|Crop Knowledge Master||Fungi|
|coffee berry disease (Plant Disease Pathogen)|
Stephen A. Ferreira, Extension Plant Pathologist
Rebecca A. Boley, Educational Specialist
Department of Plant Pathology,CTAHR
University of Hawaii at Manoa
Occurs on all Coffea species. Several species or strains of Colletotrichum occur on coffee, but only C. coffeanum causes coffee berry disease. On the immature green berry the disease is also known as brown blight.
In one study with isolates from 28 hosts (non-Rubiaceae or coffee family hosts), none were shown to be pathogenic on coffee beans. Similar results were obtained for isolates from coffee twigs, leaves, flowers, and ripe berries; these isolates were not pathogenic on green berries. These results suggest that host specialization exists among Colletotrichum strains, and that CBD isolates occur only on coffee.
Coffee Berry Disease was first reported from Kenya in 1922. It has since been recorded from most of the other coffee areas in Africa. It is not known outside of Africa, although a leaf spot and ripe berry anthracnose caused by related Colletotrichum species has been reported from Guatemala and Brazil.
Although this pathogen is capable of infecting leaves, stem bark, and twigs of the coffee plant, only the strain or species of Colletotrichum that can infect immature or green berries is the causal organism of CBD. Direct losses occur as a result of flower and young fruit infection. Flowers are susceptible at all developmental stages from the pale green unopened spike. Immature fruit or berries are most susceptible during their expansion phase which occurs from 4-14 weeks after flowering. Earlier berry stages or pin head berries and the mature green beans are fairly resistant, becoming susceptible again as the berry begins to ripen.
CBD on green berries has two forms. Active lesions are initially evident as small dark sunken spots which spread rapidly to involve as much as all of the berry. The pathogen sporulates readily and is evidenced by a pale pink crust on the lesion surface. If infection occurs early and climatic conditions favor disease development, berry development is arrested, resulting in mummified berries on the fruiting branch. When the berry ripens and anthracnose fully develops, the bean can become infected and seed borne.
In some instances, scab lesions can also occur. They develop slowly and are buff colored. The pathogen grows sparsely and sporulates poorly or not at all. Some researchers consider this scab reaction to be a resistant reaction, but is not known conclusively at this time.
Damage and Losses:
Where the virulent strains of CBD occur, serious losses have been reported. When first reported from Kenya, 75% losses were noted in some farms. The disease has been responsible for the abandonment of coffee growing in several districts of Kenya and Ethiopia. In other areas, losses of up to 80% are not unusual. More conservative estimates of losses occurring in Kenya are 20%. Successful fungicide control programs frequently double or triple yields (Griffiths, 1971)
Species and Strains:
Several species or strains of Colletotrichum have been reported from coffee. The organism pathogenic on green coffee berries is C. coffeanum while the other strains have been identified as C. gloesporioides (perfect stage=Glomerella cingulata) and in some instances C. acutatum (Hindorf, 1970, 1973a,b). None of these latter strains are pathogenic on green berries and many occur epiphytically in the bark. The different coffee strains of Colletotrichum are distinguished on the basis of colony characters while growing on agar and conidial dimensions. The CBD strains have a green to dark mycelium after 2-3 days on all media used (coffee bean extract agar, carrot agar, and coffee wood blocks) while the saprophytic forms exhibit white mycelium. It has been observed to produce abundant greyish black aerial hyphae on prune agar and was quite variable in spore shape from oval through elliptic to somewhat irregularly clavate, and was different from other species (strains) of Colletotrichum occurring on coffee.
Many workers have clearly demonstrated the existence of at least 4 strains (species) of Colletotrichum from coffee on the basis of morphology, but only one of them is capable of causing CBD or green berry anthracnose. In culture, strains are not stable and numerous color and morphological variants have been observed. Pathogenicity on green berries is a trait that CBD strains can lose.
Non CBD strains of Colletotrichum can be recovered easily from all coffee tissues and have been reported from all coffee growing areas. Many of them are able to cause a ripe berry anthracnose which also occurs in many coffee growing regions. None of these strains are CBD strains, and the vast majority of these strains are considered to be C. gloeosporioides according Hindorf.
Conidial Germination and Infection:
Spores germinate to produce germ tubes and appressoria from which infection pegs emerge to penetrate the surface cuticle. Typical black sunken anthracnose lesions of CBD initially appear about 1 week after inoculation and most by 2 weeks for berries 5-14 weeks after flowering. In the field, similar to slightly longer incubation periods were observed.
Infection and disease development is influenced by several factors: temperature, moisture, plant parts, and other microorganisms. Optimal temperature for germination and lesion development is 22 C, although the germination optimum could be increased to 27 C if nutrients were present. Maximum temperature for germination was 30 C (35 C with nutrients) with a minimum of 10 C. This information is based both on laboratory and field observation.
In practice, as with most Colletotrichum pathogens, germination and infection requires free water from rain, mist, or dew. Water is also required for dispersal of spores. Studies with alternating cycles of desiccation indicate that infectivity is generally reduced with desiccation and is completely lost after 10-12 days.
Certain tissues of coffee appear to be extremely susceptible to CBD, with flowers, young berries, and fully ripe fruit being particularly susceptible.
Saprophytic strains of Colletotrichum occur commonly in coffee, but exerted little effect on infection of the virulent CBD strains.
Because CBD isolates are rarely recovered from tissues other than berries, infected berries are the major source of inoculum. Control of latent infection, infection during the berry expansion when it is most susceptible, removal of infected or mummified berries in situations where overlapping crops occur due to bimodal flowering patterns, are the main components for developing disease management strategies. Also important to disease management is the control of inoculum dispersal. Sporulation occurs under high humidity conditions and under saturated conditions spores are readily exuded into water films on plant surfaces. Dry spore masses require thorough wetting before exudation and spread can occur. As a result, rain is a major environmental factor responsible for disease spread. Dew is generally not important for dispersal, but under certain conditions, and only in tops of trees, enough dew formation can occur resulting in water movement and dispersal of inoculum. In either case, dispersal is mainly downward and in the form of stem flow or droplets from diseased berries. Therefore, tree tops are extremely important as sources of inoculum. Wind blown rain also results in local dispersal from tree to tree or over relatively short distances.
Medium and long distance dispersal or spread must depend on vectors such as man (coffee pickers), birds, and possibly insects. Interestingly, in one study, after half an hour of coffee picking, up to 103 spores /cm2 could be washed from fingers and palms of coffee pickers.
Dispersal during periods of irrigation in dry weather was negligible probably because wetting was inadequate and few spores were exuded to be washed away with the irrigation cycle.
Differences in susceptibility to CBD are known. Cultivars considered to be resistance are: Geisha, Geisha 10, Blue Mountain, Rume Sudan, progenies of Hibrido Timor, and K7. Particularly susceptible cultivars are: Harar, SL selections, and Boubon.
Programs to develop resistant cultivars have been developed in Kenya and appear to be successful. Inoculation of seedlings and subsequent analysis of hypocotyl infection is the method for selecting resistant varieties. Because seedling susceptibility from resistant trees is known, this screening approach may be too severe a screening procedure. In Ethiopia, a program of selecting resistant cultivars from semi-wild and genetically diverse coffee populations has been used. Several of their hybrids show high levels of field resistance.
Hedgerow planting and improved pruning practices to open the canopy improves fungicide penetration and coverage. The more open canopy is also less conducive to prolonged wetting and spore exudation and spread, resulting in lower CBD incidence.
Since CBD is limited to Africa, precautions should be taken with coffee seed from this region which may be imported into countries free of the disease. In Hawaii, concern about importing beans from Africa for roasting and by potential growers interested in new cultivars from Africa could risk introducing CBD. While the risk of seed-borne infection occurring on the surface of beans is very small, mycelial penetration of the seed during the berry anthracnose phase of CBD could occur and survive processing and drying. If an infected seed is planted, systemic seed-borne inoculum could easily infect the bark of young seedlings and become established in mature orchards.
As an added precaution, seed imported for propagation purposes should only be harvested from healthy berries and treated with a fungicide seed dressing. Importation of cultivars should occur only through appropriate quarantine facilities.
Although several different and effective fungicides for control of CBD are known, their use in the field has been inconsistent. Numerous studies suggest that fungicide applications early in the season are effective only in those seasons when both flowering was early and the rainy season finished early. The key appeared to be protection of the immature crop throughout the rainy season. In years when flowering was normal or late, and the rainy period extended longer into the season, early season fungicide applications were ineffective and CBD became worse during the season.
Numerous fungicides have been evaluated for CBD control. The most effective are: 50% copper formulations, captafol, chlorothalonil, benomyl, thiophanate-methyl, thiabendazole, and dithianon. The newer systemic fungicides are also quite effective, but fungicide tolerance has been noted with carbendazim, cypendazol, and to a lesser extent benomyl. Mixture or rotation with nonsystemic protectant fungicides is recommended.
An interesting finding with respect to fungicide applications for CBD control has been noted. Although overhead applications of fungicides in coffee results in poor coverage, CBD control with this method was as good as conventional spraying that resulted in complete coverage. This success of overhead spraying probably occurred because fungicide was concentrated in the most important part of the tree where CBD inoculum is produced and from which further development within the canopy and possible spread to other trees is initiated. In addition, redistribution of fungicide from the top portion of the canopy occurred and was responsible for CBD control even though coverage with the fungicide was not as good with overhead sprays.
In some instances, fungicide applications have resulted in higher disease incidence. It has been reported that some plantations that never used fungicides for CBD have lower disease incidence than neighboring plantations with regular spray programs for CBD (Furtado, 1969). It is generally believed that changes in the coffee bark ecology for microorganisms antagonistic to the CBD pathogen may have resulted from repeated fungicide applications. It this is true, it may be possible to recover antagonistic organisms for possible use as biological control agents in order to manage CBD.
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Griffiths, E., J. N. Gibbs & J. M. Waller. 1971. Control of coffee berry disease. Annals Appl. Biol. 67:45-74.
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