Late Blight

• The Irish potato blight was caused by a fungal pathogen called Phytophthora infestans. It originated in central Mexico and caused an epidemic in the U.S. in the early 1840s before spreading around the world. P. infestans reproduces in living plant tissue, especially when moisture is high and temperatures are moderate. The spores are then transmitted to healthy plants through wind, rain splash and aphid infestations. Because late blight doesn't show up until late in the season, the existence of spores may be hidden. But when those potatoes are planted the following year, they already carry the spores.
  
  

Symptoms of Late Blight

• After a period of cool, wet weather, you may see lesions on the leaves and stems that resemble dark, water-soaked spots. In time they expand until the entire leaf or branch dies. You may not notice that the disease has taken up residence in the tubers until they have been stored. At that time it will show as a cork-like rot.
  
  

A New Threat

• Recent strains of late blight have proven to be more aggressive and more severe than their predecessors. The one blessing was the P. infestans wasn't able to overwinter and couldn't survive outside living tissue. It is now known that when two particular types of pathogen mate, they produce spores capable of overwintering and surviving for several years without a host. It has already happened in Europe and Scandinavia and, as of 2010, both types exist in Florida.
  
  

Seed Potatoes

• Seed potatoes are tubers that are commercially grown from seed. Certified seed potatoes undergo a process of regular inspection and testing to ensure that they are free of P. infestans and other viral and bacterial pathogens. Planting regular potatoes is risky business because you can never be sure they do not harbor the spores that may destroy the entire crop. Only by planting certified seed potatoes can you be confident that you are starting with disease-free tubers.
  
  

Disease Prevention

• If you choose to start potatoes from your own tubers, watch for signs of late blight. As soon as you see anything resembling the symptoms of blight, remove the affected parts. Work around the potatoes only when the foliage is dry, and disinfect any tools you use. If the disease progresses, remove all affected plants, placing them in a plastic bag. Set the bag in full sun for several days to kill the spores before throwing the bag in the trash. Keep an eye on your tomato plants, as they are susceptible to late blight as well.