Coffee is the name give to several species of plant in the genus Coffea (family Rubiaceae), including C. arabica and C. canephora which are cultivated for their beans (seeds) that are used to make the stimulatory drink. Coffee plants are small evergreen trees or shrubs often with multiple stems and smooth leaves. The leaves are oval in shape and dark, glossy green. Coffee plants produce clusters of cream-white flowers and a fruit, commonly referred to as a berry, which normally possesses two seeds. The fruit is green to begin with but ripens to a crimson red and turns black when dry. C. canephora can reach a height of 15 m (49 ft), but C. arabica is smaller, reaching only 4–5 m (13–16 ft). The trees can live for 20–30 years. Coffee may also be referred to as Arabian coffee (C. arabica) or robusta coffee (C.cenephora) and originates from Africa.
Coffee beans are usually cured, roasted and ground before being brewed with hot water to produce the coffee beverage. The ground beans are often dehydrated to produce instant coffee.
Coffee plants grow best in warm, humid environments. Arabica coffee varieties grow best at temperatures between 18 and 22°C (64–72°F), while Robusta coffee grows best in slightly warmer temperatures of 22–26°C (72–79°F). The plants do not tolerate cold and freezing temperatures will kill the leaves instantly. The plants can grow in a wide range of soils but generally prefer a deep, well-draining loam with a pH between 5 and 6.
Most coffee varieties are self pollinating and can be propagated from seed. Coffee seeds are usually pre-germinated in sand beds before planting in a nursery. The seeds are spread in the sand and covered to keep them moist. The seeds usually germinate within 4-5 weeks. When shoots begin to emerge, they are removed and planted in polyethylene bags or prepared nursery beds. The seedlings are grown in shade and are are ready to be transplanted when they are 6 to 12 months old.
Young coffee plants are planted in the field in large, pre-dug holes. (50cm × 50cm × 50cm). Various spacings are used in different regions of the world and coffee can be grown as hedgerows or in high density squares depending on the variety. Coffee will usually grow best if planted in rows and plants should be spaced 3 m (9.8 ft) apart. Young trees are delicate and require protection by shading. Shade trees are usually planted before the coffee trees are transplanted (up to a year in advance).
Water-soaked spots on leaves which dry out and become brown and necrotic with yellow halos; necrosis of shoot tips which spreads rapidly down branches; leaves turn black and die off but remain attached to tree
Disease can be spread long distance by the movement of infected seedlings or within the field by water splash; bacteria can enter the plant through wounds
Protective sprays of copper should be applied to the plants just before the onset of the rainy season and should be continued right through to the short rains
Wilting and yellowing of foliage, often at end of twigs and branches (termed "flagging"); o pin sized hole can often be found on the underside of the flagging stems or twigs where the insect has entered the plant; twigs and stems are hollowed out and can be seen by cutting open the affected tissue; the adult beetle is small and black, approx. 2 mm in length and is rarely seen; eggs and pupae are creamy white in color
Damage caused by the beetles promotes secondary infestation by bacteria and other fungi; adult beetles overwinter in the plant
Prune out infested twigs and stems and destroy; flagging branches should be pruned back a few inches from the beginning of symptomatic areas; adequate fertilizer and irrigation to ensure vigorous plants can speed recovery from pruning injury
Brown spots on foliage which enlarge and develop gray-white center and a red-brown margin; lesions may also be surrounded by a yellow halo or may have a burned appearance if lesions are very numerous; infected leaves may drop from plant prematurely; lesions on green berries are brown and sunken and may have a purplish halo; infected red berries may have large black sunken areas
Disease can be spread by wind, water-splash and through human movement through fields, particularly when plants are wet
Ensure crop is adequately fertilized as nutrient deficient plants are more susceptible to the disease; remove all crop debris from filed after pruning to prevent build up of inoculum; good plant spacing and pruning to open up the canopy promotes good air circulation around foliage and protects against disease; if disease does occur then it can be controlled with the use of copper fungicides where available
Fruit dropping from plants; small holes may be evident on red cherries; when the insect is feeding, debris is pushed out of the hole and forms a brown or grey deposit on top of the hole; adult beetle can be found by cutting open the berry; adult is a tiny black beetle approx. 1.5-2.5 mm in length; larvae are white grubs with brown heads
Female beetle lays clusters of eggs inside the berries; insect undergoes up to 5 generations per year
Removal of dropped berries and debris on plantation floor can help reduce sources of new infections; remove any berries remaining on plants after harvest; insecticide application is only effective if applied when the female beetle is still in the entry tunnel and has not yet penetrated deep into the berry
Dark sunken lesions on green berries; berries dropping from plant; mummified berries
Very serious diseases; can destroy up to 80% of crop
Protective sprays of copper containing fungicides can help to control the disease; any diseased berries should be removed from plants; resistant varieties are available and should be planted in areas where disease is present
Small, pale yellow spots on upper leaf surfaces followed by powdery orange-yellow lesions on the undersides of leaves; symptoms commonly develop on lower leaves of plant first and then spread; infected leaves drop from the plant and twigs and branches become defoliated
Origins and spread
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).
The collapse of the coffee industry in Ceylon
In the nineteenth century, Ceylon was one of the largest coffee producing regions in the world, responsible for the export of approximately 42 million kilos of coffee per year. In the 28 years following the arrival of rust, export ceased and production was reduced to less than 3 kg/year. It wasn’t until 1879 that the government of Ceylon set up a commission to investigate the crisis and the British government sent Harry Marshall Ward to the plantations. Ward’s work on the coffee rust fungus would establish him as one of the most important figures in the field of plant pathology. Ward was able to link the collapse of the coffee crop to the Hemileia vastatrix fungus and, identify characteristics of both the fungal spore and agricultural practices that caused such a catastrophic loss. Unfortunately the investigation came too late and the rust epidemic was too far advanced. Ward could do little other than document the complete collapse of the coffee crop, as has been recounted in many histories of the disease (Large, 1940, Carefoot and Sprott 1967, Money 2007). Ward’s observations however, would provide the crucial basis for the development of future control strategies, discussed below.
Biology and ecology of coffee rust
The collapse of the Sri Lankan coffee industry and Ward’s investigation of the agricultural practices being employed highlighted the problems created by planting coffee at such high densities. The proximity of the plants to one another created optimal conditions for rust transmission over short distances while the reduced genetic diversity resulting from the practice of monoculture meant that once the rust pathogen broke down the inherent host resistance, little could be done to prevent its spread. The pathogen, Hemileia vastatrix, evolved within the forest and adapted to the widely dispersed nature of the wild host by producing highly mobile spores that are capable of travelling large distances via wind currents, water splash and on the bodies of insects. The practice of removing native trees to plant coffee side by side, removed a natural barrier to the movement of the rust spores and helped compound the catastrophic crop losses witnessed in Ceylon.
Rust transmission and infection
Coffee leaf rust is an obligate parasite and is transmitted when urediniospores (spores produced from the brown-red rust pustules) disperse from one part of the plant to another, or to a new, uninfected plant. The spores are produced on the underside of the leaf from uredinia which make up part of the red/orange pustules on the undersides of the leaves. When the spores erupt, they enter the air current where they can travel a few centimeters to the next leaf, or hundreds of kilometers to another site (spores have been recorded travelling 1,000 m up in the high altitude air streams). The spores are also known to travel over shorter distances by rain-splash, which is a common way for plant pathogens to travel from leaf to leaf of the same tree. There are also documented cases of spores being transported to new sites by small insects such as Thrips and parasitoid wasps.
When the spores reach a leaf, they attach to the surface using the spines on their rough side. In order for the spores to germinate, they require the presence of liquid water on the leaves and a temperature of 17 to 25°C (62.6 to 77°F), with 22°C (71.6°F) being optimal. Heavy rains can wash the spores from the leaves and prevent infection occurring. When conditions are favorable, the spores produce a long tubes known as germ tubes which move over the leaf searching for a stomata (tiny openings in the leaf surface where plants breathe and release water). The germ tubes produce appressoria (flattened fungal structures that produce ‘pegs’ to puncture through host tissues) on, or close to the stomata, from which infection hyphae grow and puncture the host cells. The entire infection process is completed in 24 to 48 hours and new urediniospores erupt from the stomatal openings after 10 to 14 days. One rust lesion will produce 4–6 spore crops over a 3–5 month period releasing 300–400,000 spores into the environment to repeat the process.
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 4th 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.
Commercially grown coffee has, through the practice of monocultures, lost much of the genetic diversity of its wild ancestors. Sadly, due to the effects of deforestation, wild coffee has also lost much of its genetic diversity outside of its evolutionary center in Ethiopia. The breeding of crop varieties which are resistant to key pathogens has proven to be a very successful method of controlling diseases and inIn the late 1950s, a natural coffee hybrid was discovered growing wild in East Timor. The plant was found to be a hybrid of C. arabica and C. canephora and was named Hibrído de Timor (HDT). The plant was found to possess full or partial resistance to all known races of the rust pathogen and five genes were subsequently elucidated from the hybrid and from other coffee varieties that were responsible for conferring the resistance Varieties expressing some of these genes have been grown commercially but the the resistance was broken-down after a few years when new virulent races of the rust pathogen emerged. Crosses of the hybrid with other commercial cultivars produced the ‘Colombia’ cultivar which is now widely planted. Colombia managed to reduce its losses during the 2012/13 epidemic because of new plantings. Many Colombian farmers are now replanting with Castillo or Colombia varieties.
Copper-containing fungicides remain one of the most effective and economical methods of controlling the rust pathogen in susceptible coffee varieties and during conditions which are favorable to the development of rust. They have the added advantage of being active against a number of other fungal pathogens and have also been shown to increase coffee yields. Examples of copper-containg fungicides used in coffee include copper oxychloride and cuprous oxide which have largely replaced the use of Bordeaux mixture in most commercial plantations. These chemicals are applied protectively with plants being sprayed in advance of infection and work by adhering to the plant and producing a toxic barrier to invading fungal pathogens. They pose limitations due to their need to be reapplied at regular intervals to protect new growth flushes and also pose environmental concerns over the accumulation of copper to toxic levels in the soil. Copper-containing fungicides can be alternated with systemic fungicides to reduce the amount of copper build-up.
Systemic fungicides used in coffee include pyracarbolids such as triadimefon and propiconazole and strobilurins such as azoxystrobin. Systemic fungicides are transported around the plant in the vascular tissue after application thus requiring lower doses and less frequent application than copper-based fungicides. They can be applied after infection has occurred to treat the symptoms of the disease and eradicate it from the host plant. Systemic fungicides tend to be more expensive and some have been shown to induce severe defoliation of the coffee plant. They have been shown to be very effective at controlling rust when used in combination with copper-containing fungicides.
Only one organic fungicide is widely used in coffee - triadimefon. Triadimefon is a systemic fungicide which is applied to the foliage and works to inhibit the rust infection. It can be alternated or combined with other chemicals and is generally very effective at controlling rust infections.
Organically certified control methods
Most commercially grown coffee varieties are susceptible to coffee rust fungus and because organic farmers cannot use chemical approaches controlling the rust is extremely hard. (note that in some growing regions copper based fungicides are allowed). Here we discuss a few methods and we encourage others to share knowledge by emailing PlantVillage or answering questions on the forum.
i) Planting spore traps. The fungal spore has a rough side that attaches to plant tissue. Wind-break trees can be used to reduce the spore load. Organic coffee is often grown using shade trees which may act to reduce inoculum reaching the coffee plants.
ii) Spraying organic formulations that impacts the ability of the spore to germinate or of new spores to be produced. We have heard that some farmers had success with this strategy but we do not know the details. Dr Peter Baker of CABI has reported to us that some farmers are using lime sulphur because of the expense of copper. We will try and find more information. Please contact PlantVillage if you have information.
iii) Spraying water. It is feasible that high pressure water can wash the spores from the leaves and reduce the spore load. Heavy rains may also have the same effect. As humidity on the leaves actually promotes fungal growth then washing is best done when the water is likely to evaporate.
Biological control is the use of one living organism to control another living organism that is considered a pest species. In addition to breeding new and better genetic material and the use of good crop husbandry, the development of an effective biological control strategy could provide another tool to manage coffee rust which would allow for organic certification and the continued use of heirloom varieties. If a suitable agent(s) can be identified in the short term, then this approach would be available in significantly less time than that needed to develop a new variety. CBC of fungi exploits the ability of coevolved fungal natural enemies in order to produce massive quantities of inoculum on the host plant and allow them to spread and propagate continuously within the host population. It offers a sustainable control method but has and has, surprisingly, never been used for crop pathogens (diseases). The concept is simple and follows the enemy-release hypothesis whereby an exotic or alien species increases its fitness, and hence its invasiveness, because it arrives without its guild of co-evolved natural enemies.
Bacteria such as Bacillus and Pseudomonas are known to produce compounds that negatively affect fungal pathogens of plants. Such bacteria evolved in the soil and utilize antifungal compounds to compete with soil dwelling fungi. A number of studies have shown how coffee rust development in greenhouse settings or in the lab can be retarded by Bacillus and Pseudomonas. For example, a study by Haddad et al, 2009 showed for the first time that certain strains ofBacillus and Pseudomonas reduced coffee rust on organic farms in Brazil. In follow up work the same team (Haddad et al 2014) found 17 different bacterial isolates collected from leaves, leaf debris, and soil reduced both the infection frequency and the number of H. vastatrix urediniospores produced per leaf by more than 70%.
ii) Other fungi
White halo fungus, Lecanicillium lecanii, has been suggested as a potential biological control agent of coffee rust by Prof. John Vandermeer and collaborators at the University of Michigan (Vandermeer et al 2009). White halo fungus has been shown to be hyperparasitic on Hemileia vastatrix in laboratory conditions and it has also been observed attacking the fungus in the field. White halo fungus often infects green coffee scale which feed on coffee. These insects are frequently tended by ants which collect the sugar that they excrete. The ants often create clusters of scale insects on the plants which are infected with white halo disease. It is postulated that white halo fungus may attack and kill the coffee leaf rust fungus or may simply reduce its abundance due to crowding effects or produce chemicals to attack it. Currently, the fungus does not appear be a viable biological control agent because it has not evolved to parasitize the fungi, rather it evolved to infect insects. Promising attempts have been made to culture the fungus and apply it as a topical spray to control the rust fungus.
Currently, no CBC program has focused explicitly on controlling coffee rust but pathogenic rusts have themselves been used to control other pests. For example, rubber vine is considered to be a major pest plant in Australia as it is highly invasive and causes millions of dollars of damage to agriculture and massive ecological damage. A team led by Dr. Harry Evans, a scientific officer with CAB International, identified a rust called Maravalia (which is taxonomically close to coffee rust) in the center of genetic origin for rubber vine in Madagascar which showed potential for use as a CBC agent. Before the rust could be released in the environment in Australia, it had to be quarantined. This process removed the rust from its natural enemies and had the effect of making the rust fungus extremely pathogenic. Dr. Evan’s stated that the rust went ‘berserk’ and when it was eventually deployed in Australia, it was extremely successful at controlling the rubber vine, even killing off young seedlings. In 2014, another team led by Dr. Evans and Dr. Roberto Barretto of the Federal University of Vicosa in Brazil will begin to explore genetic centers of origin of Arabica coffee with the aim of identifying similar co-evolved natural enemies of Hemileia vastatrix. It is believed that CBC holds great promise for the future control of coffee rust.
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