Disease Distinction
Late blight was first identified in the United States in 1842, but is most famous for its role in the Irish potato famine of the mid 19th century. Since that time it has been considered the most destructive disease to affect agricultural non-cereal crops. Late blight is caused by Phytophthora infestans, an organism which resembles a fungus but is closer, genetically, to brown algae. While the disease most commonly develops late in the season, it is also able to cause significant damage early or mid growing season as well and is dangerous anywhere potatoes grow. Tomato plants are also susceptible.
Moisture is an important factor in the development of late blight, zoospores; the asexual, motile spores produced by fungi, bacteria and algae, require water for one stage of their life cycle. Free water on the leaf surface allows the zoospores their brief, but necessary, swim prior to invading leaf tissue. Water can also assist with dispersal of sporangia or zoospores, although aerial transit will also get the job done. Temperature is also critical in pathogen development and spread, at temperatures less than 59 degrees fahrenheit disease development will be slowed, temperatures of 77 degrees or above also slow development due to the increase in evaporation, optimal temperature for disease growth is between 64 and 71 degrees.
White sporangiophores underneath the leaf surface can be indicative of a Phytophthora infestans infection. This pathogen reproduces predominantly asexually, forming the sporangia on host tissue which will either germinate directly and infect the host or zoospores will be released to spreading disease to additional plants. Sporangia can travel long distance via aerial or hydro transit, but the disease has been known to move even farther due to infected tomato and potato transplants. Mycellium of the pathogen can overwinter in tubers or culled plants and can play a major role in the development of late blight on subsequent crops.
Potatoes affected with late blight develop reddish, brownish to blackish lesions.
Wet, blackish lesions occur on leaves infected with Phytophthora infestans, and white sporulation can be visible on leaf margins mostly on the underside.
Late blight symptoms on tomatoes include; circular, greasy looking lesions appear on skin, while fruit may remain firm eventually leathery brown spots pervade.
Symptoms of late blight on tomatoes leaves and stems are similar to those seen on potatoes; irregular shaped, wet looking lesions are visible on young leaves near the top of the plant, if conditions remain moist lesions brown and sporulation will occur. Leaves eventually become necrotic, shrivel and plants die.
As of this last week sightings of late blight have been reported on potato and tomato plants in Franklin County and Hampshire County in Massachusetts, Long Island and in southern Maine. The late blight found on tomatoes and potatoes in Massachusetts this year is more virulent and aggressive than in the past. Growers and gardeners should monitor plants closely and report suspicious lesions to their arborists or local agricultural extensions for help with identification and control.
