Effective Management Tools for Septoria Leaf Spot of
Pistachio in Arizona
Robert E. Call,
Associate Agent, Soil and Water Science
Michael E. Matheron,
Associate Research Scientist, Plant Pathology, Yuma Agricultural Center
Septoria leaf spot was detected in the United States for the first
time in 1964 within an experimental pistachio planting at Brownwood, Texas.
The first observation of the same disease in Arizona pistachio trees did not
occur until 1986. In 1988, a survey of the 2,000 acres of pistachio orchards
in southeastern Arizona revealed a widespread incidence of the disease.
Since the initial discovery of the disease, Septoria leaf spot has appeared
annually in some of the Arizona pistachio acreage. The onset and severity of
the disease is influenced by summer rainfall that occurs in this region. Disease
management trials conducted since 1992 have shown that as few as two applications
of chlorothalonil in July and August can virtually prevent disease development.
Applications of copper hydroxide or benomyl alone or in combination also effectively
arrest disease development. Leaves around nut clusters on infected trees not
receiving fungicide treatments were usually senescent at crop maturity, whereas
leaves on treated trees showed no sign of senescence. Pistachio trees infected
with Septoria leaf spot and not treated with an effective fungicide can defoliate
in the autumn up to 2 months prematurely.
Pistachio plantings were first established in North America around 1890 in
Fresno, California (Maas et al., 1971). Since then, pistachio trees also have been
grown in Arizona, Nevada, Utah and Texas. Septoria leaf spot, caused by the pathogenic
fungus Septoria pistaciarum, has been reported in various Mediterranean countries where
pistachios are grown commercially. The disease was first detected in the United States
in 1964 within an experimental pistachio planting at Brownwood, Texas (Maas, et al., 1971).
From 1965 to 1967, the first symptoms were observed during mid- May in this Texas planting
of pistachios, with the disease remaining mild during the remainder of the growing season.
In contrast, the spring seasons from 1968 to 1970 were very moist, followed by relatively
dry summers. Under these environmental conditions, leaf spots were observed as early as
late April with subsequent disease development leading to defoliation of trees during
these years. Disease ratings in Texas revealed that leaves of Pistacia vera and all
budded trees of the Kerman variety were severely infected, whereas a more moderate level
of disease was recorded on leaves of P. chinensis, P. atlantica and P. terebinthus.
A moderate level of Septoria leaf spot was first observed on leaves of pistachio trees
in Arizona in 1986 (Young and Michailides, 1989). A 1988 survey of the 2,000 acres of
pistachios in Arizona revealed that the disease was widespread. Adequate control of
Septoria leaf spot has been reported in some Mediterranean countries with applications
of copper sulfate (Hallage, 1927; Pupillo and Di Caro, 1952; Sarjanni, 1935). The objective
of our studies was to evaluate some fungicides and timing of applications to develop
effective management strategies for Septoria leaf spot of pistachio in Arizona.
Materials and Methods
A commercial pistachio orchard (Kerman on P. atlantica rootstock) was used for yearly
fungicide studies conducted from 1992 to 1996. Trees were 16-yr-old in 1992 and were
irrigated with a surface drip system. Tree spacing was 13 ft x 22 ft. All treatments
were replicated four times in a randomized complete block design. Foliar application of
fungicides was performed with a commercial orchard sprayer.
(1992) Treatments included chlorothalonil (Bravo 720F) at 3.0 or
4.5 lb a.i./acre and copper hydroxide (Kocide 101 50W) plus benomyl (Benlate 50W) at
8.2 lb a.i. and 1.0 lb a.i./acre, respectively. These treatments were applied 28 Apr,
26 May, 23 Jun, 21 Jul and 18 Aug. Each replicate plot consisted of two trees with treated
trees separated by nontreated trees. In this and all subsequent studies, one treatment
consisted of pistachio trees receiving no fungicides.
(1993) Treatments consisted of chlorothalonil (Bravo 825 WDG)
at 3.0 lb a.i./acre, applied 26 May, 23 Jun, 21 Jul and 20 Aug; chlorothalonil at
4.5 lb a.i./acre, applied at the same four times; benomyl (Benlate 50W) at 2.0 lb a.i./acre,
again applied at the same spray dates; and chlorothalonil at 4.5 lb a.i./acre, applied
26 May, 21 Jul and 20 Aug. Each replicate plot consisted of two trees separated by
(1994) Treatments for 1994 were chlorothalonil (Bravo 500)
at 4.5 lb a.i./acre, with spray volumnes of 25, 100, and 400 gallons/acre applied 7
Jul and 11 Aug; chlorothalonil (Bravo 825) applied at the same rate, spray volumes
and dates as listed for Bravo 500. Single tree replicates, separated by nontreated
trees, were used in 1994.
(1995) Two different formulations of chlorothalonil
(Bravo 720 and Bravo Ultrex 82.5DG) were compared at rates of 3.0 or 4.5 lb a.i./acre in 1995.
An additional treatment was copper hydroxide (Kocide 101 77W) at a rate of 1.9 lb a.i./acre.
Spray dates were 15 May, 7 Jul and 6 Aug and the spray volume in all cases was 100 gal/acre.
Single tree replicates, separated by nontreated trees, were used in 1995.
(1996) The Septoria leaf spot management study conducted in 1996
included chlorothalonil (Bravo Weather Stik) at 2.25 lb a.i./acre, applied either three
(12 Jun, 10 Jul and 11 Aug) or four times (25 May, 12 Jun, 10 Jul and 11 Aug), all with
a spray volume of 100 gal/acre. Each replicate plot contained three trees.
In late September of each year, disease severity was assessed by collecting 10 leaves
at random from trees within each replicate plot and counting the number of leaf spots
caused by Septoria pistaciarum . Monthly rainfall data was collected from 1992 to 1996.
Results and Discussion
In each of the five years that fungicides were evaluated for control of Septoria leaf
spot of pistachio, highly significant reduction of disease was achieved compared to no
treatment, regardless of fungicide or formulation tested, rates of each compound applied,
spray volume, or number of applications. The average number of leaf spots per leaf caused
by Septoria during 1992, 1993, 1994, 1995 and 1996 was 492, 109, 44, 138 and 483, respectively.
In comparison, the average number of leaf spots recorded on leaves treated with a fungicide
ranged from 1 to 5 in 1992, 1 in 1993, 0 in 1994, 3 to 65 in 1995 and 1 to 3 in 1996. The
average monthly rainfall in inches from 1992 to 1996 was 1.8 in Jan, 1.2 in Feb, 0.7 in Mar,
0.1 in Apr, 0.6 in May, 0.2 in Jun, 1.3 in Jul, 4.0 in Aug, 1.1 in Sep, 0.8 in Oct, 0.9 in
Nov and 2.0 in Dec.
These studies have demonstrated that chlorothalonil at a rate as low as 2.25 lb a.i./acre
and as few as two applications applied in July and August can effectively inhibit the devel
opment of Septoria leaf spot on pistachio in Arizona. Applications of copper hydroxide or
benomyl alone or in combination also were very efficacious. Septoria leaf spot typically
appeared on pistachio trees in southeastern Arizona during the month of August. It is
interesting to note that August on average is the wettest month of the year in this region,
with an average recorded rainfall of 4.0 inches during 1992 to 1996.
We were not able to assess the effect of fungicide treatments on yield, due to the
small number of trees within each treatment, the fact that the disease usually did not appear
until just before harvest and because different trees were used for each yearly fungicide trial.
However, in Arizona, healthy pistachio trees normally leaf out the first week in April and
do not completely defoliate until the last week of November. In comparison, trees heavily
infected with Septoria, as occurred in 1992 and 1996, can defoliate in September.
Presumably, up to a 2-month premature defoliation could adversely effect the overall
growth of infected trees, resulting in possibly reduced size and/or total yield of the
subsequent nut crop.
- Hallage, M.R. 1927. Fungus diseases of the pistachio tree in Syria. Internat. Bull.
Plant Prot. 1(3):38-39.
- Maas, J.L., van der Zwet, T., and Madden, G. 1971. A severe Septoria leaf spot of
pistachio nut trees new to the United States. Plant Dis. Reptr. 55:72-76.
- Pupillo, M., and Di Caro, S. 1952. Alcune osservazioni sulle Septoria del Pistachio.
Ann. Sper. Agr. N.S. 6(3):623- 634.
- Sarjanni, J.A. 1935. Notes phytopathologiques; les septorioses du Pistachier.
Ann. Inst. Phytopath. Benaki 1(3):67-76.
- Young, D.J., and Michailides, T.J. 1989. First report of Septoria leaf spot of
pistachio in Arizona. Plant Dis. 73:775.
The authors wish to thank the Pistachio Corporation of Arizona for allowing us to
conduct our studies in their pistachio orchard and especially Mr. Jim Cook for his invaluable
assistance during these studies.
This is a part of publication AZ1051:
"1998 Citrus and Deciduous Fruit and Nut Research Report," College of
Agriculture, The University of Arizona, Tucson, Arizona, 85721.
This document located at http://ag.arizona.edu/pubs/crops/az1051/az105112.html
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