Disease Management

Corn Rust

Common Rust of Sweet Corn (Puccinia sorghi)

The high susceptibility of many popular sweet corn hybrids and the staggered planting schedule can lead to rust epidemics. High concentrations of spores in the air from early plantings arrive at late-planted fields when they contain young, actively growing susceptible plants. Symptoms of Common rust are elongate to oval brown pustules on both leaf surfaces which rupture and release red spores (repeating spores). In severe epidemics, pustules may also appear on tassels and ears, and the leaves may yellow and become tattered. Pustules darken with maturity and produce dark brown, overwintering spores.

P. sorghi like most other rust diseases has a complex life cycle, involving two hosts and five spore types. All stages of this life cycle occur in Mexico; in northern temperate regions only the repeating spores or uredinospores are important; an earlier spore stage arrives from the south each year and infects the corn. Urediniospores are incapable of overwintering. Rust epidemics are rare in field (dent) corn because many resistant varieties are available. Moderate temperatures (60-75° F) and high relative humidity favor disease development by affecting spore germination.

MANAGEMENT

• Use disease resistant cultivars where available. A partial list of resistant cultivars includes ‘Xtra Tender 277’, Snowmass’, ‘Cabaret’, ‘Delectable’, ‘Silverado’, ‘Zenith’, ‘Lancelot’, and ‘Argent’.
• Scout for pustules beginning at whorl stage and spray when 80 % of leaves observed have one or more pustules.
• Avoid planning near corn fields of different maturity.
• Avoid fields with poor air circulation.

Chemical Management

azoxystrobin (Quadris F): 6.2 to 9.2 fl oz/acre. 7 dh, REI 4h, (Group 11). Do not make more than one sequential application of Quadris or Amistar before alternating with a fungicide with a different mode of action. Do not spray where product may drift to apple trees.

chlorothalonil (Bravo, Equus 720): 0.75 to 2 pt/acre.14 dh, REI 12h, (Group M5).

maneb (Maneb 75 DF): 1.5 lb/acre. 7 dh, REI 24 h. (Group M3).

propiconazole (Tilt): 4 oz/acre. 7 dh, REI 12 h, (Group 3).

pyraclostrobin (Headline EC): 6-9 fl oz/acre. 7 DH, REI 12 h. (Group 11). Do not make more than one sequential application before alternating with a fungicide with a different mode of action. Do not alternate with Bravo or Amistar.

- M. Bess Dicklow, UMass Extension 2007