Estimates indicate the value of sweet basil (Ocimum basilicum) grown in the UK to be in the order of tens of millions of UK sterling. Sweet basil crops under protection and outdoors are now subject to infection by basil downy mildew, caused by the oomycete pathogen Peronospora belbahrii. The disease was first reported in the UK during the summer of 2010 and given quarantine status, with infected crops being subject to statutory action. The status was lifted in 2012. AHDB-funded project PE 024 aims to improve knowledge and control of this disease.
Work to date suggests that where the downy mildew pathogen is detected in seed, it is carried internally rather than as an external contaminant. Agastache, lavender, sage and catnip can all act as alternate hosts for P. belbahrii so growers could consider growing these separately from basil. Weed species tested were not found to be alternate hosts.
Incubation in the light for 18 hours after inoculation resulted in no plants developing downy mildew symptoms, yet more than 50 per cent incubated in the dark developed the characteristic yellow discoloured areas on the upper leaf surface and brown downy sporulation on the underside of leaves. Infections are therefore most likely to occur overnight. Infection occurred when plants were incubated at temperatures of 5-25 degsC and required a minimum of four hours leaf wetness duration.
An outdoor fungicide efficacy trial, on a crop with low disease pressure, showed that a programme with Revus (mandipropamid) and a coded product HDC F226 was most effective. Other effective programmes were Fubol Gold (metalaxyl-M + mancozeb)/Revus or Fenomenal (fenamidone + fosetyl-aluminium)/Revus.