Late Blight of Potatoes

Potato plant with late blight.
Photo courtesy W. E. Fry

Causal Agent
Phytophthora infestans

Class of Pathogen
Fungus

Order of Fungal Pathogen
Oomycota

Host Plant
Potato, Tomato

Geographical Distribution
Late blight disease is found in nearly all areas of the world where potatoes and tomatoes are grown.  As learned through the Irish Potato Famine, late blight is most commonly found in areas of cool, moist climate.  Total destruction of a field may occur within a week or two if favorable weather patterns persist.  Also, potatoes harvested may become infected and rot while in storage.

Symptoms
The first noticeable symptom of late blight comes as water-soaked spots form, usually at the edges of the lower leaves.  Chlorotic borders may also be present during periods of moist weather.  White fungal growth 3 to 5 mm wide will appear on the underside of leaf lesions.  Infected leaves soon become limp and die. 

Tubers infected will show purple/brown areas of flesh exteding 5 to 15 mm into the potato.  Later on, these areas may dry and become firm and sunken in.  This rotting of the potato continues after the tubers are harvested and become prone to other fungi and bacteria.  Rotting potatoes will have an offensive odor. 

Infected tubers have a purplish/brown color.
Photo courtesy of http://www.apsnet.org/

Dissemination and Disease Cycle 
The pathogen Phytophthora infestans overwinters as mycelium in infected potato tubers, debris, and soil.  Reproduction occurs both sexually and asexually.  However, sexual reproduction in nature is extremely rare.  In asexual reproduction, come spring, sporangia are produced.  Sporaniga are carried to new plants through both air and water.  If cold conditions persist, sporangia will germinate indirectly through zoospores.  If warmer temperatures persist, germination will occur via germ tubes.  These sporangia and zoospores infect potato leaves.  After infection, sporangium may continue to be produced by sporangiophores, which may continue to infect other portions of the plant.  As the disease develops, older lesions continue to enlarge as new ones develop. 

The second phase of late blight occurs with the infection of the actual tubers.  As sporangia are washed down from the leaves they carry through the soil and infect the tubers.  Zoospores germinate and penetrate the tubers through either scarred tuber tissue or lenticels.  It is common for tubers to rot in the ground and in storage.

For disease cycle of late blight of potato, CLICK HERE.

Control Measures
Late blight of potatoes can be controlled successfully through a combination of various measures.  No cultivars are immune to late blight, and cultivars with greater resistance should be planted.  Destruction of volunteer plants should be conducted as well, since such plants may be a source of disease.  New varieties may be resistant to vine infection, but tuber infection is still a possibility. 

Moderate disease control is often obtained through cultural practices.  Selecting a site with adequate drainage and sufficient air movement will help reduce moist conditions.  Rotation of crops will help to minimize pathogen levels.  Along with crop rotation, residue management will help to keep pathogen levels low.

Chemical options exist for the control of late blight as well.  Materials include mancozeb, metalaxyl, chlorothalonil, and copper materials similar to Bordeaux mixture. Chemical control helps prevent tuber losses.  However, it is still necessary to combine chemical control with cultural control methods. 

References
Agrios, George N.  Plant Pathology Fourth Ed. Academic Press 1997. pp 275-278.

http://www.bcc.orst.edu/lateblight/   Accessed 2/10/01

Related Journal Article
Powelson, M. L., and Inglis, D. A. 1999.  Foliar fungicides as protective seed piece treatments for management of late blight of potatoes.  Plant Dis.  83:265-268

Summary
 Planting seed tubers contaminated with Phytophthora infestans is one of the causing factors of late blight epidemics.  Thus, it is important to insure that seeds are free of tuberborne pathogens.  The research conducted by Powelson and Inglis looked at several treatments that possessed Section 18 (Emergency Use) labels.  Under greenhouse conditions three Section 18 products were studied; Acrobat MZ, Curzate M-8, and Tattoo C.  Of the three studied, Curzate M-8 was frequently the most effective.  Overall, controll of tuber borne inoculum of P. infestans with approriate seed piece treatments resulted in an increase in plant emergence and improved crop uniformity.  Effective control of P. infestans was compounded when use of resistant cultivars was coupled with the use of fungicide sprays.