Effects of ultraviolet radiation, simulated or as natural sunlight, on conidium germination and appressorium formation by fungi with potential as mycoherbistats

Solar radiation, particularly its UV wavelengths, greatly influences conidium survival and this study looked at the impact of radiation and its interactions with temperature on Plectosporium alismatis and Colletotrichum orbiculare, two fungi that are potential mycoherbistats. UV radiation, rather than temperature, was found to be the primary cause of conidium mortality; however, there were interactions between these factors leading to the enhancement of the lethal effects of UVB on conidium germination at high temperatures. C. orbiculare was more sensitive than P. alismatis with conidium germination being halved by UVB doses of 1.47 and 13.1 kJ m-2, respectively, for the two pathogens. Conidium mortality was dose-dependent and for P. alismatis exposed to a dose of 3.7 kJ m-2 reciprocity was observed. However, for C. orbiculare equivalent doses were not reciprocal as higher doses for short periods were more lethal than lower doses of longer duration. Low UVB doses only caused delays in conidium germination, whereas higher doses killed conidia and caused delays in the germination of any survivors. Radiation also affected appressorium formation. Appressorium formation was stimulated by UVA and was dose dependent with P. alismatis requiring a higher dose than C. orbiculare to initiate formation. Microcycle conidiation by P. alismatis was observed following exposure to sunlight. This knowledge of how conidia of these potential mycoherbistats react to climate suggests that rapid conidium germination and appressorium formation could be achieved by manipulation of the time at which they are applied in the field. Conidia could be applied so that they receive sufficient UVA to stimulate appressorium formation but without receiving a dose that would significantly affect conidium germination. However, for this, additional protection from UVB may be needed.