Jeffrey W. Hoy
A sugarcane variety begins as a single seedling. Stalks from that initial plant are then cut and planted, and the buds along the stalks germinate and grow to produce new plants. This increase through cutting and planting of stalks, or “seedcane,” continues until the variety may be grown in many fields across the state. In each field, all plants are identical to the first. This increase process known as “vegetative propagation” creates the potential for disease problems, because some diseases can be spread to new fields and increased during the planting process.
Certain types of plant pathogens become distributed throughout infected plants. When this occurs, any infected stalk cut and planted will produce multiple new diseased plants. To control this systemic disease, farmers need to be able to obtain and plant healthy seedcane. Extensive, long-term research has been directed toward developing methods that will allow growers to control systemic diseases on their farms with a healthy seedcane program. In recent years, a successful disease control program has been developed through a partnership between the public and private sectors.
Ratoon Stunting Disease
A healthy seedcane program is needed to manage ratoon stunting disease (RSD), the most important disease affecting sugarcane in Louisiana. The term ratoon refers to the crop of stalks that re-grows from the base of plants left in the soil after harvest. In Louisiana, farmers typically obtain four annual cuttings of stalks from one planting—the plantcane (first year) crop and three sequential ratoon crops. The pathogen that causes RSD is distributed throughout the plant in the water-conducting vessels (Figure 1). Under adverse growing conditions, particularly drought, the growth of infected stalks is stunted and yield is reduced. The stunting and losses become progressively more severe in the ratoon crops. Because there are no visible symptoms of the disease, only reduced growth, farmers do not know when they have RSD or when they are spreading it.
One factor contributing to RSD persistence was the lack of a reliable detection method that would allow disease monitoring. However, since 1997, a detection method for RSD monitoring has been available through the Sugarcane Disease Detection Lab, which is operated under the auspices of the LSU AgCenter with financial support from the American Sugar Cane League and the company producing Kleentek seedcane. The detection method used is a tissue-blot immunoassay in which cross-sections of stalks are blotted onto a membrane, and antibodies produced against the RSD pathogen are used to determine whether a stalk has RSD or not (Figure 2).
Control Requires Healthy Seedcane
Many sugarcane diseases have been controlled through the development of resistant varieties, but this has not been the case for RSD. This disease can be controlled only with healthy seedcane programs. The first program developed used heat treatment on stalks to be used for planting. Dipping stalks in water at 122 degrees F for two hours can eliminate most but not all RSD infection without causing extensive damage to the stalks. Heat treatment must be performed carefully. Lower temperatures will not control the disease effectively, and higher temperatures will damage stalks. The logistics of treatment (time, personnel and unit capacities) limit seedcane output, and multiple field increases are needed to provide enough seedcane to meet planting needs. These factors, along with occasional stand problems, resulted in limited use of heat treatment in Louisiana and persistence of RSD.
The development of a technique to mass propagate plants in the laboratory known as “tissue culture” provided another option for producing healthy seedcane for sugarcane farmers. The potential use of seedcane produced through tissue culture began to be examined in 1984 with the introduction of a commercially produced seedcane using the product name Kleentek. Experiments comparing agronomic performance between Kleentek and traditional sources demonstrated Kleentek yields were similar or superior to the original.
Concerns about quality assurance for a commercially produced seedcane were addressed with the development of certification standards to be regulated by the Louisiana Department of Agriculture and Forestry (LDAF). Input from Kleentek, the American Sugar Cane League, the LSU AgCenter, the U.S. Department of Agriculture and LDAF was used to formulate regulations covering accession of source material for tissue culture, limitations for stand eligibility and limits for tissue culture variants, insect damage, weeds and certain diseases.
Even though RSD control was the primary objective, it was not included in the certification standards because of the lack of reliable symptoms. However, since 1997, an independent assessment of the RSD status of Kleentek seedcane has been performed by the Sugarcane Disease Detection Lab. After several changes in the parent company for Kleentek, local representatives negotiated a partnership with the LSU AgCenter in which the tissue culture lab became an LSU facility operated by Kleentek. Seedcane production and distribution remained the responsibility of Kleentek. A “local quarantine” operated by the Sugarcane Disease Detection Lab now provides healthy plant material of experimental varieties from the Louisiana Sugarcane Breeding Program for tissue culture.
LCP 85-384’s Effect
A final component of the research effort to control RSD was the determination that varieties vary in rates of spread and increase of RSD. Experiments demonstrated that the high-yielding sugarcane variety, LCP 85-384, has some resistance to the spread of RSD. This type of disease resistance has had a positive impact on RSD control. The RSD testing done by the Sugarcane Disease Detection Lab over the last four years has documented that RSD levels in the industry have fallen. The factors associated with reductions in RSD are increased planting of LCP 85-384 and use of Kleentek seedcane.
A large-scale RSD survey conducted statewide during 2000 on more than 120 farms detected RSD in 14 percent of 535 fields tested, and the average stalk infection level within fields was 2 percent. This is in contrast to a survey in 1986 that showed that 59 percent of the fields were infected with infection levels in the fields averaging 22 percent. Although the 2000 survey results indicate significant progress in RSD control, RSD was detected in at least one field on 35 percent of the farms tested. This means RSD is still present in the in-dustry, and active control measures still need to be continued.
Several additional systemic diseases can be spread during sugarcane planting, including leaf scald, mosaic and yellow leaf. Mosaic has been a problem for many years. Leaf scald first appeared in 1992. Yellow leaf has been observed only within the last few years. These diseases have other means of spreading besides planting. Leaf scald can be spread in wind-blown rain, and mosaic and yellow leaf are spread by insects. Having a source of seedcane containing little or no disease can reduce the impact of these diseases as well as RSD.
In Louisiana, on-farm healthy seedcane programs using commercial seedcane produced through tissue culture and the growth of varieties with lower rates of spread of RSD have brought about a high degree of control of what has historically been the most damaging disease of sugarcane. This has been accomplished through a partnership among farmers, the LSU AgCenter, a state regulatory agency (LDAF) and a commercial seedcane company. In addition, a joint approach of breeding for disease-resistant varieties, together with a healthy seedcane program based on tissue culture, is providing control of the other important sugarcane diseases in Louisiana.
Jeffrey W. Hoy, Professor, Department of Plant Pathology & Crop Physiology, LSU AgCenter, Baton Rouge, La.
(This article appeared in the fall 2001 issue of Louisiana Agriculture.)