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.
Highlights
Downy mildew, caused by the biotrophic oomycete Peronospora belbahrii Thines [1,2], has recently become a major disease of sweet basil (Ocimum basilicum L) in many countries [3,4,5,6,7,8,9,10,11].It was first observed in Northern Israel in November 2011
Mersha et al [14] reported that acibenzolar-S-methyl (ASM, Actigard) and DL-3-aminobutyric acid (BABA) were effective in controlling the disease under greenhouse conditions in Florida
Because of the strict regulations imposed by the Ministry of Agriculture on applications of chemicals to basil for the control of downy mildew, we looked for alternative methods to combat this disease
Summary
Downy mildew, caused by the biotrophic oomycete Peronospora belbahrii Thines [1,2], has recently become a major disease of sweet basil (Ocimum basilicum L) in many countries [3,4,5,6,7,8,9,10,11]. It was first observed in Northern Israel in November 2011. In Israel, two field experiments showed that the best performing fungicides, against a mefenoxam-sensitive isolate, are (in order) mefenoxam, mandipropamid, dimethomormph, and azoxystrobin (Cohen, Ben-Naim and Vaknin, unpublished data)
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