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September 18, 2007
Source: Alison Robertson, Department of Plant Pathology, Iowa State University via Cattle Network [edited] <http://www.cattlenetwork.com/content.asp?contentid=160747>

Severe leaf blighting due to southern rust has occurred throughout central and southern Iowa [in early September 2007]. The last severe outbreak of southern rust in Iowa was in 1999. This disease was reported in Nebraska and Kansas earlier in the growing season; however, it was only in mid- to late August [2007] that we started to notice a few lesions in our field trials. Temperatures and precipitation in Iowa throughout August [2007] were well above normal and thus highly favorable for southern rust.

Southern rust is caused by the fungus _Puccinia polysora_. Like all rusts, _P. polysora_ spores are windblown to Iowa from the South. High humidity and warm temperatures favor the development of the disease. Under these conditions, new infections can occur every 7 days, resulting in numerous new rust lesions and extensive leaf blighting.

Two types of rust occur on corn. Common rust is widespread each year in Iowa. Southern rust occurs less frequently. Southern rust can be distinguished from common rust in 2 ways. Color: common rust spore pustules are brick red; southern rust spore pustules are bright orange. Development of lesions/pustules: common rust pustules develop on both upper and lower leaf surfaces; southern rust pustules develop predominantly on the upper leaf surfaces.

Southern rust can develop rapidly, resulting in severe and early senescence and considerable yield loss. In Iowa this year [2007], we "dodged the bullet." It is unlikely that this southern rust epidemic will affect yield noticeably since disease development started late in the grain-fill period. Registered fungicides are effective against southern rust, but with disease occurring so late in the season, an application was unnecessary. Fields in which a fungicide application was made earlier in the growing season are still at risk for southern rust since fungicides are only effective against foliar pathogens for 14 to 21 days after application.

--
Communicated by:
J. Allan Dodds
Former ProMED-mail plant disease moderator
<dodds@ucr.edu>

[_Puccinia polysora_ causes southern rust (also called polysora or tropical rust) in maize. The fungus occurs in most tropical and subtropical regions and can cause significant losses of over 50 percent. Southern rust is favoured by warm, humid conditions making it an important disease in tropical areas.

Infection reduces the photosynthetic ability of the affected leaves interfering with kernel development. With heavy infections, leaves may die prematurely, and defoliate from the base up. Therefore infection before grain fill has been completed is more damaging to crop yield than infection late in the crop cycle. _P. polysora_ needs living host tissue for survival and spores are wind dispersed. No alternate hosts of the fungus are known. Resistant or tolerant ('slow rusting') varieties are being developed.

Common rust of maize is caused by the related species _Puccinia sorghi_. The disease is found worldwide in subtropical, temperate, and highland environments with cooler temperatures and high humidity. Rust development is much more likely in pre-tassel stage corn, because a large whorl provides a humid, protected environment, and young leaf tissue is more susceptible to infection than emerged leaves. Resistant maize varieties are available. This fungus can also affect _Oxalis_ species as alternate hosts producing light orange-coloured pustules, which are another stage of the same fungus.

Map of US states:
<http://www.census.gov/geo/www/us_regdiv.pdf>

Pictures
Southern rust symptoms:
<http://www.ipm.iastate.edu/ipm/icm/node/2596>
Southern rust, photo gallery:
<http://www.ent.iastate.edu/imagegal/plantpath/corn/southrust/>
Common rust symptoms:
<http://www.ipm.iastate.edu/ipm/icm/node/2594>
Common rust, photo gallery:
<http://www.ent.iastate.edu/imagegal/plantpath/corn/comrust/>

Links
Information on southern and common rusts of maize:
<http://edis.ifas.ufl.edu/BODY_VH051> and
<http://www.dpi.qld.gov.au/cps/rde/xchg/dpi/hs.xsl/26_4721_ENA_Print.htm>
Information on common rust:
<http://www.lgseeds.com/lg_tech2/commonrust.asp>
Field guide to maize foliar diseases, with pictures:
<http://www.cimmyt.org/english/docs/field_guides/maize/pdf/mzDis_foliar.pdf>
_Puccinia polysora_ taxonomy:
<http://www.indexfungorum.org/Names/SynSpecies.asp?RecordID=179511>
_P. sorghi_ taxonomy:
<http://www.indexfungorum.org/Names/SynSpecies.asp?RecordID=194826>
Plant Pathology, Iowa State University:
<http://www.plantpath.iastate.edu>. - Mod.DHA]

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