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Background on Late Blight	General Disease Symptoms
Reproduction	New Challenges of Late Blight
Researchers Working on Late Blight

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Background on Late Blight

 

General Disease Symptoms  

Late blight disease spreads very rapidly under moist humid weather especially when the days are warm with cool nights.  Foliage infected with P.  infestans initially consists of purple black or brownish black water-soaked lesions at the margins of the leaf and later on appearing anywhere on the leaf, petiole, and stem, thus killing the plant.  (Image at right:  Diseased potato leaves;  image courtesy of  Dr. Deahl.)  Under cool, moist conditions, whitish mycelia and sporangia appear on the underside of the leaf.  The tubers are affected later in the season.  The early stages show slightly brown or purple blotches on the skin.   As the disease progresses, tuber infection is followed by secondary invasion by bacteria and fungi.

Dissemination of infection is by airborne sporangia and water splash of sporangia from infected leaves.  Infection can be a result of inoculum dispersal from distant infected fields or from infected seed pieces. (Image at left:  A diseased tuber;  lab photo.  Image at bottom left: Potato field devestated by late blight;  image courtesy of Dr. Deahl.) 

Reproduction

P. infestans reproduces either by asexual or sexual means.  Asexual reproduction  occurs when a sporangium germinates by releasing zoospores (indirect) or by the formation a of germ tube (direct).  (Image at right: A Phytophthora sporangia releasing a zoospore;  lab photo.)

P. infestans is heterothallic which means that sexual reproduction occur when the two mating types (called A1 and A2) are paired forming oospores.   Oospores provide the means for long-term survival due to its resistant structure (thick-walled).  (Image at right:  Oospores in liquid culture;  lab photo.)  Oospores are often difficult to germinate and therefore can persist in the soil and serve as both inoculum and as a source of pathological variability due to sexual recombination.

New Challenges of Late Blight

     Despite various improvements in disease management strategies, late blight continues to cause annual losses in crop yield and quality.   Scientists have generally believed that the A1 mating type was distributed worldwide whereas the A2 mating type was confined to Mexico by means of a strict quarantine effort.  However, in the early 1980’s, some areas in Europe, the United States and Canada confirmed the presence of the A2 mating type.  The introduction of these new strains from central Mexico into Europe and North America is believed to have resulted in increased disease problems because it provided an avenue for the development of new, more virulent, and aggressive forms of the pathogen that are resistant to effective systemic fungicides.  The dissemination of  the new P. infestans strains worldwide becomes a serious threat to the control of late blight and once again focuses the world’s attention on this disease.  Here in the Vegetable Laboratory, late blight research focuses on the origin of these new strains as well as the molecular basis of the pathogenicity of the organism.

 

The following scientists in the Vegetable Lab conduct research on late blight:

Dr. Kenneth Deahl

Dr. Kathleen Haynes

Dr. Richard Jones

Dr. Leslie A. Wanner