Yu-Chen Wang

Research update on organic fungicide evaluation for management of celery late blight

Yu-Chen Wang, Renée L. Eriksen, Chris Greer, and Alex Putman

Celery late blight can cause significant problems during the fall and spring production period in the Salinas and Santa Maria Valleys. The disease is caused by the fungus, Septoria apiicola. Early disease symptoms include small, irregularly shaped, chlorotic spots on the leaves and petioles. Older leaves and stalks are usually infected first. As the disease progresses, the lesions enlarge, later merge to blight large areas of tissue, leading to plant death (Photo 1). A characteristic feature of these lesions is the small, black round structures in the center (Photo 2). These structures are the reproductive bodies of the fungus, and the size and color are similar to black pepper.

Management. The primary source of the pathogen is contaminated celery seeds and undecomposed celery debris. Cultural controls include using pathogen-free seeds and rotating away from celery for more than 1 year. The disease is favored by cool temperatures and prolonged wetness from rain, dew, fog, and sprinkler irrigation. Dense crop canopies reduce air movement and can also increase disease development by prolonging leaf wetness periods. Fungal spores are spread by splashing water from rainfall or overhead sprinklers. Movement of equipment or workers can also spread the spores from diseased to healthy plants. 

There are a number of fungicides that have activity against the disease. Apply protectant fungicides, especially if sprinkler irrigation is used or rain has occurred or is forecasted. Managing this disease in organic production is even more challenging as fungicide options are limited. The following section addresses research needs for organic management of celery late blight by evaluating the field efficacy of some organic fungicides. 

Research update on organic fungicide evaluation.

A fungicide trial was conducted at the USDA-ARS Spence Road research farm to evaluate the efficacy of select organic fungicides for managing celery late blight in the summer of 2025. Celery ‘Sonora’ was transplanted on May 1. Eleven organic fungicide treatments, a standard conventional rotation treatment, and an untreated control (UTC) were arranged in a randomized complete block design with four replications (Table 1). Each 20 ft-long plot consisted of two lines of celery on a narrow bed (40-inch centers). Plant spacing was 7 inches. A buffer bed on either side of the plot was left untreated to ensure there was no drift between treatments. Septoria inoculum (1.4 x 10⁵ spores/mL) was prepared by soaking and stirring dried infected celery leaves for 30 min in water then straining through two layers of cheesecloth. Inoculum was applied with a hand-pump backpack sprayer to all treatments on July 3 using a single hollow cone nozzle at 15 gal/A followed by light moistening the plant canopy. Fungicide treatments were applied with a CO₂-pressurized backpack sprayer calibrated to deliver 35 gpa at 30 psi using TeeJet 8004E flat fan nozzles on a two-nozzle boom. Fungicide applications were made on June 10, June 26, July 10, and July 24. Initial celery late blight severity was evaluated on July 28 on 10 plants from the middle of each treatment plot based on a 0-5 rating scale (0=no symptoms, 1=1%-10% leaf blight, 2=10%-20% leaf blight, 3=20%-30% leaf blight, 4=30%-50% leaf blight, 5=petiole blight). The severity evaluation was made on an outer leaf, a middle leaf, and an inner leaf on each plant. Disease severity was expressed as the average rating of the 10 plants. Data were third root transformed, analyzed using analysis of variance (ANOVA), and the Tukey HSD test to separate means at P<0.05. 

Results. Late blight was slow to develop after inoculation, but with the conducive environmental conditions created by overhead irrigation twice a week and adequate dew periods for sporulation and infection, disease severity was low to moderate near harvest maturity. On the July 28 rating date (88 days after planting), the average disease severity of the nontreated control was 1.7 (Fig. 1). There were significant differences in the average disease severity among treatments. All treatments reduced celery late blight disease severity numerically, but Kocide 3000 and the conventional fungicide rotation treatment had significantly lower celery late blight severity than the nontreated control. The conventional standard treatment had the lowest late blight severity and was therefore the most effective treatment against disease symptoms. Phytotoxicity was not observed with any treatment in this trial. 

Table 1. Treatment list 

^zApplication timing: A=June 10, B=June 26, C=July 10, and D=July 24

Tolerance of lettuce varieties to Fusarium wilt – 2024

Alex Putman, Yu-Chen Wang, and Chris Greer

Summary

Fusarium wilt of lettuce, caused by Fusarium oxysporum f. sp. lactucae (FOL), is an economically significant disease on the Central Coast of California.  We conducted field trials to evaluate 30 iceberg and 21 romaine varieties for tolerance to Fusarium wilt.  The trials were located in commercial fields in Greenfield, CA (wet date of May 27, 2024) and Salinas, CA (wet date of June 8, 2024). Foliar disease severity was visually evaluated on July 31-August 2, 2024 at Greenfield and on August 5-7, 2024 at Salinas and converted to a marketability (yes or no) rating.  Disease pressure was high at both locations. Iceberg varieties that exceeded 50% marketability at both locations were Powerball and two coded entries from Vilmorin-Mikado.  Nine varieties exceeded 50% marketability at Greenfield but not Salinas, and two varieties exceeded the same threshold at Salinas but not Greenfield.  For romaine, 19 out of 22 varieties exceeded 90% marketability at Greenfield, but only two of those varieties (Holbrook and Momentus) also exceeded the same threshold at Salinas.  In a greenhouse experiment, isolates from Greenfield showed a susceptible reaction on variety Costa Rica #4, which is consistent with the Costa Rica FOL race variant.  Although FOL race 1 is suspected to be present at the Salinas location, greenhouse testing is not complete.  These trials provide public data on the tolerance of iceberg and romaine varieties to Fusarium wilt.

Methods

Field trials were conducted in Greenfield, CA and Salinas, CA to evaluate both in-slot and out-of-slot varieties (30 iceberg and 21 romaine) for tolerance to Fusarium wilt in commercial fields with disease history.  At Greenfield, bed center spacing was 80 inches, and plots were 1 plant line wide by 100 ft. long.  Due to space constraints, 4 iceberg and 10 romaine varieties were not included in the Salinas trial.  At Salinas, bed center spacing was 40 inches, and plots were 1 plant lines wide by 40 ft. long.  Iceberg and romaine varieties were evaluated separately, and plots of each type were arranged in a randomized complete block with four replications.  Treatments were direct seeded using the grower-cooperators’ planters at Greenfield and using single-line push planters at Salinas.  The wet dates were May 27, 2024 for Greenfield and June 8, 2024 for Salinas.  The Greenfield trial was maintained to commercial standards for lettuce production, whereas the Salinas trial was not.  After thinning by commercial crews, 50 plants at Greenfield and 30 plants at Salinas in the center of each plot were counted, and the section was marked with stakes. Data were collected from this center section.  Evaluations were performed on July 31-August 2 at Greenfield and August 5-7 at Salinas, which was before maturity at the Salinas trial.  Foliar disease severity was assessed on a 0 to 4 scale where: 0 = healthy; 1 = wilting or chlorosis of one to three outer leaves; 2 = up to moderate stunting and wilting or chlorosis of <25% of leaf area; 3 = head is severely stunted or absent and between 25% and 75% of leaf area is wilting or chlorotic; and 4 = head is absent and >75% of leaf area is chlorotic and nearly dead, or plant is entirely dead.  For analysis, foliar disease severity was converted to marketability, where: disease severity of 0 or 1 = marketable; and disease severity of 2, 3, or 4 = not marketable.  Marketability data was analyzed by an analysis of variance (P < 0.05), and variety means were separated using Tukey’s honestly significant difference test.

Results – Race of the FOL pathogen present

Two races of FOL are present on the Central Coast: race 1, and a novel race variant (Nayak et al., 2024). We are using the temporary name “Costa Rica FOL variant” for the novel race variant until it is officially named following completion of the upcoming ring test, which is a collaborative experiment between researchers and seed companies.  To determine the FOL race present, isolates from each location were evaluated in a race typing experiment in the greenhouse.  Variety Costa Rica #4 showed a susceptible reaction to both Greenfield isolates, which supports the observation that the Costa Rica FOL race variant is present at the Greenfield location.  We suspect FOL race 1 is present at the Salinas location.  However, greenhouse testing of the Salinas location isolates is ongoing.

Results – Marketability

Disease pressure was high at both locations.  Iceberg varieties that exceeded 50% marketability at both locations were Powerball and two coded entries from Vilmorin-Mikado (Table 1).  Nine varieties exceeded 50% marketability at Greenfield only: Balboa, Fontinas, Meridian, Paraiso, San Andreas, San Miguel, two coded entries from Salinas Valley Seeds, and one coded entry from Sakata. In contrast, two varieties exceeded the same threshold at Salinas only: Fredonia and a coded entry from Takii.  This pattern of some varieties showing large differences in performance between locations whereas others showed similar performance suggests that a different race is present at each location, but this has not yet been confirmed by greenhouse testing. 

Of the 21 romaine varieties evaluated, two varieties exceeded 90% marketability at both locations: Holbrook and Momentus (Table 2).  A total of 17 out of 21 varieties exceeded the same threshold at Greenfield but not Salinas.  At the Salinas location, Holbrook and Momentus were not statistically different from four varieties (Boronda, Copious, Patton, and Solid Heart) with average percent marketability ranging from 77% to 88%. 

If you have additional questions about these trials, please contact Alex Putman at 951-522-9556 or aiputman@ucr.edu.

Please Send Us Samples

We are continuing to collect samples of lettuce Fusarium wilt to determine the distribution of races and to monitor the pathogen. To support this research, please contact the person in your region. Your help would be greatly appreciated.

• Monterey, San Benito, or Santa Cruz Counties – Yu-Chen Wang (831-201-9689 or yckwang@ucanr.edu)
• San Luis Obispo, Santa Barbara, or Ventura Counties – Chris Greer (805-888-1355 or cagreer@ucanr.edu)
• Any other California county – Alex Putman (951-522-9556 or aiputman@ucr.edu)

Acknowledgements

We are grateful to D’Arrigo Brothers Co. of California and an anonymous grower for the space and maintenance of the variety field trials.  We thank seed producers for providing seed for the trial.  Funding for this project was made possible by a grant from the U.S. Department of Agriculture (USDA) Agricultural Marketing Service. Its contents are solely the responsibility of the authors and do not necessarily represent the official views of the USDA.  Funding was also provided by the California Leafy Greens Research Program.

References

Nayak, S., K.L. Richardson, A.I. Putman, N.R. LeBlanc, F.N. Martin, N. Li, and J.D. McCreight. 2024. Detection of novel pathogenic variants of Fusarium oxysporum f. sp. lactucae in California. Plant Pathology Early View. doi:10.1111/ppa.14019

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