36. Whip Smut of Sugarcane and Their Control

 Sugarcane

Introduction:

    Sugarcane is a valuable crop for growers as it serves as the primary source of white crystal sugar, and also provides alternative sources of sugar such as 'gur' and 'khandsari' (brown sugar). In addition, sugarcane tops can be used as cattle fodder, bagasse and leaf trash can be used as fuel, and the stubble and roots can serve as organic manure. Crop residues can also be utilized for mulch and compost. It is worth noting that sugarcane leaves are also used as a substrate for growing edible mushrooms.

    In Pakistan, sugarcane cultivation faces numerous challenges, including significant losses caused by various diseases. Over 50 diseases have been reported in sugarcane, with fungi, bacteria, viruses, and nematodes causing the most damage. The extent of losses varies depending on the crop variety and location, and cannot be ignored, neglected, or taken lightly as they can significantly impact the crop. These diseases can affect different plant parts, and their occurrence is influenced by a range of factors such as geography, weather, and time. Symptoms or signs of the diseases raise a concern about their potential effects on the quantity and quality of the sugarcane crop.

    Sugarcane, a crop grown in over 120 countries on around 26.27 million hectares, is the largest commercial crop globally. It yields approximately 1.90 billion tonnes worldwide, providing 80% of the world's sugar. The crop produces hundreds of tons of green matter per hectare annually, giving 75,000 million calories. However, sugarcane takes 12-14 months to mature and harvest, making it vulnerable to many biotic and abiotic factors, such as pathogens and insect pests that can reduce production by up to 20%. Fungal pathogens, in particular, pose a significant challenge, with over 100 fungi reported to cause sugarcane diseases.

Whip Smut of Sugarcane

Causal Organism:

    The fungal disease, known as smut, is attributed to Ustilago scitaminea. The fungus infects sugarcane plants via buds present on erect stems or sprouting buds in the ground. Once inside, the fungus establishes itself near the plant's growing points or meristems.

Symptoms:

• Sugarcane smut is identifiable by a black, whip-like structure that replaces the spindle leaf and grows from the sugarcane plant's growing point. Abnormal whips, sometimes containing flower parts, can also develop.
• The whips are produced in shoots arising from infected cane cuttings, contaminated soil, or side shoots from mature stalks. 
• The thin membrane of the whip eventually ruptures to release a mass of black spores, leaving behind a straw-colored core.
• Before whip formation, the youngest leaves undergo shortening and crinkling. 
• Infected plants tend to be stunted, and individual stalks appear thin and grass-like.

Disease Cycle:

    Sugarcane smut spreads through teliospores produced in the smut whip found in soil or on plants. Teliospores germinate in water and produce promycelium, which undergoes meiosis to form four haploid sporidia. The fungus is bipolar, producing two different spore types that must mate to create a dikaryon that infects sugarcane's meristematic tissue by penetrating bud scales. The fungus grows within the tissue, inducing flowering structures that it colonizes to produce teliospores. These structures transform into a whip-like sorus, covered by a silvery peridium that peels back to expose black-brown teliospores. These spores disperse via wind and repeat the cycle. Teliospores are reddish-brown, round or subovoid, smooth to moderately echinulate, and 6.5-8 um. Susceptibility to S. scitamineum populations in each area should be tested before distributing sugarcane cultivars.

Spread or Transmission:

    Sugarcane smut is mainly transmitted via wind-dispersed spores or by planting contaminated or infected cane cuttings. Typically, spores spread over short distances, up to 10-15 meters, but they can travel many kilometers during strong winds or cyclones. Spore transport on machinery and shoes is also feasible.

    Moist soil can support spore survival for only 2-3 months, whereas dry soil or other arid environments can sustain them for longer periods. The fungus remains viable within infected sugarcane plants as long as the host plant is alive, as it requires a living plant to produce spores.

Control:

1. It is advisable to avoid using sets from sugarcane plants affected by smut for planting.
2. Seed sets can be disinfected by immersing them in a solution of 0.1% mercuric chloride or formalin for 5 minutes, followed by covering them with a moist cloth for 2 hours. Other effective chemicals available in the market can also be used.
3. Internal infection can be eliminated by exposing sets to hot water treatment at 52°C for 18 minutes.
4. Sugarcane plants affected by smut should be uprooted and burned before the spores burst.
5. It is recommended to avoid ratooning of crops affected by the disease.
6. Practicing suitable crop rotations with non-host crops can help in controlling the disease.
7. Planting sugarcane in healthy soil is essential to avoid infection.
8. Where the disease is prevalent, it is advisable to use dry sowing for the crop.
9. Autumn planting of sugarcane should be avoided.
10. Encouraging the use of resistant varieties is recommended to help control the spread of the disease.

References:

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