Diseases

 

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Sugarcane Rust Disease (Puccinia melanocephela)

Sugarcane rust is caused by Puccinia melanocephela, an obligate parasitic fungus. Changes in varietal susceptibility to rust have been observed over the years, suggesting the existence of fungal variants. Since 1978, sugarcane production in Florida has been threatened by sugarcane rust and the pathogen is now found almost everywhere sugarcane is grown.

Because the disease has had considerable economic impact, screening for resistance has become an integral part of Florida sugarcane breeding programs. Yield loss assessment due to rust is difficult, but estimates range from a conservative 20% to 40% during severe infestations.

Identification

Sugarcane rust is mainly a disease of the leaf. The earliest symptoms are small, elongated yellowish spots that are visible on both leaf surfaces. The spots increase in length, turn brown to orange-brown or red-brown in color, and develop a slight chlorotic halo. Lesions typically range from 2-10 mm in length but occasionally reach 30 mm. They are seldom more than 1-3 mm in width.

Infections are usually most numerous toward the leaf tip, becoming less numerous toward the base. Pustules, which produce spores, usually develop on the lower leaf surface. Certain cultivars, however, may have some pustules on the upper surface.

On a highly susceptible variety, considerable numbers of pustules may occur on a leaf, coalescing to form large, irregular, necrotic areas. High rust severities may result in premature death of even young leaves. Severe rust has caused reductions in both stalk mass and stalk numbers, thereby reducing cane tonnage.

Pustules

Rust

Control

The best means of control for sugarcane rust is to grow resistant varieties. However, resistance has not been stable or durable on certain varieties, presumably because of rust variants. For this reason, it is highly recommended that growers diversify their varietal holdings.

Varietal diversification may play an important role in holding down the overall area-wide disease pressure, thereby reducing the natural selection pressure for one particular rust variant and preserving the durability of host plant resistance in current resistant varieties. Sulfur is the only chemical currently registered for control of rust on sugarcane.

Studies have shown control of the rust fungus using sulfur is either not effective or does not offer enough economic return to be used. No effective biological control has been found to date. Since soil factors have been identified as being associated with rust infection levels on sugarcane, avoid growing susceptible varieties in areas with low soil pH and/or high levels of P and K nutrients.

Sugarcane grown in fields receiving recent applications of mill mud is typically very prone to rust. If possible, plant these fields with varieties that have demonstrated, durable rust resistance.

Summarized from EDIS document 'SC011 Sugarcane Rust Disease' for more information please refer to the full length document at http://edis.ifas.ufl.edu/SC011.


Sugarcane Mosaic Virus Disease

Sugarcane Mosaic Virus Disease

The disease caused by sugarcane mosaic virus (SCMV) is commonly referred to "mosaic." It has, at one time or another, occurred in virtually every important sugarcane growing country. Estimated yield losses due to the disease vary greatly depending on the time period and sugarcane growing area involved. Until 1996, mosaic had not been a problem in Florida. In 1996, sugarcane mosaic was observed in grower fields on CP 72-2086, a major commercial cultivar. The incidence of mosaic in other cultivars is either undetectable or very low.

Symptoms

Mosaic is identified primarily by its leaf symptoms. As with most sugarcane diseases, the symptoms may vary in intensity with the cane variety, growing conditions, and the strain of the virus involved. The most distinctive symptom is a pattern of contrasting shades of green, often islands of normal green on a background of paler green or yellowish chlorotic areas on the leaf blade (Figure 1). Generally, the chlorotic areas are diffuse, but they may be sharply defined in some clones infected with certain strains of the virus. The infection may be accompanied by varying degrees of leaf reddening or necrosis. Chlorotic areas are most evident at the base of the leaf. Chlorotic areas may also be present on the leaf sheath, but rarely on the stalk. Young, rapidly growing plants are more susceptible to infection than more mature, slower growing plants.

Spread

There are three principal modes of spread of SCMV: (1) by aphid vectors, (2) by infected seed cane and (3) by mechanical inoculation. However, only aphid vectors and infected seed cane are important in the field.

Figure 1 

Prevention and control

The use of resistant varieties is the most effective method of mosaic control. Periodic surveys of SCMV strains are necessary so that all clones may be tested against prevalent strains. Natural infection tests are being conducted to evaluate clones in the development program at the Sugarcane Field Station and US Sugar Corporation.

Summarized from EDIS document ‘SC009 Sugarcane Mosaic Virus Disease’ for more information please refer to the full length document at: http://edis.ifas.ufl.edu/SC009.