Effect of Foliar Application of Benomyl on Severity of Septoria Leaf Spot on Pistachio in Southeastern Arizona

Abstract

The fungicide, benomyl (Benlate) was foliar applied by a commercial air blast sprayer at the rate of 1.0 lb. a.i. per acre in early to late August. Treatments varied with a number of applications i.e. one or two and were compared to an untreated control. Benomyl significantly reduced leaf necrosis surrounding nut clusters and the number of leaf spot lesions when compared to control. One or two applications were equally effective in controlling Septoria leaf spot.

Introduction

Since 1986, a moderate leaf spot caused by Septoria pistaciarum has been observed on leaves of pistachio trees during the rainy season (July through September) in southeastern Arizona pathogenic fungus have been observed in Mediterranean countries in Europe as well as Texas in the U.S. (1,2). A fungicide trial was established in 1991 to examine the effect of benomyl (Benlate) on disease development and severity.

Materials and Methods

The fungicide trial was conducted in a commercial pistachio orchard in Bowie, Arizona. The test consisted of three treatments: one application of benomyl at 1.0 lb. a.i. in 33 gal. of water per acre; two applications of Benlate at the indicated rate at a 14-day interval; and an untreated control. Each treatment contained five randomized complete block design, with a row of trees representing a block. Each plot within a row was separated by a single buffer tree, while two buffer rows of trees separated each treatment row. Foliar applications of the fungicide were made August 5 and 19, 1991. Disease severity was determined at crop maturity by rating the incidence of leaf necrosis around clusters of pistachio nuts on the trees within each plot. Also, five leaves were collected at random from each plot and the average number of leaf spots caused by Septoria was determined.

Results and Discussion

The results of our rating of leaf necrosis and quantitative determination of leaf spots are shown in Tables 1 and 2. Trees treated with one or two applications of benomyl at 1.0 lb. a.i. had significantly less necrotic leaves around the nut clusters as well as a significantly less necrotic leaves around the nut clusters smaller number of leaf spot lesions when compared to untreated trees. The effect on yield of Septoria leaf spot and the associated premature necrosis and death of leaves adjacent to nut clusters is not known at this point.

The significant reduction of Septoria leaf spot by one application of benomyl suggests further studies on the efficacy of this material for disease control. Other fungal diseases of pistachio, such as Botryosphaeria panicle and shoot blight and Alternaria leaf and fruit spot, can also occur during the summer rainy season in southeastern Arizona. Timely application of benomyl could provide significant protection against potential crop loss due to these diseases as well.

References

1. Chitzandis, A. 1956. Ann. Inst. Phytopath. Benaki 10:20-44
2. Maas, J.L., et al. 1971. Plant Dis. Rept. 55:72-76
3. Young, D.J., and Michailides, T.J. 1989. Plant Dis. 73:775.