From the numerous samples of leaf spots submitted to the Yuma Plant Health Clinic this season high numbers of these cases have been diagnosed as Stemphylium in spinach, and cases are now increasing in onion. These trends are not unexpected given the environmental conditions that have prevailed in the region over recent weeks. The combination of warm daytime temperatures, mild nights, and extended periods of leaf wetness have created highly favorable conditions for pathogen infection, disease development, and spread across production fields in the Yuma County area.

Although Stemphylium species are generally regarded as minor or secondary pathogens in desert vegetable production systems, seasons such as this serve as an important reminder of the critical role that the environment plays in host-pathogen interactions and disease outcomes. Even pathogens that are typically of limited economic concern can emerge as significant issues when conditions align favorably, reinforcing the importance of the disease triangle as the central dogma of plant pathology and the need for continued vigilance and disease management options during atypical seasons.

It’s a good thing Jim Correll is in town to plant his annual downy mildew field trials at the Yuma Ag Center. I recently had the opportunity to meet with him in-person, and our discussions provided valuable insight into Stemphylium leaf spot development in spinach.

To date eleven species of Stemphylium have been associated with spinach tissues, meaning they’ve been isolated from spinach seed, leaves, or field residues but have not yet been demonstrated to be capable of causing disease symptoms on the live host (Spawton and du Toit, 2024). Isolation of a fungus from plant tissue alone does not confirm pathogenicity, to do so we must also demonstrate that these isolates can consistently recreate the disease symptoms seen on the original host when inoculated on a healthy host. Of those eleven species only Stemphylium beticola, S. drummondii, and S. vesicarium have been conclusively demonstrated as causal pathogens driving leaf spot of spinach in the field. This does not mean the other eight species are non-pathogenic, simply that evidence has not been provided yet that they are causal pathogens of spinach leaf spots.

Across two replicated field studies in Washington, several fungicides including Amistar, Rovral, Sovran, Pristine, and Cabrio performed significantly better than untreated controls in reducing disease severity (du Toit et al. 2004; du Toit et al. 2005). However, it is important to note that none of these products are currently registered for the control of Stemphylium leaf spot on spinach in Arizona. While Stemphylium leaf spot is typically considered to be of minor economic importance, disease-conducive seasons such as the one we are currently in highlight the ongoing need for research focused on effective, legally available management options and improved understanding of pathogen behavior under desert growing conditions.

If you have any concerns regarding the health of your plants/crops please consider submitting samples to the Yuma Plant Health Clinic for diagnostic service or booking a field visit with me:

Chris Detranaltes, Ph.D.
Cooperative Extension – Yuma County
Email: cdetranaltes@arizona.edu
Cell: 602-689-7328
6425 W 8th St Yuma, Arizona 85364 – Room 109