How to Manage Pests

UC Pest Management Guidelines

Almond

Alternaria Leaf Spot

Pathogen: Alternaria alternata

(Reviewed 3/09, updated 11/12)

In this Guideline:


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 dews form, 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 2 to 3 weeks later.

Disease severity values graph

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 temperatures
(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+
DVS 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/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. 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 4 applications per season and no more than two sequential applications before rotation to a different mode of action fungicide.
 
B. 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 3 applications per season and no more than two sequential applications before rotation to a different mode of action fungicide.
 
C. 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 4 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.
 
D. FLUOPYRAM/TRIFLOXYSTROBIN
  (Luna Sensation) 5-7 fl oz 12 60
  MODE OF ACTION GROUP NAME (NUMBER1): succinate dehydrogenase inhibitor (11) and Quinone outside inhibitor (11)
  COMMENTS: Do not apply after hullsplit. Do not apply more than 2 sequential sprays before alternating with a fungicide that has a different mode of action. Do not apply more than 4 applications of strobilurin fungicides per year or apply more than 27.1 fl oz/product/acre/season.
 
E. FLUOPYRAM/TEBUCONAZOLE
  (Luna Experience) 6-8 fl oz 12 35
  MODE OF ACTION GROUP NAME (NUMBER1): succinate dehydrogenase inhibitor (7) and demethylation (sterol) inhibitor (3)
  COMMENTS: Do not apply after hullsplit. Do not apply more than 2 sequential sprays before alternating with a fungicide that has a different mode of action. Do not apply more than 4 applications of strobilurin fungicides per year or apply more than 34 fl oz/product/acre/season.
 
F. DIFENOCONAZOLE/CYPRODONIL
  (Inspire Super) 7 oz 12 60
  MODE OF ACTION GROUP NAME (NUMBER1): demethylation (sterol) inhibitor (3) and AP (9)
  COMMENTS: Do not apply more than 2 sequential sprays before alternating with a fungicide that has a different mode of action. Do not apply more than 4 applications of strobilurin fungicides per year or apply more than 2.5 qt/product/acre/season.
 
G. 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.
 
H. AZOXYSTROBIN
  (Abound) 11–15.4 oz 4 28
  MODE OF ACTION GROUP NAME (NUMBER1): Quinone outside inhibitor (11)
  COMMENTS: Do not apply more than 2 sequential sprays before alternating with a fungicide that has a different mode of action. Do not apply more than 4 applications of strobilurin fungicides per year or apply more than 2.88 qt/product/acre/season.
 
I. IPRODIONE/NARROW RANGE OIL
  (Rovral 4F) 1 pt 24 see comments
  MODE OF ACTION GROUP NAME (NUMBER1): Dicarboximide (2)
  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.
 
+ 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 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 actions (For more information, see 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: Almond
UC ANR Publication 3431

Diseases

J. E. Adaskaveg, Plant Pathology, UC Riverside
W. D. Gubler, Plant Pathology, UC Davis
Roger Duncan, UC Cooperative Extension, Stanislaus County
J. J. Stapleton, UC IPM Program, Kearney Agricultural Center, Parlier
B. A. Holtz, UC Cooperative Extension, San Joaquin County

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