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Pathogen
Common rust is caused by the fungus, Puccinia sorghi.
Hosts
The main economic hosts are field and sweet corn. The fungus is able to infect other hosts (see Disease Cycle) to complete its lifecycle in Mexico.
Symptoms
Symptoms of common rust initially occur as yellow flecks on the leaf surface. These lesions rapidly develop into oval to elongate shaped reddish-brown and powdery pustules that break through the surface giving the leaf a bumpy appearance. Rust pustules are typically 1/8th inch in length and may appear by themselves or as clusters on the leaf surface. As the rust pustule emerges from the leaf the tissue usually appears torn around the outsides of the pustule. The leaf tissue around the pustules often yellows or dies leaving areas of dead tissue. As the pustules become older, they turn dark brown to black in color of approximately the same size and again cause the leaf surface to appear bumpy. Rust pustules of either color may occur on the leaves, leaf sheaths, stalks, ears, tassels, and husks. Older, black rust pustules may easily be confused with tar spot. Rust pustules do not extend through the leaf and sit on top of the leaf surface, while tar spot lesions extend through the leaf and are visible on both sides.
Figure 1: (A, C) Dark, reddish-brown pustules scattered over both the upper and lower surfaces of the corn leaves and (B) then turn dark brown or black when they age.
Disease Development
The fungus, P. sorghi, does not overwinter locally in New York. Each year inoculum to initiate rust epidemics is introduced by long-distance wind currents from the south as the fungus is only able to survive and complete its lifecycle in tropical and subtropical regions (e.g., Mexico). Infections and disease spread are promoted by high humidity, moderate day temperatures (60 to 77 °F) and night temperatures of 65 to 70 °F. Infection also requires leaves to be wet for at least 3 hours. The disease is therefore usually observed for the first time in New York sweet corn crops from mid-June onwards and is prevalent in late season plantings.
Disease Cycle
The disease cycle of common rust of sweet corn is complicated! In tropical and subtropical regions, P. sorghi completes the lifecycle by infecting a completely different alternate host, Oxalis spp. Infections of Oxalis spp. give rise to a spore type called aeciospores that move long distances in the wind. These aeciospores are those that infect corn leaves to initiate the epidemics. After infecting corn, the pustules (called uredinia) that emerge on the corn leaves produce a different type of spore called urediniospores. These spores occur on corn leaves throughout the growing season and are responsible for the rapid disease spread throughout the growing season. However, as the rust pustules turn from rust red to brownish-black in color, the uredinia develop into telia which produce teliospores. Both urediniospores and teliospores are able to infect corn. These teliospores cannot survive the New York winter. In tropical regions, the teliospores are able to infect the alternate host to complete the lifecycle.
Figure 2: Common rust of sweet corn disease cycle.
Disease Management
The management of common rust should be considered when designing an integrated program for foliar diseases of sweet corn.
Inoculum Sources
- Susceptible Hosts. Corn (sweet and field) is the only host of the pathogen.
- Seed. The pathogen, P. sorghi is not seed transmitted.
- Infested Crop Residues. P. sorghi does not survive between growing seasons on infested corn residues as it cannot survive the New York winter.
Management
Cultural Practices
- Crop rotation and residue management. Crop rotation and residue management will not influence the severity of common rust epidemics as the fungus does not survive in infested crop residue. However, we recommend a rotation of at least three years between field or sweet corn crops for management of other foliar diseases.
- Irrigation. If irrigation is used, applying at midday to minimize the time that leaves are wet will reduce infection and disease spread. Ideally, leaves should not be wet to touch for longer than 3 hours.
Cultivar Selection
Consult local seed catalogues for more information on cultivar susceptibility to common rust. Resistant or moderately resistant cultivars should be used for late season plantings.
Fungicides
In sweet corn, fungicides are typically applied at V7-V8 and tassel. The aim of applying fungicides for the control of common (and other foliar diseases) is to reduce defoliation during reproductive stages of growth and protect the ear leaves.
Conventional Fungicides. Consult the Cornell Vegetable Guidelines for the latest registered fungicides for common rust control in sweet corn in New York (Table 1). Fungicides containing active ingredients belonging to the strobilurins (Fungicide Resistance Action Committee [FRAC] Group 11) and demethylation inhibitors (FRAC Group 3) are likely to provide adequate control of common rust. Members of FRAC 7 are likely to provide lesser control.
Table 1. Selected conventional fungicides registered for common rust control in sweet corn in New York.
OMRI-listed Fungicides. No efficacy data is currently available for common rust control in sweet corn in New York.
Additional Information
Dr. Sarah J. Pethybridge (sjp277@cornell.edu)
Cornell AgriTech, 211 Barton Laboratory
Geneva, New York
(315) 744-5359 (cell)
Dr. Julie R. Kikkert (jrk2@cornell.edu)
Cornell Cooperative Extension
Cornell Vegetable Program
Canandaigua, New York
(585) 394-3977 x 404 (office)