Light Suppresses Sporulation and Epidemics of Peronospora belbahrii

Abstract

Peronospora belbahrii is a biotrophic oomycete attacking sweet basil. It propagates asexually by producing spores on dichotomously branched sporophores emerging from leaf stomata. Sporulation occurs when infected plants are incubated for at least 7.5h in the dark in moisture-saturated atmosphere at 10-27°C. Exposure to light suppresses spore formation but allows sporophores to emerge from stomata. Incandescent or CW fluorescent light of 3.5 or 6 µmoles.m2.s-1 respectively, caused 100% inhibition of spore formation on lower leaf surface even when only the upper leaf surface was exposed to light. The inhibitory effect of light failed to translocate from an illuminated part of a leaf to a shaded part of the same leaf. Inhibition of sporulation by light was temperature-dependent. Light was fully inhibitory at 15-27°C but not at 10°C, suggesting that enzyme(s) activity and/or photoreceptor protein re-arrangement induced by light occur at ≥15°C. DCMU or paraquat could not abolish light inhibition, indicating that photosystem I and photosystem II are not involved. Narrow band led illumination showed that red light (λmax 625 nm) was most inhibitory and blue light (λmax 440 nm) was least inhibitory, suggesting that inhibition in P. belbahrii, unlike other oomycetes, operates via a red light photoreceptor. Nocturnal illumination of basil in the field (4-10 µmoles.m2.s-1 from 7pm to 7am) suppressed sporulation of P. belbahrii and reduced epidemics of downy mildew, thus reducing the need for fungicide applications. This is the first report on red light inhibition of sporulation in oomycetes and on the practical application of light for disease control in the field.

Morphological characterization and optimization of conditions for conidial production of Elsinoë ampelina, the causal organism of grapevine anthracnose

“…The relationship between light and sporulation varies with fungal species. While some fungi require exposure to light for sporulation (Trione & Leach, ), other fungi sporulate better in the dark (Cohen, Vaknin, Ben‐Naim, & Rubin, ), as observed for E. ampelina in our study. A main reason for better sporulation in the dark is that most Elsinoë fungal species can produce elsinochromes (Weiss, Merlini, & Nasini, ).…”

Red Light Increases Suppression of Downy Mildew in Basil by Chemical and Organic Products

“…A pre-inoculation application of Actigard was found to be effective in reducing severity of basil downy mildew (Mersha et al 2012). Peronospora belbahrii produces sporangia on sporangiophores under dark and humid conditions (Cohen et al 2013). Our greenhouse experiments in this study documented that basil plants treated with Actigard or Organocide and maintained under red light during night had significantly reduced disease severity and sporangia production compared to basil plants treated with same fungicide treatments but maintained in dark overnight.…”

“…Cohen et al (2013) reported that the effect of light was restricted to the exposed area of the leaves. Cohen et al (2013) reported that the effect of light was restricted to the exposed area of the leaves.…”

“…None of the organic fungicides have been sufficiently proven effective for control of basil downy mildew (Allen 2013;McGrath 2015;Patel et al 2013). Recently, research conducted in Israel has shown that basil plants treated with light provided with incandescent lamps that supplied 3.5 lmol photons/m 2 /s or cool white fluorescent lamps that supplied 6 lmol photons/m 2 /s completely inhibited sporangia production (Cohen et al 2013). It was reported that red light at 625 nm was most inhibitory and blue light at 440 nm was least effective in sporangia formation of P. belbahrii.…”

Characterisation and risk assessment of the emerging Peronospora disease on Aquilegia

“…Initial infections might be controlled by prompt removal of infected plants. Cool damp conditions favour rapid onset of disease symptoms and light levels may also be of importance, as with other downy mildew species (Cohen et al 2013). Deeper knowledge of the environmental conditions that favour Aquilegia downy mildew progression may facilitate an integrated management approach to this disease in a garden situation.…”