NUMBER1): Demethylation inhibitor (3)
 
F. FLUOPYRAM/TRIFLOXYSTROBIN 4–7.6 fl oz 12 0
  (Luna Sensation)
  MODE-OF-ACTION GROUP NAME (NUMBER1): Succinate dehydrogenase inhibitor (7) and quinone outside inhibitor (11)
 
G. FLUOPYRAM
  (Luna Privilege) 6.84 fl oz 12 See label
  MODE-OF-ACTION GROUP NAME (NUMBER1): Succinate dehydrogenase inhibitor (7)
 
H. TETRACONAZOLE
  (Mettle 125ME) 3–5 fl oz 12 0
  MODE-OF-ACTION GROUP NAME (NUMBER1): Demethylation inhibitor (3)
 
I. CYFLUFENAMID
  (Torino) 3.4 oz 4 0
  MODE-OF-ACTION GROUP NAME (NUMBER1): Phenyl-acetamide (U6)
 
J. PYRACLOSTROBIN/BOSCALID
  (Pristine) 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 five applications of strobilurin or anilide fungicides per season. Do not make more than three sequential applications of this fungicide before rotating to a fungicide with a different mode of action.
 
K. 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.
 
L. AZOXYSTROBIN
  (Quadris Top) 12–14 fl oz 12 0
  MODE-OF-ACTION GROUP NAME (NUMBER1): Quinone outside inhibitor (11)
  COMMENTS. Do not apply more than two consecutive foliar applications before switching to alternative chemistry. Do not apply more than 1 lb a.i./acre per season.
 
M. INSECTICIDAL SOAP#
  (M-Pede) 1.25–2.5 fl 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 pesticides in 200 gal water/acre to ensure adequate coverage.
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.
# Acceptable for use on 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. 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 a fungicide with a different mode-of-action group number.
Text Updated: 07/18
Treatment Table Updated: 07/18