Late July/August is the time to be extra vigilant, especially when checking potatoes and tomatoes.
If the weather's mild, wet and humid, these are ideal conditions for a deadly attack of phytophthora infestans (late blight) for which there is no chemical cure.
The pathogen's carried on the wind and first indications that your plants have been attacked are dark patches on leaves with a white powdery mould - particularly underneath.
If left unchecked, the plant will be turned into a rotting mess and within a day or two it will have spread through the crop.
Once spotted on potatoes, the remedy is to cut all top growth to ground level (do not compost) and dispose of in the Council waste bin to prevent the spores being washed into the crop, which if caught quickly will still be edible.
For indoor tomatoes good ventilation is essential. Affected plant leaves must be disposed of as with potato tops.
At the end of the season it should be safe to add old stems and fruit to the compost as the blight will only survive on living material.
