Warning and Advisory Service for the Control of Potato Late Blight Phytophthora infestans: PhytoPRE
Potato late blight is caused by the pathogen Phytophthora infestans. Primary infestation can be detected in the spring on individual plants scattered throughout the crop, and can usually be traced back to infested planting material, although potato-waste dumps and second growth are also possible sources.
Further spread to healthy plants (secondary infestation) from primary outbreaks occurs mainly downwind. Infected plants can normally be recognised by the browning of the base of the stem, or of individual parts of the stem. The yellowish-green patches on the lower leaves soon change to chocolate-brown.
Mostly forming from the leaf edge or leaf tip, these patches quickly increase in size in damp weather. In the early hours of the morning, a greyish-white layer of fungus can be detected on the underside of the leaf, at the light-green zone of transition from diseased to healthy tissue. The Phytophthora infestans pathogen can also occur at shoot tips, stalks and petioles. With susceptible varieties, an infested potato crop can completely fail within a few days in humid and warm weather.
The cultivation of largely immune varieties, the use of healthy seed and the control of second-growth potatoes from the previous year in the successive crops are the most effective preventive measures.