Effects of UV-B radiation on growth, pigmentation, and spore production in the phytopathogenic fungus Alternaria solani

The effects of mild UV-B radiation doses on growth, pigmentation, and spore production in the leaf spot pathogen Alternaria solani Sorauer grown under a polychromatic (UV-A plus visible) radiation environment were investigated. UV-B radiation reduced significantly the radial growth of the fungus. It also had a negative effect on dry mass production and stimulated increased hyphal density. The highest values for UV-B absorbing compounds and melanin content (expressed on a dry mass basis) were observed in the treatment without UV-B. Low doses of UV-B radiation depressed UV-B absorbing capacity and melanin content with a gradual increase with increasing doses. However, the final attained values never surpassed those of the treatment without UV-B. Spore production was significantly enhanced by UV-A radiation. UV-B radiation had either a stimulatory or suppressive effect on spore production depending on the fluence rates of background UV-A radiation. The results indicate that A. solani employs mainly a morphological (i.e., increased density) rather than chemical (i.e., UV-B absorbing compounds) protective strategy against UV-B radiation damage. They also suggest that UV-B radiation reaching the surface of the earth may adversely affect the growth of this highly virulent fungus, although additional studies are required to test this hypothesis.