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UC Pest Management Guidelines

Alternaria leaf blight symptoms on pistachio leaves and fruit.


Alternaria Late Blight

Pathogens: Alternaria alternata, A. tenuissima, A. arborescens, and Stemphyllium spp.

(Reviewed 2/07, updated 3/10)

In this Guideline:


Alternaria late blight occurs as black angular or circular lesions on leaves of both male and female trees. When the pistachio fruit are maturing, black lesions with a red halo appear on hulls. Black spores develop in the center of the leaf lesions when humid conditions prevail in orchards. Rubbing the leaf lesions with a finger will blacken the finger. In contrast, rubbing a finger on a lesion cause by Botryosphaeria dothidea does not blacken fingers because there are no spores produced on the surface of these lesions. However, late in the season both fungi can be present in the same lesion and microscopic identification will be needed. Leaf infections can cause severe premature defoliation and create problems during shaking the trees at harvest.

Black lesions are also present on petioles and main veins of leaf blades. On immature fruit the symptoms appear as small black lesions about 1 mm in diameter. On mature fruit, both small (1–2 mm) and large (5 mm) black lesions are present on the epicarp, usually surrounded by a reddish purple margin. Multiple lesions on leaves and fruit cause leaf blight and deterioration of hulls, respectively. Deterioration of hulls results in shell staining.


Alternaria is a problem in orchards irrigated by sprinklers or flooding, although it can also be a problem in orchards with low soil infiltration and those irrigated with microsprinklers, particularly in lower areas in these orchards where relative humidity can be high and dew formation frequent during late August and September. The pathogen causes latent infections on leaves and fruit. The disease is more severe on leaves from fruit-bearing shoots than those without fruit.

Losses occur mainly because of fruit staining and from early defoliation, which can be severe enough to cause difficulties during harvest. In addition, the fungus can colonize the inner surface of the shell and endocarp, causing moldy nuts. Leaf and fruit lesions are common on both Kerman and Red Aleppo cultivars, as well as on the leaves of Peters and on leaves of 02-16 and 02-18 male selections.


Early August to mid-September is the critical period for disease development. Orchards with cover crops have more Alternaria blight than disced orchards. Sunburned fruit become more susceptible to Alternaria blight than nonsunburned fruit.

Alternaria blight is difficult to control and requires a combination of management approaches. If the disease is serious, adjust the irrigation schedule so the period from August 1 to 10 is irrigation-free. Subsurface irrigation can reduce this disease significantly. However, disease severity must be weighed against the impact of deficit irrigation on shell splitting. Avoid delaying harvest of mature nuts.

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.
  (Pristine) 10.5-14.5 oz 12 14
  MODE OF ACTION GROUP NAME (NUMBER1): Quinone outside inhibitor (11) and Carboxamide (7)
  COMMENTS: Use allowed under a FIFRA Section 2(ee) permit. Do not apply in orchards where resistance to boscalid has been detected.
  (Abound) 2EC 12.3–15 fl oz 4 7
  MODE OF ACTION GROUP NAME (NUMBER1): Quinone outside inhibitor (11)
  COMMENTS: Apply at 2- to 3-week intervals beginning late May to early June. Resistance of Alternaria spp. to Group 11 fungicides has been detected in some pistachio orchards.
  (Cabrio) EG 16 oz 12 14
  MODE OF ACTION GROUP NAME (NUMBER1): Quinone outside inhibitor (11)
  COMMENTS: Apply before disease onset and repeat at a 10-to-30 day interval.
  (Gem) 500 SC 2.9–3.8 oz 12 28
  MODE OF ACTION GROUP NAME (NUMBER1): Quinone outside inhibitor (11)
  COMMENTS: Apply at 2- to 3-week intervals beginning at late May to early June. Do not apply more than 4 times/season or more than 15.2 fl oz/acre/season. Resistance of Alternaria spp. to Group 11 fungicides has been detected in some pistachio orchards.
  (Switch) 62.5 WG 11–14 oz 12 7
  MODE OF ACTION GROUP NAME (NUMBER1): Anilinopyrimidine (9) and Phenylpyrrole (12)
  COMMENTS: Make first application at early bloom and a second 14 days later. Do not apply more then 56 oz product/acre/year.
  (Scala) SC 18 fl oz 12 30
  MODE OF ACTION GROUP NAME (NUMBER1): Anilinopyrimidine (9)
  COMMENTS: Do not apply more than 54 fl oz/acre/crop.
+ 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 two intervals is the minimum time that must elapse before harvest.
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.



[UC Peer Reviewed]

UC IPM Pest Management Guidelines: Pistachio
UC ANR Publication 3461
T. J. Michailides, Kearney Agricultural Center, Parlier
Acknowledgment for contributions to Diseases:
B. L. Teviotdale, Kearney Agricultural Center, Parlier

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