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. 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
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** | 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. | SULFUR# | |||
| (various products) | Label rates | 24 | See label | |
| MODE-OF-ACTION GROUP NAME (NUMBER1): Multi-site contact (M2) | ||||
| COMMENTS: Do not use if an oil was applied recently or will be soon. | ||||
| B. | FLUTRIAFOL | |||
| (Rhyme) | 7 fl oz | 12 | 0 | |
| MODE-OF-ACTION GROUP NAME (NUMBER1): Demethylation inhibitor (3) | ||||
| C. | AZOXYSTROBIN/DIFENOCONAZOLE | |||
| (Quadris Top) | 8 fl oz | 12 | 0 | |
| MODE-OF-ACTION GROUP NAME (NUMBER1): Quinone outside inhibitor (11) and Demethylation inhibitor (3) | ||||
| D. | FLUXAPYROXAD/PYRACLOSTROBIN | |||
| (Priaxor Xemium) | 6–8 fl oz | 12 | 0 | |
| MODE-OF-ACTION GROUP NAME (NUMBER1): Succinate dehydrogenase inhibitor (7) and Quinone outside inhibitor (11) | ||||
| E. | CYFLUFENAMID | 3.4 fl oz | 4 | 0 |
| (Torino) | ||||
| MODE-OF-ACTION GROUP NAME (NUMBER1): Unknown (U6) | ||||
| F. | PYRIOFENONE | 4 fl oz | 4 | 0 |
| (Prolivo 300SC) | ||||
| MODE-OF-ACTION GROUP NAME (NUMBER1): Aryl-phenyl-ketones (50) | ||||
| G. | TETRACONAZOLE | |||
| (Mettle 125 ME) | 6–8 fl oz | 12 | 0 | |
| MODE-OF-ACTION GROUP NAME (NUMBER1): Demethylation inhibitor (3) | ||||
| H. | MYCLOBUTANIL | |||
| (Rally 40 WSP) | 2.5–4 fl oz | 24 | 0 | |
| MODE-OF-ACTION GROUP NAME (NUMBER1): Demethylation inhibitor (3) | ||||
| I. | MEFENTRIFLUCONAZOLE | |||
| (Cevya) | 3–5 fl oz | 12 | 0 | |
| MODE-OF-ACTION GROUP NAME (NUMBER1): Demethylation inhibitor (3) | ||||
| J. | PYDIFLUMETOFEN/ FLUDIOXONIL | |||
| (Miravis Prime) | 9.2–11.4 fl oz | 12 | 0 | |
| MODE-OF-ACTION GROUP NAME (NUMBER1): Succinate dehydrogenase inhibitor (7) and Phenylpyrrole (12) | ||||
| K. | FLUOPYRAM/ TRIFLOXYSTROBIN | |||
| (Luna Sensation) | 5–7.6 fl oz | 12 | 3 | |
| MODE-OF-ACTION GROUP NAME (NUMBER1): Succinate dehydrogenase inhibitor (7) and Quinone outside inhibitor (11) | ||||
| L. | WETTABLE SULFUR | |||
| (Various products) | Label rates | 24 | See label | |
| MODE-OF-ACTION GROUP NAME (NUMBER1): Multi-site contact (M2) | ||||
| COMMENTS: Do not use if oil was applied recently or will be soon. | ||||
| ** | See label for dilution rate. |
| ‡ | 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 until harvest can take place. In some cases, the REI exceeds the PHI. The longer of the 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 action. Fungicides with different group numbers are suitable to alternate in a resistance management program. In California, make no more than one application of a fungicide with a mode-of-action group number associated with high resistance risk before rotating to a fungicide with a different mode-of-action group number; for other fungicides, make no more than two consecutive applications before rotating to fungicide with a different mode-of-action group number. |