|Symptoms of coffee rust caused by Hemileia vastatrix on foliage|
Berk. & Broome (1869)
Wardia vastatrix J.F.Hennen & M.M.Hennen (2003)
Hemileia vastatrix is a fungus of the order Pucciniales (Previously also known as Uredinales) that causes coffee rust (roya disease), a disease that is devastating to coffee plantations. Coffee serves as the obligate host of coffee rust, that is, the rust must have access to and come into physical contact with coffee (Coffea arabica) in order to survive.
The mycelium with uredinia looks yellow-orange and powdery, and appears on the underside of leaves as points ~0.1 mm in diameter. Young lesions appear as chlorotic or pale yellow spots some millimetres in diameter, the older being a few centimetres in diameter. Hyphae are club-shaped with tips bearing numerous pedicels on which clusters of urediniospores are produced.
Telia are pale yellowish, teliospores often produced in uredinia; teliospores more or less spherical to limoniform, 26–40 × 20–30 µm in diameter, wall hyaline to yellowish, smooth, 1 µm thick, thicker at the apex, pedicel hyaline.
Urediniospores are more or less reniform, 26–40 × 18-28 µm, with hyaline to pale yellowish wall, 1–2 µm thick, strongly warted on the convex side, smooth on the straight or concave side, warts frequently longer (3–7 µm) on spore edges.
Hemileia life-cycle begins with the germination of uredospores through germ pores in the spore. It mainly attacks the leaves and is only rarely found on young stems and fruit. Appressoria are produced which in turn produce vesicles from which entry into the substomatal cavity is gained. Within 24–48 hours, infection is completed. After successful infection, the leaf blade is colonized and sporulation will occur through stomata. One lesion produces 4–6 spore crops over a 3–5 month period releasing 300–400,000 spores.
Probably it does not complete its life-cycle on Coffee tree, but lives on some stage on some other, as yet unknown plant (heteroecious life-cycle).
Hidden meiosis and sexual reproduction (cryptosexuality) has been found within the asexual urediniospores. This finding may explain why new physiological races have arisen so often and so quickly in H. vastatrix.
Hemileia vastatrix is an obligate parasite, that lives mainly on the plants of genus Coffea, reportedly also on Gardenia in South Africa. It needs suitable temperatures to develop (not less than 10 °C and not greater than 35 °C). The presence of free water is required for infection to be completed. Loss of moisture after germination has been initiated inhibits the whole infection process.
Sporulation is most influenced by temperature, humidity, and host resistance. The colonization process is not dependent on leaf wetness, but is influenced greatly by temperature and by plant resistance. The main effect of temperature is to determine the length of time for the colonization process (incubation period).
The fungus is of East African origin, but nowadays widely spread in Africa, tropical Asia, Central- and South-America. Coffee originates from high altitude regions of Ethiopia, Sudan and Kenya and the rust pathogen is believed to have originated from the same mountains. The earliest reports of the disease hail from the 1860s. It was reported first by a British explorer from regions of Kenya around Lake Victoria in 1861 from where it is believed to have spread to Asia and the Americas. Rust was first reported in the major coffee growing regions of Sri Lanka (then called Ceylon) in 1867 and the causal fungus was first fully described by the English mycologist Michael Joseph Berkeley and his collaborator Christopher Edmund Broome after an analysis of specimens of a “coffee leaf disease” collected by George H.K. Thwaites in Ceylon. Berkeley and Broome named the fungus Hemileia vastatrix, Hemileia referring to the half smooth characteristic of the spores and vastatrix for the devastating nature of the disease.
It is unknown exactly how the rust reached Ceylon from Ethiopia but over the years that followed, the disease was recorded in India in 1870, Sumatra in 1876, Java in 1878, and the Philippines in 1889. During 1913 it crossed the African continent from Kenya to the Congo, where it was found in 1918, before spreading to West Africa, the Ivory Coast (1954), Liberia (1955), Nigeria (1962–63) and Angola (1966).It spread from East Africa first to Sri Lanka and then West Africa before Uredospores are disseminated to long distances mainly by wind, and over short distances by both wind and rain. Other agents such as animals, mainly insects and man, occasionally have been shown to be involved with dissemination.
Historically found in coffee-growing areas of Africa, the Near East, India, Asia, and Australia, the disease was discovered in 1970 to be widespread in Brazil, marking the first known spread to the Western Hemisphere. Hence it is now found worldwide in virtually all coffee-producing countries.
The disease was first reported in Kenya in 1861. By 1869 it had spread to Sri Lanka, and by the 1920s it was widely found across much of Africa and Asia. In the last decades of the nineteenth century, coffee rust did serious damage to the coffee plantations of Sri Lanka, Philippines, Java and Malaya, which led to the collapse of the coffee industry in Lipa, Batangas and almost brought about the extinction of the Arabica variety. This led to increased cultivation of alternative crops, notably tea in Sri Lanka, and rubber in all three places. Liberica, a species cultivated in the Philippines since early in the 19th Century, was found to be resistant and thus rose to prominence in Philippine coffee production by the mid-20th Century. Liberica coffee grown in Batangas is locally known as "Barako" coffee, named after the laborers who worked harvesting and processing the coffee.
Roya disease is a big problem in coffee plantations in Peru, declared in sanitary emergency by government (Decreto Supremo N° 082-2013-PCM).
The 2012 Coffee leaf rust epidemic In 2012 there was a major increase in coffee rust across ten Latin American and Caribbean countries. The disease became an epidemic and the resulting crop losses pushed coffee prices to an all time high amid concerns for supply. The reasons for the epidemic remain unclear but an emergency rust summit meeting in Guatemala in April 2013 compiled a long list of shortcomings. These included a lack of resources to control the rust, the dismissal of early warning signs, ineffective fungicide application techniques, lack of training, poor infrastructure and conflicting advice. In a keynote talk at the “Let’s Talk Roya” meeting (El Salvador, November 4, 2013), Dr Peter Baker, a senior scientist at CAB International, raised several key points regarding the epidemic including the proportional lack of investment in research and development in such a high value industry and the lack of investment in new varieties in key coffee producing countries such as Colombia 
- Carvalho CR, Fernandes RC, Carvalho GM, Barreto RW, Evans HC (2011). "Cryptosexuality and the genetic diversity paradox in coffee rust, Hemileia vastatrix". PLoS ONE 6 (11): e26387. doi:10.1371/journal.pone.0026387. PMC 3216932. PMID 22102860.
- Waller JM, Bigger M, Hillocks RJ (2007). Coffee pests, diseases and their management. Wallingford, Oxfordshire: CABI. p. 171. ISBN 1-84593-129-7.
- Guatemala declares national coffee emergency February 08, 2013 BusinessWeek
- Coffee rust plagues farmers in Mexico; Climate change seen as a factor in spread of fungus, which puts many small growers at risk 26 March 2013 The Guardian
- Coffee Research Institute: Coffee rust
- University of Nebraska-Lincoln: Coffee rust
- The University of Hawaii page on Hemileia vastatrix 
- U.S.Dept.Agriculture page on Coffee Leaf Rust 
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