|Symptoms of coffee rust caused by Hemileia vastatrix on foliage|
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 leaf rust (CLR), a disease that is devastating to susceptible 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 sp.) in order to survive. There is no cure at the moment, although farms have managed to reduce their impact by replanting infected farms with hybrids that have a strong genetic resistance to rust.
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 lifecycle 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 the stomata. One lesion produces 4–6 spore crops over a 3–5 month period releasing 300–400,000 spores.
While the predominant hypothesis is that H. vastatrix is heteroecious, completing its life cycle on an alternate host plant which has not yet been found, an alternative hypothesis is that H. vastatrix actually represents an early-diverging autoecious rust, in which the teliospores are non-functional and vestigial, and the sexual life cycle is completed by the urediniospores. Hidden meiosis and sexual reproduction (cryptosexuality) has been found within the generally asexual urediniospores. This finding may explain why new physiological races have arisen so often and so quickly in H. vastatrix.
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 starts 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, and 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. 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. 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).
Uredospores are disseminated across 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.
The disease was first described and named by Berkley and Broom in the November 1869 edition of the Gardeners Chronicle.:171 They used specimens sent from Sri Lanka, where the disease was already causing enormous damage to productivity. Many coffee estates in Sri Lanka were forced to collapse or convert their crops to alternatives not affected by CLR, such as tea.:171–2 The planters nicknamed the disease "Devastating Emily" and it affected Asian coffee production for over twenty years. By 1890 the coffee industry in Sri Lanka was nearly destroyed, although coffee estates still exist in some areas. Historians suggest that the devastated coffee production in Sri Lanka is one of the reasons why Britons have come to prefer tea, as Sri Lanka switched to tea production as a consequence of the disease.
By the 1920s CLR was widely found across much of Africa and Asia, as well as Indonesia and Fiji. It reached Brazil in 1970 and from there it rapidly spread at a rate enabling it to infect all coffee areas in the country by 1975.:171–2 From Brazil, the disease spread to most coffee-growing areas in Central and South America by 1981, hitting Costa Rica and Colombia in 1983.
As of 1990, coffee rust has become endemic in all major coffee-producing countries.:171–2
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 led to a fall in supply, outstripping demand. Coffee prices rose as a result, although other factors such as growing demand for gourmet beans in China, Brazil and India also contributed.
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.
Coffee Leaf Rust (CLR) has direct and indirect economic impacts on coffee production. Direct impacts include decreased quantity and quality of yield produced by the diseased plant. Indirect impacts include increased costs to combat and control the disease. Methods of combating and controlling the disease include fungicide application and stumping diseased plants and replacing them with resistant breeds. Both methods include significant labor and material costs and in the case of stumping, include a years-long decline in production (coffee seedlings are not fully productive for three to five years after planting).
Due to the complexity of accurately accounting for losses attributed to CLR, there are few records quantifying yield losses. Estimates of yield loss vary by country and can range anywhere between 15-80%. Worldwide loss is estimated at 15%.
Some early data from Ceylon documenting the losses in the late 19th century indicate coffee production was reduced by 75%. As farmers shifted from coffee to other crops not affected by CLR, land used for growing coffee was reduced by 80%, from 68,787 to 14,170 ha.
In addition to the costs mentioned above, additional costs include research and development costs in producing resistant cultivars. These costs are normally borne by the industry, local and national governments and international aid agencies.:174
Colombia’s National Federation of Coffee Growers (Fedecafe) set up a research lab specifically designed to find ways to stop the disease, as the country is a leading exporter of the Coffea arabica bean that is particularly prone to the disease.
CLR disease is a big problem in coffee plantations in Peru, declared in sanitary emergency by government (Decreto Supremo N° 082-2013-PCM).
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- 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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