In 2020, a number of numerous cases of basket anomalies of sunflower were recorded in Ukraine, which due to the degeneration of tubular flowers into leaf blades in the middle of the basket are unable to form functional generative organs, leading to partial or complete sterility of plants. This developmental anomaly has been known to phytopathologists for half a century (Meyer, 1966) and is called “phyllody”. It has been found not only on sunflower (Bidari et al., 1987), but also on other plants, such as sesame (Cagirgan et al., 2013), roses (Yan et al., 2016), tomatoes, rapeseed and corn, and in nature, in particular, on plantain, cornflowers, hops and other species.
External signs: typical morphological aberrations, accompanied by chlorosis, drying of the leaf edge, which can be mistaken for calcium or boron deficiency.
Infectious and stress etiology: Common biological agents that cause the development of philodium are representatives of intracellular bacteria of the genus Phytoplasma (“Candidatus Phytoplasma”), so named by analogy with mycoplasmas for lack of cell walls, variable shape, small cell size and <1 μ genome. Phytoplasmas are not capable of free existence in the environment without host plants, because they are obligate parasites. Vectors that carry phytoplasmas are various species of insects (including cicadas or leafhoppers), which mechanically damage the outer coverings of plants and feed on phloem sap (Akulov, 2020). Phytoplasmas can affect more than 500 species of cultivated and wild plants, causing a number of other symptoms in addition to philodias, including dwarf leaves, broom cancellations, and vortexing (abnormal green pigmentation of parts of the plant that usually have pigments of other colors). In addition to phytoplasmas, phylodia can be caused by viruses, smut fungi, rust and oomycetes, so in each case to determine the treatment strategy requires targeted diagnosis of the pathogen.
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Spontaneous formation of leaf blades instead of flowers can also occur due to the complex action of environmental factors, which leads to impaired expression of genes responsible for flower formation (eg, whorl and RcPISTILLATA) (Yan et al. 2016), as well as phytohormone imbalance. An interesting fact is that such developmental disorders may have some decorative significance, as, for example, in the tea rose Rosa chinensis cv. Viridiflora, which is considered a spontaneous mutant (Yan et al. 2016). Adverse climatic conditions (prolonged drought, sharp fluctuations in day and night temperatures), mineral nutrition disorders (excess nitrogen, boron deficiency, etc.), oversaturation of the soil with fertilizers or herbicides can affect plants at certain stages of development critical to flower formation and lead to abnormal phenotype.
Diagnosis: macroscopic examination; microscopy of ultrathin sections of the phloem with signs of phytoplasma damage; and molecular genetic analysis to detect phytoplasmas, in particular using Multiplex Real-Time qPCR Assay, Loop mediated isothermal amplification (LAMP) in both affected plants and vector insects (Ikten et al., 2016) . Before choosing a treatment strategy, it is necessary to identify the pathogen and determine its nature: viral, bacterial, fungal and others, and determine the extent of exposure to abiotic stressors.
Treatment: Currently, phytopathologists and environmental scientists are working to develop effective approaches to the treatment of phyllodes of microbiological origin, as well as to study the species of insects that are carriers of phytoplasma. Tested methods are conducting the affected material through aseptic culture of plants to obtain clones of virus-free planting material, treatment with tetracycline antibiotics (the method is dangerous for the environment and due to the risk of tetracycline-resistant strains of mycoplasmas).
Prevention: 1) control of biotic factors (use of tested seed and genetically resistant varieties of cultivated plants; targeted search for philodiums by agronomists; timely destruction of weeds in fields, their edges and separate strips; tracking fluctuations in the population of vector insects; fields with sunflowers of plants that can be affected by philodias (for example, roses, clover, asters or strawberries), or regular removal of flowers with an abnormal phenotype).
2) control of abiotic factors (sufficient irrigation regime; selection of minimum effective concentrations of mineral fertilizers and herbicides that would not exceed acceptable norms and would not affect plant health; timely crop rotations and other approaches to optimize growing conditions for cultivated plants).
List of primary sources:
- Akulov O. 2020. Expert opinion: philodias on sunflower baskets. https://superagronom.com/blog/724-oleksandr-akulov-filodiyi-na-koshikah-sonyashnika
- Bidari, V.B., Sangappa, H.K. and Mahalinga, D.M., 1987. Phyllody—a new menace to the sunflower leading to sterility. Current Science, 56(18), pp.954-956. https://www.jstor.org/stable/24091368?seq=1
- Cagirgan, M.I., Mbaye, N., Silme, R.S., Ouedraogo, N. and Topuz, H., 2013. The impact of climate variability on occurrence of sesame phyllody and symptomatology of the disease in a Mediterranean environment. Turkish Journal of Field Crops, 18(1), pp.101-108. http://www.field-crops.org/assets/pdf/product51d3c5b42aeb6.pdf
- Ikten, C., Ustun, R., Catal, M., Yol, E. and Uzun, B., 2016. Multiplex real-time qPCR assay for simultaneous and sensitive detection of phytoplasmas in sesame plants and insect vectors. PloS one, 11(5), https://journals.plos.org/plosone/article?id=10.1371/journal.pone.0155891
- Meyer, V. G. (1966). Flower abnormality. Bot. Rev. 32, 165–195. doi: 10.1007/BF02858659
- Yan, H., Zhang, H., Wang, Q., Jian, H., Qiu, X., Baudino, S., Just, J., Raymond, O., Gu, L., Wang, J. and Bendahmane, M., 2016. The Rosa chinensis cv. Viridiflora phyllody phenotype is associated with misexpression of flower organ identity genes. Frontiers in plant science, 7, p.996. https://www.frontiersin.org/articles/10.3389/fpls.2016.00996/full