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
Dark sunken lesions on green berries; berries dropping from plant; mummified berries
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
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
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
. For example, a study by Haddad et al, 2009 showed for the first time that certain strains of< i>Bacillus 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.