Influence of phototropic response of spore germ tubes on infection process in Colletotrichum lagenarium and Bipolaris oryzae

The germ tubas of Colletotrichum lagenarium showed negative phototropism to UV-blue (300–520 nm) and far-red (> 700 nm) regions with maximum in the near ultraviolet (NUV) region, while monochromatic radiations of 575–700 nm (yellow-red region) induced positive phototropism with maximum in the red region. Green light (520–575 nm) was ineffective. Negative phototropism-inducing wavelength regions inhibited germ tube growth and positive phototropisminducing wavelength regions promoted it significantly. Bipolaris oryzae did not show any phototropic response and light did not affect the germ tube growth. These results indicate that the lens effect, in combination with the light growth reaction and light growth inhibition, is the mechanism of the phototropism of germ tubes of C. lagenarium. NUV radiation, which induced negative phototropism of C. lagenarium, promoted appressorium formation, while red light, which induced positive phototropism, suppressed it significantly. In the case of B, oryzae, light did not affect the infection structure formation. These results indicate that negative phototropism of germ tubes of C. lagenarium favors the infection process by facilitating the contact of the tips of germ tubes with the host surface, while positive phototropism has the opposite effect.