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Fungicide Evaluation for Alternaria Head Rot (Pin Rot) of Broccoli

Broccoli head rot, also known as pin rot, can cause significant problems, especially in fall broccoli production in Salinas Valley. Two types of head rot are affecting broccoli, including bacterial head rot and Alternaria head rot (Koike 2010). Differences between those two types can be seen at: https://ucanr.edu/blogs/blogcore/postdetail.cfm?postnum=3861. Here we focus on the Alternaria head rot, caused by the fungi Alternaria spp.

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 (photo 1). The infection can spread from buds to stems (photo 2). With secondary bacteria or fungi infection, further decay occurs.The initial yellow spots resemble brown bead (photo 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 (photo 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.

Photo 1. Broccoli Alternaria head rot

Photo 3. Broccoli brown bead

Photo 4. Alternaria leaf spot on broccoli

Management. The disease is favored by prolonged wetness from rain, dew, and fog. 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. An early harvest before rainfall could also reduce disease risk. Variety effects on disease tolerance might 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. More frequent fungicide applications should occur during wet weather which is favored by the disease.

Research update: fungicide evaluation.

Methods. One fungicide trial was conducted in a commercial broccoli field to test the efficacy of select fungicides for controlling broccoli head rot in 2023 fall. Broccoli ‘Centennial’ were direct seeded on 27 July, 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 that was 30-ft long on the 40-inch wide bed. 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 a double TeeJet 8004E flat fan nozzles. Fungicide applications were made on 4 October and 16 October. All treatments were applied with non-ionic surfactant Dyne-Amic 0.08% v/v. Alternaria head rot incidence was evaluated at harvest on 23 October, 2023. 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°F, 53°F, and 75°F, respectively.

Results (Table 1). The disease pressure in this trial area was low with nontreated control having 14.0% head rot. 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 that 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.

Table 1. Disease incidence of Alternaria head rot at harvest

zProduct and Rate/A in this trial: Boscalid (Endura 9 oz), Penthiopyrad (Fontelis 30 fl oz), Pydiflumetofen+ Fludioxonil (Miravis Prime 11.4 oz), Azoxystrobin (Quadris 15.5 fl oz), Fluxapyroxad+ Pyraclostrobin (Priaxor 8.2 fl oz), Fluopyram+ Trifloxystrobin (Luna Sensation 7.6 fl oz), Pyraclostrobin (Cabrio 16 oz). xNumbers in a column followed by the same letter are not significantly different based on Tukey’s significant difference test (P<0.05). yFRAC: Fungicide Resistance Action Committee. — By Yu-Chen Wang, Plant Pathology Farm Advisor, UC Cooperative Extension

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

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