HPIPM:Helminthosporium Leaf Blight FC

Author: Howard F. Schwartz, David H. Gent, and William M. Brown, Jr.

Taxonomy

Domain	Eukarya
Kingdom	Fungi
Phylum	Ascomycota
Subphylum	Pezizomycotina
Class	Dothideomycetes
Subclass	Pleosporomycetidae
Order	Pleosporales
Family	Pleosporaceae
Genus	Exserohilum

Scientific Name

Exserohilum turcicum

Scientific Name Synonyms

Bipolaris turcica

Helminthosporium inconspicuum

Helminthosporium turcicum

Setosphaeria turcica

Common Name

northern corn leaf blight

Identification and Life Cycle

Helminthosporium leaf blight is a general term for several diseases caused by several fungi formerly known as Helminthosporium spp. These diseases include Southern corn leaf blight (caused by Cochliobolus heterostrophus), Northern corn leaf blight (caused by Setosphaeria turcica), and Northern corn leaf spot (caused by Cochliobolus carbonum). Infection of susceptible varieties occurs when temperatures are moderate (64 to 80ºF) to warm (68 to 90ºF) and damp, humidity, weather prevails. Helminthosporium leaf blight pathogens survive between corn crops as spores (conidia) and mycelium in and on crop debris, but can also be transported long distances on wind currents.

Plant Response and Damage

Disease symptoms of Helminthosporium leaf blights can vary among inbreds and hybrids. Southern corn leaf blight symptoms include tan, elongated lesions between veins with light brown to brown borders. Northern corn leaf blight causes gray-green, elliptical or cigar-shaped lesion approximately 0.4 to 6 inches long. As lesions age they become tan with distinct dark zones. Northern corn leaf spot lesions are varied, but typically include oblong dark brown spots on leaves of maturing plants. Symptoms may also include narrow, grayish tan with light to dark border lesions on leaf blades, sheaths, and husks. Black felty mold on kernels can also occur with some strains of the Northern corn leaf spot fungus. Yield losses from these diseases are generally minimal in the High Plains region, but can be significant on susceptible inbred lines during weather conditions favorable for disease, especially when infection occurs before silking.

Biological Control

Practical biological control strategies have not been developed for Helminthosporium leaf blights.

Cultural Control

Conventional tillage and sanitation of crop residues in the fall along with crop rotation can reduce primary inoculum, and generally are sufficient in the High Plains region, but may not be adequate under disease favorable conditions. Resistant varieties are available and should be planted. Several races of the Helminthsporium leaf blight pathogens exist, so single-gene resistance may not be effective against all strains of each pathogen. Varieties with general resistance are available that do not completely control disease, but reduce the number and size of lesions.

Chemical Control

Fungicides applied when lesions first appear effectively control Northern corn leaf blight, but may only be cost effective on high-value seed-production fields or sweet corn. Fungicides may be most effective when combined with varieties with general resistance.

Product List for Helminthosporium Leaf Blight

Pesticide	Product/Acre	Interval	Preharvest Interval, remarks
Chlorothalonil	Varies	4-7 days	Various formulations available, including Bravo Ultrex, Bravo WeatherStick, Echo 720, and Echo Zn; 7-14 day PHI
EBDC	Varies	3-10 days	Various formulations available, including Manzate, Maneb, and Dithane; Include a nonionic surfactant to improve performance; 7-40 day PHI
Propiconazole and Propiconazole/Trifloxystrobin
PropiMax	4 oz	7-14 days	Maximum of 16 ounces per season; 30 day PHI
Stratego	10-12 oz	7-14 days	Maximum of 36 ounces or three applications per season; do not make more than 2 consecutive applications before rotating to a fungicide with a different mode of action; 30 day PHI
Tilt	2-4 oz	7-14 days	Maximum of 16 ounces per season; 30 day PHI
Azoxystrobin
Amistar	2-3 oz	7-14 days	Maximum of six applications per season; rotate with a fungicide with a different mode of action; 7 day PHI
Quadris	6.2 – 9.2 oz	7-14 days	Maximum of 2.88 quarts or six applications per season; do not make more than 2 sequential applications before rotating to a fungicide with a different mode of action; 7 day PHI

The information herein is supplied with the understanding that no discrimination is intended and that listing of commercial products, necessary to this guide, implies no endorsement by the authors or the Extension Services of Nebraska, Colorado, Wyoming or Montana. Criticism of products or equipment not listed is neither implied nor intended. Due to constantly changing labels, laws and regulations, the Extension Services can assume no liability for the suggested use of chemicals contained herein. Pesticides must be applied legally complying with all label directions and precautions on the pesticide container and any supplemental labeling and rules of state and federal pesticide regulatory agencies. State rules and regulations and special pesticide use allowances may vary from state to state: contact your State Department of Agriculture for the rules, regulations and allowances applicable in your state and locality.