Southern Leaf Blight

Southern corn leaf blight is caused by the fungus Bipolaris maydis (Nisikado & Miyake) Shoemaker, teleomorph Cochliobolus heterostrophus (Drechs.)Drechs. This disease has many synonyms associated with both its common name and the scientific name of the causal organism. The imperfect stage of the fungus exists in two distinct races; Race O (old race) and Race T (virulent on corn containing Texas male sterile cytoplasm, Tm-sc). Both races produce phytotoxins. The phytotoxin produced by Race O is nonspecific to cytoplasm types and is produced in small amounts. Race T produces a phytotoxin that specifically affects corn containing Tm-sc. Common hosts of the fungus are corn, sorghum and teosinte. The fungus overwinters on crop debris, primarily on the soil surface, as mycelium, conidiospores and chlamydospores. Conidiospores are windblown or splashed by water to fresh plant tissue in the spring. The spores germinate on the leaf surface and infect the host directly and through stomata. Disease development is favored by warm ( 20-30^oC) moist weather and the presence of free moisture on the host tissue surface. The fungus is very prolific; able to complete an entire life cycle in 60-72 hours under favorable weather conditions.

Southern corn leaf blight is of great significance in the history of American agriculture because of its epidemic (or epiphytotic) proportions in 1970 and subsequent devastation of much of the corn crop that year. The monetary estimate attached to the epidemic was $1 billion.

Symptoms

Symptoms associated with this disease vary greatly depending on which race is present. Infections by both races begin on lower leaves and progress up the plant. Leaf symptoms associated with both races are similar on corn that has normal cytoplasm but vary greatly on corn containing Texas male sterile cytoplasm (Tm-sc) Race O is typically thought of as the less virulent of the two races. Symptoms that occur with this race vary somewhat in size and shape depending on the genetic makeup of the corn hybrid or inbred. The O race is primarily a leaf pathogen. Lesions typically elongated and vein-limited with dimensions ranging from 2-6 cm in width by 3-22 cm in length. Leaf spots are tan in color with a buff to brown border. Race T can infect both normal and Tm-sc corn. Symptoms on normal cytoplasm hybrids are small leaf spots that are few in number. The most dramatic symptoms are seen on corn containing Tm-sc. On these plants leaves, leaf sheaves, ear husks, kernels and stalks. Leaf lesions are elliptical to spindle shaped and may be up to several cm in length. The lesions are tan with a dark red to purple border. It is very common to have large blighted areas form when the lesions merge. The fungus also produces large numbers of dark spores in infected areas. Other infected plant tissues exhibit similar symptoms. Ear infections are characterized by black moldy growth (conidia) on the kernels and ear and cob rot can occur. Inner husks that come in contact with infected kernels are also black in color. Stalk injury may be present in two forms; fungal lesions and stalk rot that occurs when plants are predisposed to this condition by premature leaf loss.

Plant Health Management

Resistant Hybrids

Resistant crop material is readily available to both races of the pathogen and should be used as the primary means of controlling plant health.


Residue Management

The causal organism survives in plant debris and burial of this material has proven effective in some instances. However, there are parts of the country where the fungus can survive on buried debris so a rotation to a nonhost crop would be most effective. Crop rotation may also be more practical in a farming operation that is conservation tillage oriented.




Fungicide Application

There are fungicides labeled for the control of southern corn leaf blight and stalk rot. The list includes mancozeb and propiconazole, labeled for field corn, popcorn and sweet corn. Maneb and maneb plus zinc are also labeled for control on popcorn and sweet corn (1997). These fungicides do have restrictions regarding application timing and feeding residue to livestock. They may also be cost prohibitive in general production situations.

References

Schurtleff, M.C. (ed.) 1980. Compendium of Corn Diseases (2nd ed.) American Phytopathological Society, St Paul, MN, PP.15-16.


Ullstrup, A.J. 1971. Southern corn leaf blight and other corn leaf diseases. BP-5- 9,7pp. Cooperative Extension Service, Purdue Univ.


Ullstrup, A.J. 1972.The impact of the southern corn leaf blight epidemics of 1970-71. Annu Rev. Phytopathol. 10:37-50.

Useful Links

North Carolina State University

Purdue University

Caveat

Material contained on the Links from the page are the responsibility of the linked page's author(s).

This page was drafted by: Jane Christensen, Department of Plant Pathology, University of Nebraska-Lincoln

Disease images were provided by:

Dr. David Wysong, Department of Plant Pathology, University of Nebraska-Lincoln
and
Dr. John Watkins, Department of Plant Pathology, University of Nebraska-Lincoln

Questions, Comments, Complaints and Complements?

This page is authored and maintained by:
Dr. J.E. Partridge, Department of Plant Pathology, University of Nebraska-Lincoln

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Copyright (C) 2003 J.E. Partridge, University of Nebraska-Lincoln. All Rights Reserved.