• Topics: Agronomy  |  Diseases in Sugarcane (Sugarcane Handbook)  | Diseases  |  Lentini, Richard S  | Xanthomonas spp.  |  Comstock, Jack C 

Sugarcane Leaf Scald Disease¹

Leaf scald was first recognized as a bacterial disease of sugarcane in the 1920s. It is a vascular disease caused by Xanthomonas albilineans. The disease has been found in at least 55 countries. Many of these countries are in the most productive sugarcane areas of the world.

Leaf scald is a disease with the potential to seriously limit the cultivation of susceptible varieties. The disease is insidious in that it may have a latent (asymptomatic) period that lasts for years. Leaf scald is further complicated by the fact that it may be manifested in a chronic phase or acute phase.

The disease was discovered on the mainland U.S. at the USDA Sugarcane Field Station at Canal Point, Florida in 1967. The breeding station has been screening for resistance since its discovery.

The incidence of leaf scald since 1989 at the Canal Point Sugarcane Station has been cause for concern since 10 to 15% of the clones in Stage II have been discarded in recent years because of the disease. More importantly, however, the disease has increased in commercial fields in Florida. The disease first appeared in a few fields of CP 78-1247 near Lake Okeechobee. These fields have been plowed out. Presently, leaf scald is a concern on CP 80-1743, a cultivar occupying over 25% of the commercial acreage that is moderately susceptible. Although the leaf scald incidence in almost all fields of CP 80-1743 has remained low, growers are advised to monitor fields of CP 80-1743 for increase of incidence of the disease.

SYMPTOMS

Some leaf scald-infected plants do not have any external symptoms. These plants are referred to as being latently infected and the mechanism of latent infection is not understood. There are also cases of apparent recovery in which symptoms subside and do not become visible until ratoon crop regrowth or after planting infected seed cane. However, during apparent recovery, the disease is in a period of latent infection in the affected cane.

The chronic phase is characterized by several external symptoms. The most typical symptom is a white pencil-line streak about 1 - 2 mm wide on the leaf that extends from the midrib to the leaf margin running parallel to the veins (Figure 1). A diffuse yellowish border of varying widths runs parallel to the pencil line streak. The pencil line may have areas of reddish discoloration along part of its length. As the disease progresses, necrosis develops from the leaf tip or leaf margin.

Figure 1. 

Pencil-line mark on sugarcane leaf caused by leaf scald disease.

Leaf scald can also cause partial or complete chlorosis (scalding) of the leaf blade. Close inspection of these areas may reveal the diagnostic white pencil line or its reddish necrotic sections.

The disease can also cause shoots to be stunted and wilted. Usually, affected leaves turn a dull blue-green color before dense browning (a late symptom of the disease). Under stress conditions the whole stool may die. This has happened in a few fields of CP 80-1743 grown under stressed conditions.

On mature stalks, the spindle leaves become necrotic from the tips and moderate to profuse side shoots develop. Side shoots first appear at the bottom of the stalk and progress upward. These side shoots usually show the scalding and/or white pencil lines (Figure 2). The side shoots often die while quite small (<18").

Figure 2. 

Sugarcane side shoots infected with leaf scald.

Internally, affected stalks may show bright to dark red streaks caused by necrosis of the vascular bundles. These streaks are most prominent at the nodes and are nearly always present at the juncture of side shoots and the stalk.

The acute phase is characterized by a sudden wilting and death of mature stalks, often without previous symptom expression. The onset of this condition generally follows a period of stress, especially of prolonged drought.

CAUSAL AGENT

The leaf scald bacterium has been found to be restricted to the xylem elements of the vascular bundles in the white pencil line streaks. It is not found in the surrounding chlorotic tissues. A phytotoxin has been isolated from chlorosis-inducing strains of X. albilineans. It has been proposed that this phytotoxin may inhibit chloroplast development and/or in some way disrupt photosynthesis.

Variants of the pathogen have been identified. Worldwide there are at least several serological strains of the bacterium. A single serological strain is present in Florida. However, Dr. M. J. Davis has shown that there are two distinct genetic strains in Florida.

SPREAD OF THE DISEASE

Since scald is a systemic disease which may be inconspicuous (latent) for lengthy periods of time, infected seed cane is a major cause of disease spread. Cutting knives, including those on machinery, are an important source of infection.

The pathogen can also survive in stubble. The organism does not appear to survive for long periods of time in soil or undecomposed cane trash.

Alternative hosts may offer another means of pathogen survival. X. albilineans naturally infects several wild grass weeds, such as elephant grass.

Besides transmission by cutting knives, evidence is accumulating to suggest aerial transmission. This may explain, in part, the recent spread of leaf scald.

The amount of damage caused by leaf scald appears to be influenced by environmental conditions. Periods of stress such as drought, waterlogging and low temperature are reported to increase the severity of the disease. The yield of stalks that are dead or have necrotic tops and leaves with numerous side shoots is decreased to 20 to 30% of that of symptomless stalks. Fortunately, over the last ten years, only a few fields of CP 80-1743 had over 10% incidence of stalks with these severe symptoms, and these fields were environmentally stressed.

PREVENTION AND CONTROL

The best control is prevention and the replacement of susceptible varieties with resistant varieties. Due to the latency of leaf scald, however, growers should be alert for infection even in those varieties thought to be resistant.

Seedcane can be given a long-hot-water treatment to kill the pathogen. The Australians use a 24-hour presoak in flowing water, followed by a 3-hour 50° C treatment. The 2-hour 50° C treatment used for ratoon stunting disease would give partial control.

To prevent mechanical spread of the pathogen, all cane cutting knives, including those on mechanical harvesters, should be sterilized when coming from suspect fields. Disinfection of the knives can be accomplished by cleaning and immersing for several minutes in a suitable antiseptic solution, such as Lysol, alcohol or a dilute solution of bleach. Aerial transmission of the pathogen would also influence the length of time disease-free seedfields would remain disease-free. There are no known chemical or biological controls for this disease.

The Institute of Food and Agricultural Sciences (IFAS) is an Equal Opportunity Institution authorized to provide research, educational information and other services only to individuals and institutions that function with non-discrimination with respect to race, creed, color, religion, age, disability, sex, sexual orientation, marital status, national origin, political opinions or affiliations. For more information on obtaining other extension publications, contact your county Cooperative Extension service.

U.S. Department of Agriculture, Cooperative Extension Service, University of Florida, IFAS, Florida A. & M. University Cooperative Extension Program, and Boards of County Commissioners Cooperating. Millie Ferrer, Interim Dean.

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