Symptoms and Signs
Powdery mildew is marked by superficial, white, weblike growth on leaves, shoots, or fruit.
Comments on the Disease
The fungus overwinters in buds on twigs and as chasmothecia, which are spore-containing structures, on the bark of twigs and branches. Secondary spores produced in spring spread the fungus to new growth. Infections can be severe in commercial orchards during years of low rainfall, high humidity, and warm temperatures (70° to 80°F). The disease is particularly severe on new growth, such as shoots of inner scaffolds, and can infect fruit as well, causing direct crop loss. In warm, humid, coastal areas, powdery mildew can also be severe after harvest.
Management
To protect fruit, spray soon after petal fall and 2 to 3 weeks later if needed. Treat immediately if mildew is found on leaves or shoots of inner scaffolds or water sprouts. Do not use the same fungicide or fungicides with similar chemistry more than twice in one year, to reduce the potential for the development of resistance by the pathogen to the fungicide. In orchards with a history of mildew, select fungicides that are active against brown rot and Botrytis, as well as powdery mildew, during bloom and immediately before harvest, being careful to note the preharvest interval.
| 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. | AZOXYSTROBIN/DIFENCONAZOLE | |||
| (Quadris Top) | 12–14 fl oz | 12 | 0 | |
| MODE-OF-ACTION GROUP NAME (NUMBER1): Quinone outside inhibitor (11) and Demethylation inhibitor (3) | ||||
| COMMENTS: Resistance to FRAC 3 and FRAC 11 group fungicides is possible in fungal populations. Use only when alternating with a fungicide that has at least one different FRAC Group number. | ||||
| B. | AZOXYSTROBIN/PROPICONAZOLE | |||
| (QuiltXcel) | 14 fl oz | 12 | 0 | |
| MODE-OF-ACTION GROUP NAME (NUMBER1): Quinone outside inhibitor (11) and Demethylation inhibitor (3) | ||||
| COMMENTS: Resistance to FRAC 3 and FRAC 11 group fungicides is possible in fungal populations. Use only when alternating with a fungicide that has at least one different FRAC Group number. | ||||
| C. | TEBUCONAZOLE/TRIFLOXYSTROBIN | |||
| (Adament 50WG) | 4–8 fl oz | 120 (5 days) | 1 | |
| MODE-OF-ACTION GROUP NAME (NUMBER1): Quinone outside inhibitor (11) and Demethylation inhibitor (3) | ||||
| COMMENTS: Resistance to FRAC 3 and FRAC 11 group fungicides is possible in fungal populations. Use only when alternating with a fungicide that has at least one different FRAC Group number. | ||||
| D. | FLUOPYRAM/TRIFLOXYSTROBIN | |||
| (Luna Sensation) | 5.0–5.6 fl oz | 12 | 1 | |
| MODE-OF-ACTION GROUP NAME (NUMBER1): Succinate dehydrogenase inhibitor (7) and Quinone outside inhibitor (11) | ||||
| COMMENTS: Resistance to FRAC 7 and FRAC 11 group fungicides is possible in fungal populations. Use only when alternating with a fungicide that has at least one different FRAC Group number. | ||||
| E. | PYRACLOSTROBIN/BOSCALID | |||
| (Pristine) | 10.5–14.5 oz | 12 | 0 | |
| MODE-OF-ACTION GROUP NAME (NUMBER1): Quinone outside inhibitor (11) and Succinate dehydrogenase inhibitor (7) | ||||
| F. | PENTHIOPYRAD | |||
| (Fontelis) | 14–20 fl oz | 12 | 0 | |
| MODE-OF-ACTION GROUP NAME (NUMBER1): Succinate dehydrogenase inhibitor (7) | ||||
| COMMENTS: Resistance to FRAC 7 group fungicides is possible in fungal populations. Use only when alternating with a fungicide that has a different FRAC Group number. | ||||
| G. | TRIFLUMIZOLE | |||
| (Procure 48SC) | 8-16 fl oz | 12 | 1 | |
| MODE-OF-ACTION GROUP NAME (NUMBER1): Demethylation inhibitor (3) | ||||
| H. | MYCLOBUTANIL | |||
| (Rally 40WSP) | 2.5–6 oz | 24 | 0 | |
| MODE-OF-ACTION GROUP NAME (NUMBER1): Demethylation inhibitor (3) | ||||
| COMMENTS: Do not apply more than 3.25 lb product/acre per season. More effective when applied as a concentrate (80-100 gal/acre) than as a dilute spray. | ||||
| I. | FENARIMOL | |||
| (Rubigan EC) | 6–12 fl oz | 12 | 0 | |
| MODE-OF-ACTION GROUP NAME (NUMBER1): Demethylation inhibitor (3) | ||||
| COMMENTS: Apply a treatment at late bloom to petal fall and repeat at 14–21 day intervals as required for control until harvest. Do not apply more than 48 oz before harvest. | ||||
| J. | PROPICONAZOLE | |||
| (Bumper) | 4 fl oz | 12 | 0 | |
| MODE-OF-ACTION GROUP NAME (NUMBER1): Demethylation inhibitor (3) | ||||
| COMMENTS: Apply at popcorn. More effective when applied as a concentrate (80-100 gal/acre) rather than a dilute spray. Do not apply more than 8 fl oz/acre per crop (48 fl oz/acre per year). | ||||
| K. | TEBUCONAZOLE | |||
| (Elite 45WP) | 4-8 oz | 120 (5 days) | 0 | |
| MODE-OF-ACTION GROUP NAME (NUMBER1): Demethylation inhibitor (3) | ||||
| COMMENTS: More effective when applied as a concentrate (80-100 gal/acre) rather than a dilute spray. | ||||
| L. | FENBUCONAZOLE | |||
| (Indar 2F) | 6 oz | 12 | 0 | |
| MODE-OF-ACTION GROUP NAME (NUMBER1): Demethylation inhibitor (3) | ||||
| COMMENTS: Do not apply more than 8 applications per season (48 fl oz/acre per year). | ||||
| M. | QUINOXYFEN | |||
| (Quintec) | 7 fl oz | 12 | 7 | |
| MODE-OF-ACTION GROUP NAME (NUMBER1): Quinoline (13) | ||||
| COMMENTS: Treat early; this product is a protectant so do not use if disease symptoms are present. Apply at full bloom and 2 weeks after petal fall. If additional treatments are necessary, rotate with a fungicide that has a different Group number. Do not apply more than 35 fl oz/acre per year or make more than 5 applications per year. | ||||
| N. | TRIFLOXYSTROBIN | |||
| (Gem 500SC) | 1.9–3.8 fl oz | 12 | 1 | |
| MODE-OF-ACTION GROUP NAME (NUMBER1): Quinone outside inhibitor (11) | ||||
| O. | AZOXYSTROBIN | |||
| (Abound) | 12–15.5 fl oz | 4 | 0 | |
| MODE-OF-ACTION GROUP NAME (NUMBER1): Quinone outside inhibitor (11) | ||||
| COMMENTS: Do not apply more than three sequential sprays of this product before alternating with a fungicide with a different Group number. | ||||
| P. | THIOPHANATE METHYL | |||
| (Topsin-M WSB) | 1.5 lb lb | 12 | 1 | |
| MODE-OF-ACTION GROUP NAME (NUMBER1): Methyl benzimidazole (1) | ||||
| COMMENTS: Recommended rate is 1.5 lb/acre. Use only once a year. Do not use in orchards where resistance has been observed. Use only in combination or in an alternating application program with a fungicide with a different Group number. Resistant populations to benzimidazole fungicides may result in low performance or a failed fungicide treatment. | ||||
| Q. | WETTABLE SULFUR# | |||
| (80-92% Sulfur, Various) | 10–20 lb | 24 | 0 | |
| MODE-OF-ACTION GROUP NAME (NUMBER1): Multi-site contact (M2) | ||||
| COMMENTS: Do not enter treated areas for 24 hours unless protective product clothing is worn. Do not apply within 3 weeks of an oil application. May cause outbreaks of pest mites; at temperatures above 85°F, may cause phytotoxicity. | ||||
| R. | LIQUID LIME SULFUR# | |||
| (Various) | 2–3 gal per 100 gal dilute |
48 | 0 | |
| MODE-OF-ACTION GROUP NAME (NUMBER1): Multi-site contact (M2) | ||||
| COMMENTS: Lime sulfur is incompatible with most other pesticides. Check before use. Use in delayed dormant season only, not during the growing season. | ||||
| S. | POTASSIUM BICARBONATE | |||
| (Kaligreen) | 2.5– 3 lb | 4 | 1 | |
| MODE-OF-ACTION GROUP NAME (NUMBER1): Unknown (NC) | ||||
| ‡ | 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 actions. 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. |