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''Diaporthe helianthi'' is a fungal pathogen that causes ''Phomopsis'' stem canker of sunflowers. In sunflowers, ''Phomopsis helianthi'' (teleomorph =''Diaporthe helianthi'') is the causative agent behind stem canker. Its primary symptom is the production of large canker lesions on the stems of sunflower plants. These lesions can eventually lead to lodging and plant death. This disease has been shown to be particularly devastating in southern and eastern regions of Europe, although it can also be found in the United States and Australia. While cultural control practices are the primary method of controlling for Stem Canker, there have been a few resistant cultivars developed in regions of Europe where the disease is most severe. == Host and symptoms == Stem canker on sunflower is caused by the fungal pathogen ''Phomopsis helianthi'' (teleomorph = ''Diaporthe helianthi'').〔Swartz, Howard F., and David H. Gent. ''Phomopsis'' Brown Stem Canker. N.p.: High Plains IPM Guide, 2005. Print.〕 There are many "Phomopsis" species that have a broad host range, but "Phomopsis helianthi" is the primary pathogen that infects sunflower, although it has also been seen to infect other, secondary hosts. Some "Phomopsis" species have been found to be able to cause disease on several host genera.〔Rehner, Stephen A., and Francis A. Uecker. "Nuclear Ribosomal Internal Transcribed Spacer Phylogeny and Host Diversity in the Coelomycete ''Phomopsis''." Canadian Journal of Botany, 1994, 72 (11), 1666-1674〕 Sunflower hosts that found in high densities are particularly susceptible to attack by the pathogen due to the microclimate formed in crowded areas. This microclimate has a high humidity and allows for infection to proceed rapidly.〔Thompson, S. M., Y. P. Tan, and R. G. Shivas. "Stem Cankers on Sunflower (''Helianthus Annuus'') in Australia Reveal a Complex of Pathogenic ''Diaporthe'' (''Phomopsis'') Species." Persoonia: 27 (2011): 80-89. Web.〕 There has also been recent molecular evidence to suggest there are several genetically distinct species of ''Phomopsis'' that can cause disease on the sunflower hosts.〔 Initial symptoms of ''P. helianthi'' infection can be found at the attachment of petiole to the stem on the lower portion of the plant.〔Herr, L. J. "''Diaporthe'' Stem Canker of Sunflower." Plant Disease 67.8 (1983): 911-13. Print. 〕 Initial infection will cause brown necrotic regions with a chlorotic border. Eventually these regions will grow into larger, brown to tan lesions and eventually into large cankers. Cankers are found on the stem and will have a sunken border. Cankers can be long, up to 6 inches, but do not appear until flowering.〔 Other early symptoms can include necrosis and bronzing of intervenial areas of leaves at or above the stem lesion. Wilting and death of leaves above the canker also occurs.〔 Leaves above the site of inoculation can also show symptoms of being water soaked. This is sometimes accompanied by leave distortion.〔 ''Phomopsis'' stem canker is also characterized by extensive pith degradation to the point where the stalk is easily crushed under moderate thumb pressure. Wilting is also a typical symptom, and extensive wilting can eventually lead to lodging later in the season.〔 ''Phomopsis'' also produces mycotoxins, which can cause additional symptoms such as premature ripening or early plant death.〔 Disease identification is facilitated by observation of fruiting structures. Perithecia are found on cortical tissues and will produce globular to sub-cylindrical asci with two-celled ascospores. Interestingly, it has been observed that isolates from stem canker in the United States will readily form perithecia when grown in culture, but those from Europe will not. This suggests that there may be several species or biotypes of ''Phomopsis'' that cause stem canker worldwide. Pycnidia, the asexual fruiting bodies, can be detected on the stem and leaf lesions throughout the disease cycle. They are embedded in the tissue, dark brown and globular. The conidia come in two types, but the most common is beta conidia that are threadlike, hyaline and can be curled or straight.〔Kong, Gary, comp. Diagnostic Methods for ''Phomopsis'' Stem Canker ''Phomopsis Helianthi''. N.p.: Plant Health Australia, 2013. Online.〕 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Diaporthe helianthi」の詳細全文を読む スポンサード リンク
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