Introduction
Alternaria leaf blight and head rot (ABHR) is caused by Alternaria brassicicola, an economically important disease of brassicas, including broccoli. The pathogen causes lesions with concentric rings that gradually fall off as the disease progresses, giving a shot-hole appearance. This disease can cause significant yield losses in commercial broccoli production fields.
This report provides data on the efficacy of two fungicide programs and highlights the importance of early initiation of a systemic fungicide program for the management of ABHR in broccoli.
Materials and Methods
Broccoli seedlings were transplanted into two row-beds covered with 18-in. white plastic mulch on November 18, 2024, at the Blackshank Farm at the University of Georgia Tifton campus. Beds were formed on 6-ft centers. Plots were 20 ft long with 10-ft borders planted between plots.
The fertility and insecticide programs were consistent with UGA Extension recommendations. The experimental design consisted of a randomized complete block with four replications.
Treatments were applied with a backpack sprayer calibrated to deliver 40 gallons/acre at 75 to 80 psi through TX-18 hollow-cone nozzles. Fungicide applications were made on 10 dates: November 19 and 26, 2024; December 6, 13, and 26, 2024; January 2, 10, 24, and 31, 2025; and February 2, 2025.
The total rainfall received during November 2024, January 2025, and February 2025 was 3.9 in., 4.1 in., and 1.5 in., respectively. The average high and low temperatures for the month of November 2024 were 61 °F and 47 °F, respectively, and for the month of February 2025 were 54 °F and 32 °F, respectively.
Foliar disease severity was assessed on three dates: January 20 and 27, and February 7, 2025, as percentage leaf area with symptoms per plot. The area under the disease progress curve (AUDPC) was calculated from disease severity ratings collected during these assessment periods. Effects of treatments on disease severity and values of AUDPC were analyzed using R version 4.1.0 software. Treatment means were compared using Fisher’s least significant difference (LSD) at α = 0.05.
Results
Alternaria leaf blight appeared late, around a month before harvest maturity (January 20, 2025). Significantly lower foliar disease severity was observed with Program 1 compared to Program 2, and Program 2 had significantly lower disease severity than the nontreated check plots.
Within a week, the disease progressed aggressively, and the significant differences in foliar disease severity between the two fungicide programs were no longer observed during the two subsequent assessment periods (January 27, 2025, and February 7, 2025). The AUDPC values were significantly lower for Program 1 compared to Program 2, and both programs had significantly lower AUDPC values compared to the nontreated check. Phytotoxicity was not observed with any of the treatments used.
| Treatment program | Product and rate per acre | Application datesw | Mean foliar disease severity (%)x | AUDPCy | |
|---|---|---|---|---|---|
| Jan. 20, 2025 | Feb. 7, 2025 | ||||
| Program 1 | Luna Sensation 7.6 fl oz | 1,7 | 11.3 c | 53.8 b | 437.5 cz |
| Inspire Super 20 fl oz | 2,8 | ||||
| Approvia Top 13.5 fl oz | 3,9 | ||||
| Bravo Weather Stik 1.5 pt | 4,5,6 | ||||
| Priaxor 8 fl oz | 10 | ||||
| Program 2 | Bravo Weather Stik 1.5 pt | 1–3 | 35.0 b | 58.8 b | 660.5 b |
| Luna Sensation 7.6 fl oz | 4,7 | ||||
| Inspire Super 20 fl oz | 5,8 | ||||
| Approvia Top 13.5 fl oz | 6,9 | ||||
| Priaxor 8 fl oz | 10 | ||||
| Nontreated check | 70.8 a | 94.3 a | 1181.2 a | ||
| wApplication dates: 1 = Nov. 19, 2024, 2 = Nov. 26, 2024, 3 = Dec. 6, 2024, 4 = Dec. 13, 2024, 5 = Dec. 26, 2024, 6 = Jan. 2, 2025, 7 = Jan. 10, 2025, 8 = Jan. 24, 2025, 9 = Jan. 31, 2025, and 10 = Feb. 2, 2025. xDisease severity was rated on a 0 to 100 scale (0 = no infection and 100 = 100% of leaf area infected) on Jan. 20, 2025, Jan. 27, 2025, and Feb. 7, 2025. yAUDPC was calculated from ratings taken on Jan. 20, 2025, Jan. 27, 2025, and Feb. 7, 2025. zMeans followed by the same letter in each column are not significantly different according to Fisher’s LSD test at p ≤ 0.05. | |||||
Funding
This work is supported by the Specialty Crops Research Initiative Award 2020-51181-32062 from the USDA National Institute of Food and Agriculture.
