Posted: 07/10/2014 at 07:59
It looks like late blight (Phytopthora infestans), here is some info on it
Applying fungicides. Protectant fungicides are most effective if applied more frequently at low labeled dosages than less frequently at high dosages. This is partly because more frequent applications ensure better coverage.
Treating a crop exposed to inoculum. Fungicides that have systemic activity (penetrate into plant tissues) are necessary if a crop has been exposed to sporangia within the last 24 hours. Even if the first infections occurred more than 24 hours earlier, if lesions are visible in the crop and a systemic has not been applied, an effective systemic will probably provide some benefit that is not possible from a protectant. Protectant fungicides ie Bordeaux mix (those that are not systemic and cannot penetrate tissue) are ineffective against the pathogen once it has penetrated the cuticle (sometimes within two hours of germination). Thus applications of a protectant fungicide will have no visible effect on disease suppression until six to nine days after application because it takes that long for lesions to be easily visible. Unfortunately, even systemic fungicides do not suppress all infections and will have little effect on infections that are more than 24 to 48 hours old. Effects from systemic fungicides may be visible within three to four days.
Treating "hotspots." A hotspot is a group of infected plants located amid relatively healthy ones. If very little disease is present in the crop and there are only a few hotspots, the latter should be destroyed as quickly as possible Plants immediately surrounding the hotspot should also be destroyed because they are very likely infected even though the infections are not yet visible. If fungicides are being used, the remainder of the held should be treated with a fungicide that has some systemic activity, and subsequently, applications of a protectant fungicide should be applied on a tight (frequent) schedule.
Treating established infections. Once 5 to 10 percent of the foliage is infected it is usually not possible to halt the development or progress of the disease.