Abstract
A description is provided for Didymella chrysanthemi. Information is included on the disease caused by the organism, its transmission, geographical distribution, and hosts. HOSTS: Chrysanthemum cinerariifolium, C. morifolium. Also by inoculation on Chrysanthemum carinatum, Cichorium endivia, Cynara scolymus, Dahlia variabilis, Helianthus annuus, Lactuca sativa, Rudbeckia hirta and Zinnia elegans (47, 1154). DISEASE: Generally referred to as ray blight of chrysanthemum (29, 215; 35, 878) and sometimes called black rot (43, 97) or Ascochytosis of chrysanthemum (48, 185). On blossoms usually infection is observed on the side but may later spread until all the florets are involved. The fungus causes tissue discolouration of the floret progressively upward from the receptacle and the affected petals turn light brown. In most cases the fungus grows down into the peduncle for several cms causing it to turn black and weaken and finally droop. The fungus frequently infects unopened buds and their peduncles, darkening the bracts and stems tissue. Leaf infection is common on plants with diseased flowers resulting in irregular blotches 2-3 cm wide. On stems frequently black girdling lesions several cms long appear usually starting at a node (Baker, Dimock & Davis, 1949). Ray blight of chrysanthemums is one of the serious diseases of cut flowers (37, 702) and occurs in commercial plantings both on greenhouse and outdoor chrysanthemums (39, 79, 585) causing severe lossess (29, 215; 36, 102). Because of the seriousness of the disease, countries like Germany (42, 446) and Norway (43, 2217) had to amend their Plant Inspection ordinance and adopt stringent quarantine regulations so as to prohibit the importation of chrysanthemum cuttings or plants without certification of their derivation from mother plants. Recently, a computer simulator programme Mycos based on weather data has been designed and tested showing that the simulator can determine qualitatively seasons of high and low disease incidence (57, 2551). GEOGRAPHICAL DISTRIBUTION: Africa, Asia, Australasia & Oceania, Europe and North America (CMI Map 406, ed. 2, 1973). New records not mapped are: Europe (Austria, Italy, Northern Ireland). TRANSMISSION: By ascospores, conidia, mycelia and sclerotia dispersed during wet periods by rain splash, air currents and by workmen on clothing, tools or hands (Baker, Dimock & Davis, 1949). Ascospore discharge has been reported to be regulated by environmental factors such as moisture and, depending on the isolate, by light at a constant air temp. of 20°C. Water sprinkling or heavy dew have been claimed to induce explosive disharge of ascospores. Max. number of ascospores have been trapped in an outbreak of D. chrysanthemi blight in field grown chrysanthemums just after an evening thunder shower, while much lower concentrations have been present during dew periods at night (53, 1412). Ascospores have been observed to be discharged for a mean horizontal distance of 3.1 mm with a max. of 6.2 mm, requiring initial velocities of discharge of 21 and 50 m/sec., respectively. The terminal velocity of the ascospores has been calculated to be 1 × 10-3m/sec. (53, 1413). The fungus overwinters as mycelia or pycnidia or as developing pseudothecia. Pseudothecia which mature in late summer and autumn have been claimed to provide much of the primary inoculum for infection of developing buds and flowers in the autumn and winter (29, 215). Sclerotia of D. chrysanthemi buried in soil subjected to normal glasshouse watering were found dead after 30 weeks and those in compost were not pathogenic after 8 weeks (45, 3066). The fungus has been claimed to survive by colonizing the root surface of chrysanthemum cuttings and still remain pathogenic to unrooted cuttings after 12 weeks as an epiphyte on the roots. Survival on the colonized roots of other plants has been claimed to be not > 8 weeks. Also it has been claimed that the fungus is commonly distributed on rooted cuttings without causing symptoms and therefore passing inspection by growers and quarantine officers (46, 341). The thick-walled mycelium has been reported to help the fungus to survive adverse conditions (48, 328h). Cultures of the fungus maintained by storing agar blocks from the originals in tubes with 10 ml. sterile distilled water either at 1°C or 10-12°C for 20-25 months have been claimed to remain viable without loss of virulence (41, 374).
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