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Powdery mildew causes irregular yellow blotches on tomato leaves.

Tomato

Powdery Mildew on Field-grown Tomatoes

Pathogens: Leveillula taurica (Oidiopsis taurica)

(Reviewed 1/08, updated 8/08)

In this Guideline:


SYMPTOMS AND SIGNS

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. There are no lesions on stems or fruit. Rarely, a gray mycelium develops on the lower leaf surface of infected leaves. Severe infections kill leaves and result in sunburn fruit 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 occurs late in the season. High relative humidity favors disease development. Mild temperatures favor infection while higher temperatures hasten the death of infected leaves.

MANAGEMENT

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

Organically Acceptable Methods
Sulfur sprays are acceptable for use on organically certified produce.

Monitoring and Treatment Decisions
A weather-based, forecasting model is available online. The model predicts the occurrence of powdery mildew based on temperature, relative humidity, and leaf wetness. Growers can use the model to accurately time fungicide applications.

Fungicides are not needed for control unless the disease becomes extensive. If you see early symptoms, check weekly to monitor the progression of the disease. Multiple, early applications of sulfur are likely to provide control. An application of azoxystrobin may be more effective than sulfur if it is well timed. There are no immune tomato varieties in California.

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. When choosing a pesticide, also consider information relating to environmental impact. Not all registered pesticides are listed. Always read label of product being used.
 
A. PYRACLOSTROBIN
  (Cabrio) EG 8–16 oz 12 0
  MODE OF ACTION GROUP NAME (NUMBER1): Quinone outside inhibitor (11)
  COMMENTS: Do not apply more than 2 sequential applications or more than 96 oz/acre/season.
 
B. MYCLOBUTANIL
  (Rally) 40 WSP 2.5–4 oz 24 0
  MODE OF ACTION GROUP NAME (NUMBER1): Demethylation inhibitor (3)
 
C. TRIFLOXYSTROBIN
  (Flint) Label rates 12 3
  MODE OF ACTION GROUP NAME (NUMBER1): Quinone outside inhibitor (11)
 
D. SULFUR# Label rates 24 0
  MODE OF ACTION GROUP NAME (NUMBER1): Multi-site contact (M2)
 
E. 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.
 
 
**  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/Yolo counties
K. Subbarao, USDA Agricultural Research Station, Salinas
J. J. Stapleton, UC IPM Program, Kearney Agricultural Center, Parlier
Acknowledgments for contributions to the disease section:
B. W. Falk, Plant Pathology, UC Davis

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