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Effect of Conidia Production Temperature on Germination and Infectivity of Alternaria helianthi. Hamed K. Abbas, USDA Agricultural Research Service, Southern Weed Science Laboratory, P.O. Box 350, Stoneville, MS 38776; Grant H. Egley, and Rex N. Paul. USDA Agricultural Research Service, Southern Weed Science Laboratory, P.O. Box 350, Stoneville, MS 38776. Phytopathology 85:677-682. Accepted for publication 3 March 1995. This article is in the public domain and not copyrightable. It may be freely reprinted with customary crediting of the source. The American Phytopathological Society, 1995. DOI: 10.1094/Phyto-85-677.

Conidia of an isolate of Alternaria helianthi produced under different temperatures were studied for growth and infectivity on hosts and non-hosts. Infective conidia (5 104 conidia per ml) caused blight disease on 2- to 3-week-old hosts, including sunflower (Helianthus annuus), safflower (Carthamus tinctorius), and common cocklebur (Xanthium strumarium), within 24 h of treatment. Disease symptoms included necrosis, stunting, wilting, and mortality of susceptible species. The fungus grew well from 18 to 30C, but growth was more rapid at 28 and 30C. More conidia were produced at 18 to 26C than at 28 or 30C. Infectivity decreased as conidia production temperature increased. The conidia produced at 28 and 30C were noninfective on their hosts. Infectivity was greatest when conidia had thick cell walls, high percent germination, and high number of germ tubes. Conidia produced at 18 to 22C germinated more rapidly than did conidia produced at 26 to 30C. Regardless of production temperature, germination was greater on host plants than on nonhost plants or filter paper. Histochemical studies showed that conidia produced at lower temperatures stained lightly for lipids and proteins and intensely for polysaccharides. Conidia produced at 28C were either empty or had gutuoles of lipids and only traces of proteins or polysaccharides. The conclusion drawn was that cellular degeneration at the higher temperatures was responsible for the reduced germination and infectivity.

Additional keywords: Alternaria leaf blight, biological control, mycoherbicide.