February 2011. A message from the Global Crop Diversity Trust asked about a downy mildew (Peronospora belbahrii) disease outbreak in Hawaii: is it due to lack of diversity? An article appeared in the 3 February 2011 edition of the Staradvertiser.com www.staradvertiser.com/news/hawaiinews/20110203_Basil_bane_putting_bite_on_business.html
A member said that he had heard about the disease in the UK (it usually apears in late August or September). It occurred at two sites about 20 km apart at the same time, indicating that it might be seedborne. Downy mildews are a common problem in the UK because of the climate. One is a major threat to lettuce, and genetic resistance and a routine fungicide programme is normal for this crop. There is no known resistance in basil, but there are some approved fungicides for controlling it, for instance, fosetyl aluminium, propamocarb, metalaxyl M (a member from New Zealand suggested to use phosphorus acid (potassium phosphate) as its cheaper.
Another member mentioned downy mildew of tobacco, blue mold caused by P. hyoscyami. The control principles applied may be relevant for basil. These were:
- a crop free period from November to March with rogueing of any volunteer plants
- control in the seedbed with metalaxyl and
- field sprays with metalaxy
Once metalaxyl was introduced, the disease virtually disappeared from commercial tobacco crops in Australia. Note, some downy mildews can be seed-borne so the key challenges may be to ensure seed comes from disease-free crops and to control the disease in seedbeds.