EFFECT OF CONIDIA PRODUCTION TEMPERATURE ON GERMINATION AND INFECTIVITY OF ALTERNARIA-HELIANTHI

Conidia of an isolate of Alternaria helianthi produced under different temperatures were studied for growth and infectivity on hosts and nonhosts. Infective conidia (5 x 10(4) conidia per mi) caused blight disease on 2- to 3-week-old hosts, including sunflower (Helianthus annuus), safflower (Carthamus tinctorius), and common cocklebur (Xanthium strumarism), within 24 h of treatment. Disease symptoms included necrosis, stunting, wilting, and mortality of susceptible species. The fungus grew well from 18 to 30 degrees C, but growth was more rapid at 28 and 30 degrees C. More conidia were produced at 18 to 26 degrees C than at 28 or 30 degrees C. Infectivity decreased as conidia production temperature increased. The conidia produced at 28 and 30 degrees C 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 22 degrees C germinated more rapidly than did conidia produced at 26 to 30 degrees C. 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 28 degrees C 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.