Leaf rusts: Old and new threats to sugarcane

Linda Benedict  |  7/8/2008 1:31:35 AM

leaf rust

Sugarcane variety LCP85-384 showing symptoms of brown rust. The reddish-brown color is due to numerous lesions caused by the pathogen from which high numbers of spores will be released to spread the disease. (Photos by Jeffrey W. Hoy)

Jeffrey W. Hoy

Sugarcane is affected by two diseases known as rusts because of the color of the lesions caused in the leaves and reproductive spores produced by the pathogens. One disease is called brown rust, and the other is called orange rust, based on differences in lesion and spore color.

Brown rust, a disease of sugarcane worldwide, has been present in Louisiana since the 1970s and in recent years has been causing serious problems. This disease is caused by a fungus named Puccinia melanocephala. Orange rust is caused by a closely related organism, Puccinia kuehnii. It is a less widely distributed disease found for the first time in Florida sugarcane in 2007. Orange rust, which is not yet in Louisiana, represents a new potential threat to the profitability of the Louisiana sugarcane industry.

These organisms infect leaves and cause reddish-brown or orange pustules and then produce huge quantities of spores dispersed in the wind to cause more infections. A field of sugarcane severely affected by rust will develop a rusty color clearly visible from outside the field. Both diseases reduce the healthy leaf area available for photosynthesis and thereby cause reductions in plant growth and yield. Per-acre sugar yield losses in excess of 20 percent because of brown rust have been documented in Louisiana.

Historically, control of both rusts has relied on the development and release of disease-resistant sugarcane varieties. This approach has been successful, but it takes time and can be problematic. Rust pathogens are known for the ability to adapt and overcome host-plant resistance. The emergence of a new pathogenic “race” can dramatically change the impact of rust diseases and can create an emergency situation for a disease that has been present for a long time.

During the spring of 2000, a severe epidemic occurred in the major sugarcane variety LCP 85-384, indicating a probable race change in the pathogen population, and epidemics have continued to occur since 2000. This variety was rated as resistant to brown rust when it was released in 1993. A product of the basic breeding program in Louisiana, LCP 85-384 offered a 30 percent increase in yield potential compared to the previously available varieties. Therefore, it was viewed as highly desirable for planting by Louisiana sugarcane farmers. At its peak during 2004, it was planted on 91 percent of the acreage.

Rapidly replacing a widely planted variety is difficult to accomplish in sugarcane, even if resistant varieties with comparable agronomic performance are available. Six varieties with some level of resistance to brown rust have been released over the past five years, but severe disease symptoms were observed in one variety, Ho 95-988, during 2006, indicating that resistance in this variety is being overcome by the pathogen. Severe symptoms continued to be observed in Ho 95-988 during 2007, and moderate symptoms were observed at some locations in HoCP 96-540, another new variety being rapidly increased in the industry. HoCP 96-540 will be the most widely grown sugarcane variety in 2008.

The ongoing problem with brown rust suggested a need for an alternative disease-control measure. Therefore, research was initiated in 2005 to determine if fungicides could provide economic rust control, and fungicides with the ability to control brown rust have been identified. Federal funding was obtained to support residue testing for two fungicides – pyraclostrobin (Headline) and metconazole (Caramba) – during the 2008 growing season. These results will be used to request EPA approval for fungicide use for brown rust control. However, it will take several more years to get this approval.

Orange rust is now causing a problem in Florida sugarcane. And any plant pathogen spread by windblown spores occurring extensively in Florida will spread to Louisiana. This al ready has happened with brown rust. When a sugarcane pathogen arrives for the first time in a new area, it encounters new host genetic types. Whether or not the varieties being grown in Louisiana are susceptible to the pathogen is not known until they come into contact with each other. It is essentially a matter of luck if our major varieties and most of the experimental lines in the variety selection program will turn out to be resistant. If they don’t, then we could have a significant problem.

Many of our current commercial varieties are being used as parents in the breeding program in Florida. Information on natural infection and infection resulting from inoculation with the orange rust pathogen will be collected to gain valuable clues as to whether any of our current varieties are going to be susceptible to orange rust when it appears in Louisiana. In addition, experiments are under way in Florida to evaluate the ability of different fungicides to control orange rust.

Research, extension and industry personnel in Louisiana will be watching closely for the appearance of orange rust during the coming season. The impact of this disease on our sugarcane industry remains to be determined.

Jeffrey W. Hoy, Professor, Department of Plant Pathology and Crop Physiology, LSU AgCenter, Baton Rouge, La.

(This article was published in the spring 2008 issue of Louisiana Agriculture.)
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