Sugarcane orange rust, and sugarcane common rust. 

Orange rust is also known as sugarcane leaf rust, and sugarcane rust. Sugarcane common rust is also known as sugarcane rust.

Puccinia kuehnii (sugarcane orange rust). Previously, it was known as Uromyces kuehnii.

Puccinia melanocephala (sugarcane common rust). Previously, it was known as Puccinia erianthi.

Puccinia kuehnii: Asia, Africa, North, South and Central America, the Caribbean, Oceania. It is recorded from Australia, American Samoa, Cook Islands, Federated Stated of Micronesia, Fiji, French Polynesia, Guam, New Caledonia, Papua New Guinea, Samoa, Solomon Islands, and Vanuatu.

CABI considers reports of Puccina kuehnii in Africa and South America may have been confused with sugarcane common rust.

Puccinia melanocephala: Asia, Africa, North, South and Central America, the Caribbean, Oceania. Apart from it being common in Africa, and widely present in the Americas, it has limited distribution in Oceania  compared to Puccinia kuehnii). It is recorded from Australia, American Samoa, and Papua New Guinea.

Both rusts: sugarcane, Saccharum officinarum and other Saccharum species.

Puccinia kuehnii: yellow leaf spots are the first sign of infection, developing rapidly into elongated orange to yellowish-brown pustules (uredinia), 3-8 mm by 1-2 mm, in patches, mainly on the underside of leaves (Photos 1&2). As the pustules mature, masses of orange spores (urediniospores) are produced. In severe infestations, leaves become covered with pustules and die early. From a distance, crops appear brown.

This rust has only one type of spore. Orange rust is favoured by warm, humid weather (occurs in wet season), in late summer and autumn.

Puccinia melanocephala is similar: elongate yellow spots enlarging to 5-10 mm by 1-2 mm and becoming reddish-brown pustules (uredinia). As they split, the orange-brown spores are released (urediniospores). From a distance the leaves appear reddish.

This rust produces a second spore type, teliospores. Common rust is favoured by cool nights with dew, and warm sunny days (occurs in dry season), in spring and early summer.

Both species are spread by wind, and in wind-blown rain (perhaps more than 2000 km). The outbreak of Puccinia kuehnii in Queensland in 2000 showed how rapidly this rust can spread on susceptible varieties and where environmental conditions are favourable. The epidemic that occurred in Florida in 2007, is thought to have resulted from the movement of urediniospores from Africa.

Until 2000, orange rust was thought to be a minor disease of low economic impact in Queensland, Australia. In that year, a new strain appeared infecting a widely grown variety, Q124, occupying 45% of the production area. Yield losses of 35-40% were reported in some crops, and in 2000 alone financial losses were estimated at more than $A200 million. Additional costs accrued over several seasons from a need to change to resistant varieties. In Australia, it has been found that if even 10% of the leaf area of the 5th leaf from the spindle is infected that is enough to affect yield. 

In June 2007, an outbreak of the rust also occurred in the State of Florida, USA, on commercial varieties (mostly CP 80-1743 and CP 72-2086) which accounted for about 30% of the crop. Further epidemics occurred in subsequent years. Overall, losses were estimated to be about 10%, but over 40% on the susceptible variety, CP80-1743. Since that time, the rust has been identified in several countries in Central and South America and the Caribbean.

Impacts from common sugarcane rust, Puccinia melanocephala, have been equally severe. Yield losses have compared yields of varieties before and after introduction of the rust and from fields trials. Indirect costs have come from the need to breed new varieties and to replace those susceptible to infection. Wherever these factors have been considered - Cuba, Mexico, Florida, Hawaii - economic impacts have been considerable, up to 40% lost yield depending on the environment, variety, and age when infected.

Differences exist between sugarcane orange rust and sugarcane common rust in the size and colour of the pustules and spores (Photo 3). Additionally, when viewed under a microscope, the spores of Puccinia kuehnii are thicker at the apex. However, these differences are not large, and it is recommended that identifications are done by experts with knowledge of rust taxonomy.

BIOSECURITY
Because rust diseases of sugarcane are unevenly distributed throughout the world, all efforts should be made to prevent their further spread. The movement of untreated consignments of sugarcane internationally for commercial purposes (food or planting) should be avoided. Exchanges of germplasm belonging to Saccharum species for crop improvement programs should follow the FAO/IBPGR Technical Guidelines for the Safe Movement of Sugarcane Germplasm. Frison EA, Putter CAJ (eds.). 1993. Rome, Italy: (https://www.bioversityinternational.org/fileadmin/_migrated/uploads/tx_news/Sugarcane_259.pdf). The Guidelines stipulate the therapy and indexation that should be used. 

CULTURAL CONTROL
Not a method of control for either Puccinia kuehnii or Puccinia melanocephala.

RESISTANT VARIETIES
The majority of commercial varieties have resistance to both these diseases. The outbreak of Puccinia kuehnii in Australia appears to have been a genetic change in the rust, such that a widely grown variety became susceptible to infection. Fortunately, most varieties in the breeders' collection were resistant. In Florida, where a similar outbreak occurred, the situation was somewhat different. The rust was newly introduced, and most varieties were susceptible; this has meant a search for resistance and a variety development program to produce new sugarcane clones.

CHEMICAL CONTROL
Chemical control of orange and common rust is not usually required as varieties are resistant to infection, and the economics of production cannot sustain their use. However, the 2000 epidemic in Australia required growers to use them as they changed from the susceptible variety, Q124, to others with resistance. In this case, tebuconazole and mancozep were registered. To assist growers, they were informed when periods of rust infection were likely from monitoring of weather data.

In Florida, strobilurins (pyraclostrobin and azoxystrobin), triazoles (metconazole and propiconazole) and carboxamides, have been registered for use against the rust, with the strobilurins showing best efficacy. Fungicides are only recommended on susceptible varieties. Several applications at 3-4-week intervals may be required to obtain disease control.

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When using a pesticide, always wear protective clothing and follow the instructions on the product label, such as dosage, timing of application, and pre-harvest interval. Recommendations will vary with the crop and system of cultivation. Expert advice on the most appropriate pesticides to use should always be sought from local agricultural authorities.