Effective Management Tools for Septoria Leaf Spot of Pistachio in Arizona

Abstract

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.

Introduction

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 nontreated trees.

(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.

Literature Cited

1. Hallage, M.R. 1927. Fungus diseases of the pistachio tree in Syria. Internat. Bull. Plant Prot. 1(3):38-39.
2. 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.
3. Pupillo, M., and Di Caro, S. 1952. Alcune osservazioni sulle Septoria del Pistachio. Ann. Sper. Agr. N.S. 6(3):623- 634.
4. Sarjanni, J.A. 1935. Notes phytopathologiques; les septorioses du Pistachier. Ann. Inst. Phytopath. Benaki 1(3):67-76.
5. Young, D.J., and Michailides, T.J. 1989. First report of Septoria leaf spot of pistachio in Arizona. Plant Dis. 73:775.

Acknowledgments

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.