How to Manage Pests

UC Pest Management Guidelines

Tomato

Powdery Mildew on Field-grown Tomatoes

Pathogens: Leveillula taurica (Oidiopsis taurica)

(Reviewed 12/13, updated 12/13)

In this Guideline:


Symptoms

Leaves on infected tomato plants develop irregular, bright yellow blotches; severely affected leaves die but seldom drop. Spots of dead tissue, sometimes surrounded by a yellow halo, eventually appear in the blotches. Abundant white sporulation may be observed on upper or lower leaf surfaces. There are no lesions on stems or fruit. As the disease progresses leaves die, resulting in sunburn damage on fruit, reduced soluble solids, and weakened plants.

Comments on the Disease

Powdery mildew occurs in most tomato-growing areas of California. The fungus infects weeds and crops in the solanaceous family; spores are carried by wind to tomato plants. The disease usually is most severe late in the season. High relative humidity favors disease development. Mild temperatures favor infection while higher temperatures hasten the death of infected leaves. Plants stressed by other problems appear to be more susceptible to powdery mildew.

Management

When conditions are conducive to disease development and sporulation is abundant, fungicide applications may be necessary to control powdery mildew.

Organically Acceptable Methods

Bacillus pumilus and some sulfur sprays may be acceptable for use on organically certified produce. Check with your certifier before use.

Monitoring and Treatment Decisions

A weather-based, forecasting model is available online. The model attempts to predict the occurrence of powdery mildew based on temperature, relative humidity, and leaf wetness. However, since 2007 the disease has been more severe and faster developing such that disease severity and fungicide timings have not been well predicted by the model in recent years.

Fungicides may not be needed on early-season crops harvested in July or August. In the Central Valley, disease outbreaks generally start in July or August and mainly affect plants that are at full-bloom or a later stage. Multiple, early applications of sulfur dust are the most effective option; once the disease becomes severe, control is difficult. Apply fungicides if needed preventatively or during the early infection period. When disease pressure is high, repeat fungicide applications at 7-day intervals to control the disease. Stop treatments within two weeks of harvest.

There are no immune tomato varieties in California, though varieties vary in susceptibility.

Common name Amount per acre** R.E.I.‡ P.H.I.‡
(example trade name)   (hours) (days)

  Calculate impact of pesticide on air quality
When choosing a pesticide, consider its usefulness in an IPM program by reviewing the pesticide's properties, efficacy, application timing, and information relating to resistance management, honey bees (PDF), and environmental impact. Not all registered pesticides are listed. Always read the label of the product being used.
 
A. SULFUR# Label rates 24 0
  MODE OF ACTION GROUP NAME (NUMBER1): Multi-site contact (M2)
  COMMENTS: Do not use if an oil was applied recently or will be in the near future.
 
B. AZOXYSTROBIN + DIFENOCONAZOLE
  (Quadris Top) 8 fl oz 12 0
  MODE OF ACTION GROUP NAME (NUMBER1): Quinone outside inhibitor (11) + Dimethylation inhibitor (3)
 
C. FLUXAPYROXAD + PYRACLOSTROBIN
  (Priaxor Xemium) 6–8 fl oz 12 7
  MODE OF ACTION GROUP NAME (NUMBER1): Succinate dehydrogenase inhibitor (7) + Quinone outside inhibitor (11)
   
D. AZOXYSTROBIN
  (Quadris F) 5–6 fl oz 4 0
  MODE OF ACTION GROUP NAME (NUMBER1): Quinone outside inhibitor (11)
  COMMENTS: Apply on a 7- to 14-day interval; make no more than three sequential applications before alternating with a fungicide that has a different mode of action. Do not alternate or tank mix with fungicides to which resistance has developed in the pathogen population.
   
E. PYRACLOSTROBIN
  (Cabrio EG) 8–16 oz 12 0
  MODE OF ACTION GROUP NAME (NUMBER1): Quinone outside inhibitor (11)
   
F. WETTABLE SULFUR Label rates 24 0
  MODE OF ACTION GROUP NAME (NUMBER1): Multi-site contact (M2)
  COMMENTS: Do not use if oil was applied recently or will be in the near future.
   
G. PENTHIOPYRAD
  (Fontelis) See label 12 0
  MODE OF ACTION GROUP NAME (NUMBER1): Succinate dehydrogenase inhibitor (7)
 
H. MYCLOBUTANIL
  (Rally 40WSP) 2.5–4 oz 24 0
  MODE OF ACTION GROUP NAME (NUMBER1): Demethylation inhibitor (3)
 
I. BACILLUS PUMILUS STRAIN QST2808#
  (Sonata) 2–4 qt 4 0
  MODE OF ACTION: A biological antagonist.
  COMMENTS: Begin applications before disease onset or when disease pressure is low. Repeat at 7- to 10-day intervals.
 
** See label for dilution rate.
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 until harvest can take place. In some cases the R.E.I. exceeds the P.H.I. The longer of these two intervals is the minimum time that must elapse before harvest may take place.
# 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: Tomato
UC ANR Publication 3470

Diseases

R. M. Davis, Plant Pathology, UC Davis
G. Miyao, UC Cooperative Extension, Solano and Yolo counties
K. V. Subbarao, USDA Agricultural Research Station, Salinas, CA
J. J. Stapleton, UC IPM Program and Kearney Agricultural Center, Parlier
B. J. Aegerter, UC Cooperative Extension, San Joaquin County (powdery mildew on field-grown tomatoes)
Acknowledgments for contributions to Diseases:
B. W. Falk, Plant Pathology, UC Davis

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