Alternaria helianthi
Author: Maria G. Zambrano, University of Florida
Reviewed by: Jeffrey Rollins, University of Florida
Pathogen
Sunflower (Helianthus annus L.) is a crop of importance for oil production, human consumption of the seeds, and as an ornamental flower. A major threat to this crop is sunflower blight disease, also known as sunflower leaf spot. The disease is caused by the fungus Alternaria helianthi (Hansf) Tubaki & Nishih. Yield losses of up to 80% have been reported. The disease is most severe in warm and humid climates (2). The main host is sunflower, but it has proven capable of infecting safflower (Charthamus tinctorius), noogoora burr (Xanthium pungens), cocklebur (Xanthium strumarium),and bathurst burr (Xanthium spinosum) (3), all of which can serve as alternative hosts.
Alternaria helianthi is an ascomycete in the Pleosporaceae family. Most members of the genus Alternaria lack a sexual stage. For this reason, they have been classified as deuteromycetes (i.e., imperfect fungi) in the Dematiaceae family. Alternaria helianthi produces simple, rarely branched conidiophores that bear solitary conidia. The conidia are ellypsoid or broadly ovoid, light brown colored, and rarely form longitudinal septa (4).
Symptoms and Signs
Symptoms initiate with chlorotic spots on the leaves that later turn dark brown with irregular size and shape. Some may show a yellow halo around the spot. On stems, lesions are dark and elongated. Severely affected stems will ultimately break. Lesions also occur on the petals, sepals, and head. The infection causes blight and defoliation, destruction of the flowers, and early senescence (5).
Ecology and Spread
When conidia are seedborne, there can be a detrimental effect on seed germination and cause seedling blighting. Conidia can also be transported by wind and water. Temperatures that favor the development of sunflower leaf spot are 24-27°C (1). Alternating periods of high humidity with dry periods cause the fungus to sporulate profusely. Conidia germinate producing one or more germ tubes, which spread across the surface. They penetrate the host through apressoria, natural wounds, and stomates. Conidiophores emerge through stomata, trichomes, and from the vascular tissue (3). The conidia produced at this stage will cause secondary infections and spread to healthy plants. The main source of inoculum comes from mycelium and conidia overwintering on plant debris.
Geographic Distribution
Sunflower blight disease is distributed world wide, although it represents a more serious problem in humid regions.
Management
Cultural control practices include: 1) the destruction of plant residue to eliminate source of inoculum; 2) crop rotation with non Asteraceae crops; and 3) allowing for fallow periods. Additionally, adjusting the sowing time so that it does not coincide with the rainy season, oil solarization, and field sanitation are highly recommended to reduce Alternaria infections.
Chemical control is usually not necessary in zones with low humidity. In areas where the disease is most severe, timely application of fungicides such as mancozeb (0.2%), chlorothalonil (0.2%), difenaconazole (0.1%), or tebuconazole (0.1%) have proven to be effective. To achieve the best results, a combination of cultural and chemical control must be implemented. Consult your local extension specialist for legal and efficacious fungicide products available in your state. Remember, the label is the law and the product applicator is responsible for reading and following all chemical labeling.
Diagnostic Procedures
Traditional methods for detecting the disease include visual inpsection of plants in the field and seed incubation to evaluate colony characteristics. These methods are of limited value because infections of other pathogens (e.g., Macrophomina phaseolina, Diaporthe helianthi) cause similar symptoms (6). Also, other species of Alternaria are frequently associated with seeds, making it difficult to identify A. helianthi. Molecular detection methods based on PCR have shown to be sensitive and precise for the proper identification, however, no standardized methods are available yet (7).
Resources and References
- Abbas, H. K., Egley, G. H., and Paul, R. N. 1995. Effect of conidia production temperature on germination and infectivity of Alternaria helianthi. Phytopathology. 85(6): 677-682.
- Agrawat, J. M., Chippa, H. P., and Mathur, S. J. 1979. Screening of sunflower germplasm against Alternaria helianthi. Indian Journal of Mycology and Plant Pathology. 9: 85-86.
- Allen, S. J., Brown, J. F., and Kochman, J. K. 1983. The infection process, sporulation and survival of Alternaria helianthi on sunflower. Annals of Applied Biology. 102(3): 413-419.
- Alves, J. L., Woudenberg, J. H. C., Duarte, L. L., Crous, P. W., and Barreto, R. W. 2013. Reappraisal of the genus Alternariaster (Dothideomycetes). Persoonia-Molecular Phylogeny and Evolution of Fungi, 31(1): 77-85.
- Bhargav, D. K., and Meena, H. P. 2014. Alternaria Blight: A Chronic Disease in Sunflower.
- Perron G, Delos M, Hariot J, Germillon C. 1990. Symptoms on sunflower stem: beware of confusions. Phytoma 419: 31–33.
- Udayashankar, A. C., Nayaka, S. C., Archana, B., Anjana, G., Niranjana, S. R., Mortensen, C. N., and Prakash, H. S. 2012. Specific PCR-based detection of Alternaria helianthi: the cause of blight and leaf spot in sunflower. Archives of microbiology. 194(11): 923-932.
