Research Update on Broccoli Alternaria Head Rot Management

Figure 1. Head rot symptoms start as yellow spots and then turn brown and black (all photos courtesy Y. Wang.)

Broccoli head rot, also known as pin rot, continues to increase in the Salinas Valley, especially in fall broccoli production. Two types of head rot, bacterial head rot and Alternaria head rot, are affecting broccoli (Koike 2010). Here we focus on Alternaria head rot caused by the fungi Alternaria spp.

Head Rot Symptoms
All aboveground parts of broccoli are subject to infection including heads and leaves. Head rot symptoms start as yellow spots and then turn brown and black (Figure 1).

Figure 1. Head rot symptoms start as yellow spots and then turn brown and black (all photos courtesy Y. Wang.)

The infection can spread from buds to stems (Figure 2).

Figure 2. The infection can spread from buds to stems.

With secondary bacteria or fungi infection, further decay occurs. The initial yellow spots resemble brown bead (Figure 3), a broccoli disorder that can potentially be caused by excessive temperature, poor growth or nutrient and water deficiency. However, the brown bead doesn’t rot the stem, and no sign of fungi is presented on the buds. For uncertain cases, scraping the buds to see if the stem rot or fungi are presented is a useful technique. Leaf spot symptoms start as small yellow spots on the old leaves and then form dark, concentrical rings like a target (Figure 4). The old spots may become brittle and split open or fall out as shot holes. The high number of leaf spots per plant indicates a higher disease pressure and could be a signal for fungicide application.

Figure 3. Brown bead, a broccoli disorder.

Management Options
The disease is favored by prolonged wetness from rain, dew and fog. The wetness, a thin layer of water, is required for fungal spore germination. In addition, fungal spores are spread by winds and splashing water. Cultural practices to promote leaf drying or prevent leaf wetness may reduce disease severity. Some growers have seen the benefits of using drip irrigation instead of overhead irrigation to avoid wetting the foliage. An early harvest before rainfall could also reduce disease risk. Variety effects on disease tolerance play a role. Lumpy broccoli heads tend to accumulate water which may further weaken the plant tissues and become a suitable target for the pathogens. Finally, there are a number of fungicides that have activity against the disease. Preventative fungicide applications should be considered for wet weather that is favored by the disease.

Figure 4. Leaf spot symptoms start as small yellow spots on the old leaves and then form dark, concentrical rings like a target.

Research Update: Fungicide Evaluation
This study was conducted to evaluate some new and common fungicides in fall broccoli to support the growers and ag industry.

One fungicide trial was conducted in a commercial broccoli field to test the efficacy of select fungicides for controlling broccoli head rot in fall 2023. Broccoli ‘Centennial’ was direct-seeded on July 27, 2023. Seven fungicide treatments and a nontreated control were arranged in a randomized complete block design with four replications. Each plot consisted of two seedlines of broccoli 30-ft long on 40-inch-wide beds. On each side of the plot was a nontreated guard bed. Treatments were applied with a CO2-pressurized backpack sprayer calibrated to deliver 35 gpa at 30 psi using double TeeJet 8004E flat fan nozzles. Fungicide applications were made on October 4 and October 16. All treatments were applied with non-ionic surfactant Dyne-Amic 0.08% v/v. Alternaria head rot incidence was evaluated at harvest on October 23. Disease incidence was expressed as the percentage of the number of plants with Alternaria head rot in the total number of plants within the middle 15 ft of the plot. Data were analyzed using analysis of variance (ANOVA) and the Tukey test to separate means at P<0.05. The total rainfall received one month before harvest was 0.57 inches. The average, minimum and maximum temperatures were 62 degrees F, 53 degrees F and 75 degrees F, respectively.

Table 1

The disease pressure in this trial area was low with nontreated control having 14.0% head rot (Table 1). However, significant differences occurred among treatments for the % Alternaria head rot. All treatments reduced % Alternaria head rot numerically, while Pydiflumetofen+Fludioxonil, Azoxystrobin, Fluxapyroxad+Pyraclostrobin, Fluopyram+Trifloxystrobin and Pyraclostrobin had significantly lower % Alternaria head rot than nontreated control. And they had statistically similar % Alternaria head rot. These results also showed single FRAC 11, premixes with FRAC 7 and 11, and premixes with FRAC 7 and 12 provided good control of Alternaria head rot. Single FRAC 7 provided fair control of Alternaria head rot.

Thanks to the cooperating growers and PCAs for assisting the trial. Thanks to agrochemical companies for funding and the technical assistance from Carlos Rodriguez.

Koike, S. T. 2010. Looking ahead: head rot can be issue for winter and early spring broccoli. Salinas Valley Agriculture blog.

Yu-Chen Wang | UCCE Plant Pathology Advisor, Santa Cruz, Monterey and San Benito Counties
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