Cause The fungus, Puccinia sorghi, is seen only occasionally in the Pacific Northwest. Common rust overwinters as teliospores, which are resistant to weathering. Spores germinate in spring to produce basidiospores, which infect wood sorrel (Oxalis spp.). Spores produced in those infections complete the life cycle of the fungus by infecting corn. Cool (66°F optimum) humid weather favors the disease.
Symptoms Common-rust pustules (uredinia) may appear on any aboveground plant part but occur most abundantly on leaves. Circular to elongate, golden brown to cinnamon-brown pustules are scattered over both leaf surfaces. They become brownish-black as the teliospores develop.
Cultural control
- Plant resistant or tolerant hybrids or varieties.
Chemical control
- Demethylation-inhibiting (DMI) fungicides (Group 3) are labeled for use.
- Tilt at 4 fl oz/A when rust pustules first appear. Do not apply within 30 days of harvest for field corn forage, grain and stover or 14 days for sweet corn ears and forage. 12-hr reentry.
- Dithane F-45 Rainshield (Group M3) at 1.2 quart/A on 4- to 7-day intervals. Preharvest interval is 7 days for sweet corn and 40 days for field corn. Do not feed treated foliage to livestock. 24-hr reentry.
- Strobilurin fungicides (Group 11) are labeled for use. Do not make more than one (1) application of a Group 11 fungicide before alternating to a labeled fungicide with a different mode of action.
- Headline at 6 to 9 fl oz/A prior to disease development on 7- to 14-day intervals. Preharvest interval is 7 days. 12-hr reentry except if hand detasseling or hand harvesting, for which the reentry interval is 7 days.
- Quadris Flowable at 6 to 9 fl oz/A on 7- to 14-day intervals. Preharvest interval is 7 days. 4-hr reentry.
- Vertisan (Group 7) at 10 to 24 fl oz/A on 7- to 14-day intervals. Do not make more than two (2) sequential applications before alternating to a labeled fungicide with a different mode of action. Preharvest interval is 7 days for grain and stover; 0 days for forage. 12-hr reentry.
- Premixed fungicides are available for use.
- Elatus (Group 7 + 11) at 4.9 oz/A for field and popcorn and 5 to 7.3 oz/A for sweet corn as two (2) applications at least 14 days apart. Preharvest interval is 7 days. 12-hr reentry.
- Miravis Neo (Group 7 + 3 + 11) at 13.7 fl oz/A. Early application (V4-V8); late application when disease first appears or at VT or R1. May be re-applied 7- to 14-days later. Do not apply more than two (2) sequential applications. Preharvest interval is 30 days for field corn and popcorn; 14 day PHI for sweet corn. 12-hr reentry.
- Quilt (Group 11 + 3) at 7 to 14 fl oz/A or Quilt Xcel (Group 11 + 3) at 10.5 to 14 fl oz/A on 14-day intervals. Preharvest interval of Quilt is 14 days for sweet corn and 30 days for field corn. Preharvest interval of Quilt Xcel is 14 days. 12-hr reentry.
- Stratego (Group 3 + 11) at 10 fl oz/A for sweet corn or 10 to 12 fl oz for field corn and popcorn on 7- to 10-day intervals. Apply between 4-leaf stage and after silking. Do not apply within 14 days of harvest for sweet corn and 30 days of harvest for field corn or forage. 12-hr reentry.
- Trivapro (Group 3 + 7 + 11) at 13.7 fl oz/A applied early application (V4-V8) on a 14-day interval. Do not apply more than two (2) sequential applications. Preharvest interval is 14 days for sweet corn, field corn is 30 days. 12-hr reentry.