The Role of Solar Radiation, Especially Ultraviolet, in the Mortality of Fungal Spores

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Abstract
Spores of Peronospora tabacina, Uromyces phaseoli and Alternaria solani were exposed in the field either to full sunlight or were partially protected from the sun by a leaf, cheesecloth or an UV filter. The fungicidal effect of long wavelength (above 290 nm) UV radiation on mortality increased from A. solani to U. phaseoli to P. tabacina. Sporangia of P. tabacina survived better when attached to sporangiophores than when detached. Temperature was an important factor in mortality of detached, but not of attached, sporangia of P. tabacina. Spores of A. solani were killed after 5 days of exposure to a total of about 5.5 MJ .cntdot. m-2 of long wavelength UV radiation. Their mortality was not affected by temperature. Spores of U. phaseoli were more susceptible to UV than those of A. solani and their mortality was also affected by temperature. In both cases, more spores survived in samples shaded by cheesecloth, by a leaf, or protected by UV filters than in samples exposed directly to sunlight. The fact that the temperature in samples under the UV filters was higher than in samples exposed directly to sunlight, indicates that UV radiation rather than increased temperature was the main factor causing mortality. Compared to data published on survival of spores of P. tabacina, U. phaseoli and A. solani in the laboratory, exposure to sunlight shortened their longevity by 6 to 30 times. Solar radiation in general, and its UV portion in particular, is a major factor in mortality. Doses of long wavelength UV needed to kill spores in the open were about 103 times greater than the dosage of short wavelength (254 nm) UV radiation required to kill them in the laboratory.