Law on control of sugarcane ratoon dwarfing disease

Ratoon Stunting Disease (RSD) was first reported in Taiwan, China, in 1954. Subsequently, Guangdong and Fujian provinces in mainland China also documented the occurrence of this disease. To date, no reports have emerged from other sugarcane-growing regions. The disease is more prevalent in arid areas, where it causes significant economic losses due to reduced yield and plant vigor. **(A) Pathogen** The causative agent of RSD is *Clavibacter xyli* subsp. *xyli*, a coryneform bacterium. This pathogen resides within the vascular tissues of infected sugarcane plants. It exhibits a unique morphology: straight or slightly curved, slender, thin-walled, occasionally with one end swollen, and containing interstitial structures. It does not move, and it is Gram-positive. The distribution of the bacteria within the plant is uneven, with higher concentrations found in the base of the stem, followed by the lower internodes, growing point, heart, leaf blades, and leaf veins and sheaths. The severity of the disease correlates directly with the bacterial load in the plant. **(B) Symptoms** Externally, RSD does not present clear symptoms. Affected plants are shorter, have smaller stems, and exhibit delayed growth. They also show a tendency for excessive tillering. These plants are highly sensitive to water stress, which further slows their growth during droughts. Severely affected plants may appear wilted, with dry leaf margins. However, adequate irrigation can mask these symptoms and reduce damage. Internally, two key signs are observed. First, the tissue just below the growing point (within 1 cm of the node) turns orange-red, with color intensity varying by variety. Some varieties may not show this symptom even when infected, and nitrogen fertilizer application influences the depth of the discoloration. Second, the vascular bundles in mature stems change color from yellow to orange-red and eventually to dark red. This discoloration is most visible on the first 1–10 nodes, particularly between 3–8 nodes. The discoloration is usually confined to the nodes and rarely extends into the internodes. In cross-sections, the red vascular bundles appear as dots or stripes, while in longitudinal sections, they may look like punctiform, comma-shaped, or short-stripe patterns. The number and intensity of these changes depend on the variety and the plant’s age. Some sugarcane strains do not display internal symptoms after infection. **(C) Disease Spread** RSD spreads primarily through infected planting material, such as cane cuttings and ratoon shoots. The pathogen can survive in the stalks of infected seedlings or ratoon heads for extended periods. In the next growing season, these infected materials can transmit the disease to new plants. Diseased plants can spread the pathogen via contaminated tools like cane knives or harvesters, especially when handling healthy sugarcane. Soil contact and leaf-to-leaf friction also contribute to transmission. The pathogen remains infectious in diluted sugarcane juice up to 10,000 times. Contaminated tools can remain infectious for up to seven days in shaded conditions. The longer the perennial period, the more severe the disease becomes. **(IV) Control Measures** Preventing the spread of RSD starts with using disease-free seedlings. Heat treatment is an effective method to eliminate the pathogen: 1. **Hot Water Treatment**: Using 50°C water for 2 hours can prevent RSD. This method works well for double-sprouted and full-stem cuttings but may damage buds, reducing germination rates. 2. **Hot Air Treatment**: A heated oven at 54–58°C for 8 hours is another option. At 58°C, it achieves 100% control. This method is less damaging to sprouts, resulting in better germination, but it is time-consuming and increases the risk of water loss. 3. **Steam Treatment**: Mixing steam and air to maintain a constant temperature (53–54°C) for 4 hours eliminates both the risk of bud damage and water loss. It is a more balanced approach. In addition to heat treatment, establishing a disease-free nursery is essential. Seedlings should be planted in a controlled environment, and all tools must be disinfected using 70% alcohol or flame sterilization. During cutting, tools should be kept separate for different varieties, regularly sterilized, and rodents controlled to prevent disease spread. Tissue culture techniques, such as meristem tip culture, can produce virus-free seedlings. These are then used for large-scale production. Before planting, fields should be deeply plowed, soil prepared carefully, and fertilizers applied timely to enhance plant resistance. Finally, all equipment, including cane knives and seed drills, must be thoroughly disinfected before use to prevent re-infection. Rodent control is also crucial to avoid mechanical transmission of the pathogen.

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