Studies On Aternaria Crassa (SACC) Rands As A Possible Myooherbicide On Datura Stramonium

Abstract

The use of Alternaria crassa (Sacc.) Rands as a control agent of Datura stramonium L. has been studied in this work. A. ci'assa was isolated from infected seeds and leaves of D. stramonium asceptically on water agar (WA) at 25°C. Cultural studies were done to determine the best media, optimum temperature, light and pH conditions necessary for maximum growth and sporulation of the fungus. The fungus grew fast and sporulated best at 29°C, alternating light (8hr)/darkness and at a pH of 6.8. Media did not affect the rate of growth of the fungus, except host leaf decoction agar (HLDA), but influenced its sporulation. Four different media were tested and the highest sporulation was obtained on HLDA followed by potato dextrose agar (PDA), cornmeal agar (CMA) and

vegetable soup agar (VSA). For mass production of inoculum (spores and mycelial fragments) liquid medium was found to be more appropriate since large amounts of spores and mycelium could be harvested. Also more than one harvest could be obtained. Serial microtome sections of the inoculated leaves showed that the germinating conidia penetrated the leaf

in three ways: (i) by forming an appressorium followed by an infection peg which punctured the epidermal cells (ii) by forming an infection [Deg which penetrated the leaf between the guard cell and the epidermal cell (iii) through the open stomata. Development of the fungus in the host tissue was the same irrespective of the mode of penetration. The fungus ramified through the host tissue both inter- and intracellularly, destroying the host cells and forming necrotic spots within seven days of inoculation. Spore concentration, plant age and incubation period at 1007. R.H affected infection. Thirty-three day old plants inoculated with an inoculum concentration of 10,000 conidia/ml and kept at 1007. R.H for three days, had the highest infection. In the field, however, it was necessary to inoculate twice in order to avoid escapes. Within 21 days, 75.67. of the plants had died. Among the control only 0.87. died. The fungus killed the target plant but did not spread to the other crops in the garden.

The ease in sporulation of /A. crassa within four days under inexpensive conditions, the ability of the fungus to penetrate D. stramonium leaves and form necrotic spots within seven days (the spots coalesced to form large necrotic areas within nine days leading to premature defoliation), the infection of D. stramonium leaves at any age by the fungus and the host specificity of the fungus indicated the high potentiality of A. crassa as a mycoherbicide on D. stramonium.