Symptoms and Signs
Alternaria leaf spot appears as fairly large brown spots on leaves, about 0.5 to 0.75 inches (12–18 mm) in diameter. The spots turn black as the fungus produces spores. Leaf spot develops most rapidly in June and July, and trees can be almost completely defoliated by early summer when the disease is severe. The disease appears to be most severe where dew forms, humidity is high, and air is stagnant.
Comments on the Disease
Alternaria leaf spot can occur on almond trees grown anywhere in the Central Valley, but rarely is it severe enough in the northern San Joaquin Valley to require treatment. It has been most serious on trees in the southern San Joaquin Valley and in the northern Sacramento Valley.
Management
- The disease occurs first and is most severe on exposed leaves.
- Trees trained to an open and spreading canopy usually have more severe Alternaria leaf spot.
- Trees planted with rows in an east-west direction also have more severe disease than do orchard with rows planted north-south.
- Varieties that are most susceptible include Carmel, Sonora, Monterey, Winters, and Butte.
- Monitor for signs of the disease in April through June. If monitoring indicates the presence of Alternaria, begin late-spring treatments about mid-April.
- In orchards with a history of the disease, treat in mid- to late April and again 2 to 3 weeks later.
A disease severity value or DSV model has been developed on tomato and modified for almond for forecasting Alternaria leaf spot. Index values are assigned for specific ranges of average temperatures during leaf wetness periods during a day. Apply fungicide if accumulated index values over a 7–day period reach a value of 10 or higher.
| Mean temperature (C) during wetness | Leaf wetness duration (hours) | |||||
|---|---|---|---|---|---|---|
| 15–17 | 0–6 | 7–15 | 16–20 | 21 | - |  |
| 17.1–20 | 0–3 | 4–8 | 9–15 | 16–22 | 23+ | |
| 20.1–25 | 0–2 | 3–5 | 6–12 | 13–20 | 21+ | |
| 25.1–29 | 0–3 | 4–8 | 9–15 | 16–20 | 23+ | |
| DSV | 0 | 1 | 2 | 3 | 4 | |
In some orchards, Alternaria sp. resistance to quinone outside inhibitor (QoI) fungicides (also known as strobilurins) and succinate dehydrogenase inhibitor (SDHI) fungicides have been documented; do not use FRAC mode-of-action group number 7 or 11 fungicides in these orchards.
| 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. | METCONAZOLE | |||
| (Quash) | 2.5-3.5 oz | 12 | 25 | |
| MODE-OF-ACTION GROUP NAME (NUMBER1): demethylation (sterol) inhibitor (3) | ||||
| COMMENTS: Do not make more than four applications per season and no more than two sequential applications before rotation to a different mode of action fungicide. | ||||
| B. | DIFENOCONAZOLE | |||
| (Inspire) | 7 fl oz | 12 | 14 | |
| MODE-OF-ACTION GROUP NAME (NUMBER1): demethylation (sterol) inhibitor (3) | ||||
| COMMENTS: Do not make more than four applications per season and no more than two sequential applications before rotation to a different mode of action fungicide. | ||||
| C. | POLYOXIN D ZINC SALT | |||
| (Ph-D) | 6.2 oz | 4 | 0 | |
| MODE-OF-ACTION GROUP NAME (NUMBER1): glucan synthesis (19) | ||||
| COMMENTS: Do not make more than three applications per season and no more than two sequential applications before rotation to a different mode of action fungicide. | ||||
| D. | PYRACLOSTROBIN/BOSCALID | |||
| (Pristine) | 10.5–14.5 oz | 12 | 25 | |
| MODE-OF-ACTION GROUP NAME (NUMBER1): quinone outside inhibitor (11) and succinate dehydrogenase inhibitor (7) | ||||
| COMMENTS: Do not make more than four applications per season of (QoIs (strobilurins) or SDHIs and no more than two sequential applications before rotation to a different mode of action fungicide to limit the potential for the development of resistance. Note that resistant populations have developed in selected almond-producing areas within California. | ||||
| E. | PYRACLOSTROBIN/FLUXAPYROXAD | |||
| (Merivon) | 6.5 oz | 12 | 14 | |
| MODE-OF-ACTION GROUP NAME (NUMBER1): quinone outside inhibitor (11) and succinate dehydrogenase inhibitor (7) | ||||
| COMMENTS: Do not make more than four applications per season of QoIs (strobilurins) or SDHIs to limit the potential for the development of resistance. Note that resistant populations have developed in selected almond-producing areas within California. | ||||
| F. | FLUOPYRAM/TRIFLOXYSTROBIN | |||
| (Luna Sensation) | 5–7 fl oz | 12 | 14 | |
| MODE-OF-ACTION GROUP NAME (NUMBER1): succinate dehydrogenase inhibitor (7) and Quinone outside inhibitor (11) | ||||
| COMMENTS: Do not apply after hullsplit. Do not apply more than two sequential sprays before alternating with a fungicide that has a different mode of action. Do not apply more than four applications of strobilurin fungicides per year or apply more than 27.1 fl oz /acre per season. | ||||
| G. | DIFENOCONAZOLE/CYPRODINIL | |||
| (Inspire Super) | 16–20 oz | 12 | 60 | |
| MODE-OF-ACTION GROUP NAME (NUMBER1): demethylation (sterol) inhibitor (3) and AP (9) | ||||
| COMMENTS: Do not apply more than two sequential sprays before alternating with a fungicide that has a different mode of action. Do not apply more than four applications of strobilurin fungicides per year or apply more than 80 fl oz/acre per season. | ||||
| H. | AZOXYSTROBIN/DIFENOCONAZOLE | |||
| (Quadris Top) | 14 oz | 12 | 28 | |
| MODE-OF-ACTION GROUP NAME (NUMBER1): quinone outside inhibitor (11) and demethylation (sterol) inhibitor (3) | ||||
| COMMENTS: Do not use later than 5 weeks after petal fall. Not a good choice when disease pressure is severe because of label timing restrictions. | ||||
| I. | AZOXYSTROBIN | |||
| (Abound) | 12.0–15.5 oz | 4 | 28 | |
| MODE-OF-ACTION GROUP NAME (NUMBER1): quinone outside inhibitor (11) | ||||
| COMMENTS: Do not apply more than two sequential sprays before alternating with a fungicide that has a different mode of action. Do not apply more than four applications of strobilurin fungicides per year or apply more than 92.3 fl oz/acre per season. For best results, mix with a DMI (FRAC Group 3), QoI (11), or other registered fungicide. | ||||
| J. | PENTHIOPYRAD | |||
| (Fontelis) | 14–20 fl oz | 12 | 14 | |
| MODE-OF-ACTION GROUP NAME (NUMBER1): succinate dehydrogenase inhibitor (7) | ||||
| COMMENTS: Do not apply more than two sequential sprays before alternating with a fungicide that has a different mode of action. Do not apply more than three applications of SDHI fungicides per year or apply more than 61 fl oz/acre per season. For best results, mix with a DMI (FRAC Group 3), QoI (11), or other registered fungicide. | ||||
| K. | IPRODIONE | |||
| (Rovral, Nevado) | 1 pt | 24 | See comments | |
| MODE-OF-ACTION GROUP NAME (NUMBER1): Dicarboximide (2) | ||||
| . . . PLUS . . . | ||||
| NARROW RANGE OIL | ||||
| (various) | Label rates | See label | See label | |
| MODE-OF-ACTION: Contact | ||||
| COMMENTS: Do not apply later than 5 weeks after petal fall. Not a good choice when disease pressure is severe because of label timing restrictions. | ||||
| ‡ | 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 these two intervals is the minimum time that must elapse before harvest may occur. |
| 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. |
| — | No information |
| QoI | = quinone outside inhibitor or strobilurin |
| DMI | = demethylation (sterol) inhibitor |
| SDHI | = succinate dehydrogenase inhibitor |