Seed-borne Cercospora beticola can initiate Cercospora leaf spot in sugar beet (Beta vulgaris L.) fruit tissue

Authors: SPANNER, REBECCA - North Dakota State University; Neubauer, Jonathan; HEICK, THIES - Aarhus University; Grusak, Michael; HAMILTON, OLIVIA - North Dakota State University; RIVERA-VARAS, VIVIANA - North Dakota State University; DE JONGE, RONNIE - Utrecht University; PETHYBRIDGE, SARAH - Cornell University; Webb, Kimberly; LEUBNER-METZGER, GERHARD - University Of London; SECOR, GARY - North Dakota State University; Bolton, Melvin

Submitted to: Phytopathology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 11/19/2021
Publication Date: 4/6/2022
Citation: Spanner, R., Neubauer, J., Heick, T.M., Grusak, M.A., Hamilton, O., Rivera-Varas, V., De Jonge, R., Pethybridge, S., Webb, K.M., Leubner-Metzger, G., Secor, G.A., Bolton, M.D. 2022. Seed-borne Cercospora beticola can initiate Cercospora leaf spot in sugar beet (Beta vulgaris L.) fruit tissue. Phytopathology. 112:1016-1028. https://doi.org/10.1094/PHYTO-03-21-0113-R.
DOI: https://doi.org/10.1094/PHYTO-03-21-0113-R

Interpretive Summary: Cercospora leaf spot (CLS) is a disease of global importance caused by the fungus Cercospora beticola. Previous studies have provided indirect evidence that this fungus has traveled across continents. However, Cercospora spores are only known to travel relatively short distances of a few kilometers or less. Consequently, it has not been clear how the fungus has the ability to travel long distances. Seeds infested with pathogens have been shown to cause disease in other cropping systems. In this study, we attempted to grow the fungus directly from seeds from 37 commercial sugar beet seed lots. We identified Cercospora in ten of these seed lots. These Cercospora strains were resistant to two important fungicide classes. When Cercospora-infested seed was planted, CLS disease occurred indicating that the disease can be seed-borne. During the course of these studies, we noticed several other species of fungi growing from seeds. To take an inventory of all the fungi found in sugar beet seed, we used a new sequencing device called a MinION as a diagnostic tool to identify different fungal species. This indicated that several other important fungi are in sugar beet seed, including potential sugar beet pathogens. Taken together, the presence of seed-borne pathogens should be considered when implementing integrated pest management for Cercospora leaf spot and possibly other diseases of sugar beet.

Technical Abstract: Cercospora leaf spot (CLS) is a globally important foliar disease of sugar beet (Beta vulgaris ssp. vulgaris) caused by the fungus Cercospora beticola. Management of CLS has been challenging due to the rapid development of resistance to many fungicides. Long-distance movement of C. beticola has been indirectly evidenced in multiple recent population genetic studies. In this study, we provide direct evidence for seed-borne C. beticola to initiate disease in sugar beet. We confirmed the presence of viable C. beticola for nine of 37 sugar beet seed lots.Each of the 38 seed-derived C. beticola isolates had a unique genotype. All isolates contained the G143A mutation in cytochrome b conferring QoI fungicide resistance and 32 of 38 isolates showed low DMI fungicide sensitivity (EC50 > 1µg/mL). Direct planting of pelleted seed demonstrated the ability of seed-borne inoculum to initiate CLS disease in sugar beet. C. beticola DNA was detected in DNA isolated from xylem sap, suggesting that it may use the vascular system to systemically colonize the host. Additionally, this was the first study to investigate the fungal microbiome within sugar beet seed. Long-read internal transcribed spacer amplicon sequencing using the MinION platform demonstrated the presence of potentially viable fungi from 20 different fungal genera. Fusarium, Alternaria, and Cercospora were dominant taxa and comprised an average of 93% relative abundance over 11 seed lots. The presence of seed-borne inoculum should be considered when implementing integrated disease management strategies for CLS of sugar beet in the future.