TITLE: Black Spot Control Efficacy and Phytotoxicity Evaluation of Heritage® Fungicide on Rose

AUTHOR: George L. Philley, H. Brent Pemberton and William E. Roberson

SIGNIFICANCE TO THE INDUSTRY: Heritage (azoxystrobin) gave acceptable control of black spot when applied on both 7 and 14-day intervals at the 1/2 lb/100 gal rate. There was not a significant difference in control between the 1/4 lb and 1/2 lb rate. Both were acceptable under the conditions of this test, which was conducted from late August through October. Black spot did not develop as much as it would under season-long conditions. More disease pressure is needed to fully evaluate Heritage on highly susceptible varieties.

INTRODUCTION: Azoxystrobin is currently being tested for efficacy on fungal diseases affecting many ornamental crops in a commercially available product known as Heritage. It is a strobilurin, compounds consisting of analogs derived from strobilurin A which was originally isolated from naturally occuring fungi. Black spot (Diplocarpon rosae Wolf) is one of the most destructive diseases on rose worldwide. Control in disease prone areas requires frequent spraying with an effective fungicide. The objective of this study was to test the efficacy of azoxystrobin for black spot control and to assess phytoxicity to rose plants.


Phytotoxicity Study

Eleven commercial rose cultivars were treated 3 times on 14-day intervals with Heritage. The rate was 1 lb/100 gal of water. Latron B-1956, a surfactant, was added at 0.06% V/V. All control plots were treated with the surfactant and water mixture. Treatments were applied with a Birchmeier backpack sprayer pressurized by hand to approximately 50 psi. All plots were sprayed to near runoff. The application dates were 27 August, 10 September and 24 September 1997. Plots were evaluated on 1 October 1997. There were 3 replicates (treated and control) for each cultivar and the plots were approximately 6 feet long. All standard fungicide applications made by the grower to control black spot continued on the entire crop throughout the test period. The following Rosa cultivars were treated:

Medallion Gold Glow
Poinsettia Paradise
Yankee Doodle Christian Dior
Camelot Mister Lincoln
Perfume Delight Peace
Queen Elizabeth
Black Spot Control Study

The same equipment and treatment procedures used in the phytotoxicity study were used in the black spot control test. The test was established on two Rosa cultivars - Lady X and Gold Glow. Plots were approximatly 10 feet long. The treatments on each cultivar are listed below. A treatment interval comparison was done on Lady X and a rate comparison on Gold Glow. These cultivars were grown at the Texas A&M University Agricultural Research and Extension Center at Overton, TX.

Gold Glow
Treatment Rate/100 gal Interval/Day Surfactant
Control (water) -- -- +
Sentinel 40 WG 1.76 oz 14 +
Heritage 50 WG 1/4 lb 14 +
Heritage 50 WG 1/2 lb 14 +

Lady X
Control (water) -- -- +
Sentinel 40 WG 1.76 oz 14 +
Heritage 50 WG 1/2 lb 14 +
Heritage 50 WG 1/2 lb 7 +

Treatment dates were:

(14-day treatments) 20 August; 3 September; 18 September, 1 October; 15 October; and 31 October 1997.

(7-day treatments) 20 August; 28 August; 3 September; 10 September; 18 September; 24 September; 1 October; 15 October; 22 October; 31 October 1997. (Weather and other factors prevented treatment on 8 October).

Black spot control ratings were made 3 November 1997.

Phytotoxicity - No phytotoxicity symptoms were observed on the final evaluation date (1 October). Blooming characteristics, leaf size and development and leaf color were not affected.

Black Spot Control -

Rate Comparison - Gold Glow

There was no significant difference in black spot control between the two rates tested (Table 1). Both gave acceptable control.

Interval Comparison - Lady X

Like the rate study there was no significant differences between the 7 and 14-day interval (Table 2).

In both the rate and interval comparisons, Heritage gave significantly better control than the nontreated plots. At the end of the test, black spot was developing rapidly and causing noticeable defoliation in the checks. However, the control achieved with Heritage may not prove acceptable if tested over a full season. Although black spot developed, disease pressure had not reached the plateau we normally encounter with season-long tests. For example, on Lady X, two of the three replicates in treatment 3 rated 3 for black spot (data not shown), and the check averaged 7.5 (Table 2). When the check averages 9-10, which is normal at the end of a season with no treatment, the Heritage rating would likely be higher. The 2.7 average rating for treatment 3 (Table 2) and the fact the 7-day interval was not significantly better (treatment 4) indicates that control with Heritage at these rates was slowly being overpowered by the increased disease pressure. It should be noted that black spot had been controlled on the test plants through August prior to establishing the study.

Table 1. Blackspot control ratings of Heritage on Gold Glow roses - TAMU, Overton, 1997.
Treatment Rate/100 gal Interval/Days Black spot * Defoliation *
1. Control --- --- 5.2 2.2
2. Sentinel 1.76 oz 14 1.0 1.0
3. Heritage 1/4 lb 14 1.7 1.0
4. Heritage 1/2 lb 14 2.0 1.0

* Avg of 3 replications. Black spot rating 1-10, 1 = no black spot, 10 = all leaves infected and heavy defoliation. Defoliation rating 1-10, 1 = 0-10%, 10 = 91-100% defoliation.

Table 2. Black spot control ratings of Heritage on Lady X roses - TAMU, Overton, 1997.
Treatment Rate/100 gal Interval/Days Black spot* Defoliation*
1. Control --- --- 7.5 4.3
2. Sentinel 1.76 oz 14 1.2 1.3
3. Heritage 1/2 lb 14 2.7 1.8
4. Heritage 1/2 lb 7 2.3 1.2

* Avg of 3 replications. Black spot rating 1-10, 1 = no black spot, 10 = all leaves infected and heavy defoliation. Defoliation rating 1-10, 1 = 0-10%, 10 = 91-100% defoliation.