Maize disease Huang Jiang Hua 31 March, 2008

Northern Corn Leaf Blight Northern leaf blight has traditionally been one of the most damaging corn leaf diseases in Ontario. Use of resistant hybrids has limited yield losses from this disease in commercial corn, however, significant losses continue to occur in seed corn production when highly susceptible corn inbreds are planted.

Symptoms Symptoms develop on lower leaves first and progress up the plant under favorable weather conditions ( temperatures and high relative humidity). Several types of lesions may occur on leaves and husks. The type of lesion present is dependent on host resistance genes.

Symptoms The typical symptoms seen on a susceptible host are long elliptical spots up to 15 cm in length.. Spots are grayish-green to tan in color. Spore produced in the lesions are olive-green to black and may be produced in concentric rings giving the spot a target like appearance.

Symptoms Spores from the primary lesions reinfect the host producing secondary cycles of the disease. Lesions produced on hybrids with polygenic (quantitative) resistance are long and narrow resembling those of Stewart's wilt. These lesions may extend the entire length of the leaf.

Symptoms Fewer lesions are produced on these hybrids and their size, in term of surface area affected, is less than on susceptible hybrids. Lesions produced on hybrids with monogenic resistance are characterized as small necrotic spots that are surrounded by a chlorotic halo. Spore is greatly reduced or absent in these lesions.

Pathogen is caused by the fungus Exserohilum turcicum, teleomorph Setosphaeria turcica. Both the common name and causal organism have several synonyms.

Host range and races of pathogen Host of E. turcicum include corn, sorghum, Sudan grass, Johnson grass, gamagrass and teosinte. Exserohilum turcicum is divided into 3 races and infection of hosts from different genera and species is dependent on the race. In addition, two biotypes have been identified from maize.

Pathogen The fungus overwinters as mycelium, conidia and chlamydospores in and on plant debris. Spores are windblown or splashed by rain from plant debris to the current season crop.

Pathogen Conidiospores of Exserohilum turcicum have a slightly protruding hilum which aids in identification of the fungus. The conidiospores germinate and penetrate leaf tissue directly or through stomata. Infection occurs when free moisture is present on the leaf surface.

Plant Health Management 1.Resistant Hybrids Hybrids are available with both monogenic and polygenic resistance and should be used whenever possible.

Plant Health Management 2.Residue Management Severe outbreaks of northern corn leaf blight are sporadic but the potential for a major outbreak is present where corn is being continuously cropped in a conservation tillage system.

Plant Health Management Since the fungus that causes the disease survives between seasons on crop residue, reduction of the residue should reduce the amount of inoculum present in the spring. However, it is not practical (or legal in some government programs) to abandon conservation tillage. In this case a grower may find it useful to rotate to an unrelated crop such as soybeans.

Plant Health Management 3.Fungicide Application Several fungicides are labeled for control of northern corn leaf blight. Mancozeb（代森锰锌） and propiconazole（普克利、丙环唑） are labeled for field corn, popcorn and sweet corn.

Plant Health Management Maneb and maneb plus zinc are also labeled for control on popcorn and sweet corn. (1997) Fungicide application may be cost prohibitive in some situations and may be limited to seed production fields. Applications may also be prohibited past a certain growth stage and restrictions may apply to feeding crop residue to livestock.

Southern corn leaf blight One of the best known of the leaf blights is southern corn leaf blight (SCLB), which caused extensive and widespread damage to the corn crop in 1970. A new race of this fungus, designated Race T, attacked both inbreds and hybrids with the Texas male-sterile (Tms) cytoplasm. An estimated 80-85% of the dent corn grown in 1970 had Tms cytoplasm. Race T not only attacked leaves, but also leaf sheaths, ears, and stalk tissues.

Symptoms Common lesions are elongated, tan lesions between veins on leaves

Symptoms Different isolates of this pathogen will cause lesions of different sizes

Symptoms Race O causes long, tan, lesions that have brown borders on leaves. Lesions will develop differently on various inbreds and hybrids

Pathogen Cochliobolus heterostrophus (=Bipolaris maydis) (a fungus) Overwinters in corn residue Especially prevalent in areas with a warm, humid climate

Pathogen Three races known of this pathogen: race O, race T, and race C Race T and race C are known to be specifically virulent to corn with cytoplasm male-sterile T and cytoplasm male-sterile C, respectively. Since the switch from cytoplasm male sterile T to normal cytoplasm corn, race T is not considered to be a threat.

Time of Occurrence When corn has reached mid-whorl up to maturity

Conditions Favoring Disease Warm, temperatures (70 - 90° F) and wet conditions Monoculture Reduced tillage

Disease Management Use of resistant hybrids Crop rotation Tillage may be helpful