The objectives of studies undertaken here were to identify the species of Colletotrichum associated with coffee berry anthracnose in PNG, gain an understanding of the infection process and factors affecting it, assess the impact, if any, of anthracnose on coffee quality and identify suitable chemicals for anthracnose control. A total of 40 isolates were collected from PNG and their cultural and morphological characteristics on PDA were studied and used to identify the species. Species identification was further confirmed by molecular characterisation using RFLP and DNA sequencing of the ITS region of the rDNA. After species identification only two isolates were selected to represent C. gloeosporioides and C. acutatum for further studies. Studies on conidia germination and the effects of conidia concentration, temperature, relative humidity and pH affecting germination were done on TWA, followed by studies on the infection process on coffee berries and the influence of temperature on germination and appressoria formation in vivo. For assessment of effect of anthracnose on coffee quality, 100 samples of ripe berries were assessed for disease incidence, followed by processing of the berries to green bean and data on bean defects (black bean) together with anthracnose incidence subjected to appropriate statistical analysis. A similar procedure was followed using disease severity but the samples were from one farm only. The closing work on chemical control was done by screening 16 different fungicides for the control of C. gloeosporioides and C. acutatum. With the 40 isolates, 29 were identified as C. gloeosporioides and 11 as C. acutatum. This is the first report of C. acutatum on coffee in PNG. Identification of the species was further confirmed by RFLP groupings where C. gloeosporioides and C. acutatum were separated at 574bp and 584bp respectively and DNA sequence homology identified the PNG isolates with C. gloeosporioides and C. acutatum accessions. Optimum conditions for conidia germination in relation to spore concentration, temperature, and pH are 1 x 106 spores/ml, 21 - 29°C and pH 5 - 7 respectively for C. acutatum and for C. gloeosporioides 1 x 106 spores/ml, 25 - 31°C and pH 5 - 7 respectively. Humidity is not a limiting factor for activity of both species. Infection process for both species is similar where conidia germinate to produce the germ tube which swells at the tip to form the appressoria. The appressoria produce an infection peg which is responsible for berry cuticle penetration and cell colonisation (resulting in typical anthracnose symptom expression) and eventual sporulation. C. acutatum has not been reported elsewhere as a pathogen of coffee and this is the first report of C. acutatum causing infections on both ripe and mature green berries. Anthracnose incidence did not correlate well with coffee bean defects, but anthracnose severity v suggested that coffee quality could be affected by anthracnose. The most effective fungicide for anthracnose control is thiram alone or thiram alternating with propiconazole.
| Identifer | oai:union.ndltd.org:ADTP/254167 |
| Creators | Mark Kulie Kenny |
| Source Sets | Australiasian Digital Theses Program |
| Detected Language | English |
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