Integrated management of Phytophthora palmivora diseases of cocoa in Papua New Guinea. - (:unkn) unknown (Victoria : School of Botany, University of Melbourne, 1999)
Cocoa is a very important cash crop in Papua New Guinea comprising about 17 percent of the national agricultural revenue each year. Diseases of cocoa caused by P. palmivora are very serious wherever cocoa is grown with losses ranging from 10 to 90 percent worldwide. In Papua New Guinea, losses to Phytophthora palmivora diseases range from 20 to 40 percent, but estimates could be higher in different areas and at times of high rainfall. This thesis reports the findings of studies carried out to investigate the infection of the cocoa roots and the survival of P. palmivora on infected cocoa tissues under grass and natural leaf litter ground cover types. Cocoa pods which were used as baits for bioassay were the most desirable selective medium in the absence of artificial media. It was found that the pathogen attacks cocoa roots in PNG without causing any apparent external symptoms. Different strains of the same pathogen showed different levels of infectivity under different root germinating media. Relatively younger cocoa roots were more susceptible than relatively mature seedling roots. Infection of cocoa root spread relatively more rapidly towards the root tips than the stem. The investigation on survival of the pathogen on infected cocoa tissues remaining in the field illustrated that survival of the pathogen declined with time. The survival of the pathogen was longest on cocoa bark and shorter on pod and leaf respectively. The pathogen's survival on these infected host tissues declined more rapidly in leaf litter ground cover than grass ground cover. Infected cocoa pods in the cocoa canopy had the potential to supply infective inoculum until they were removed. The main survival structure of the pathogen in these mummified cocoa pods appeared to be chlamydospores which could still be recovered and remained viable six months later. There were different levels of recovery of the pathogen from infected cocoa pods of different genotypes. Higher levels of the pathogen were recovered from the clone KA2 - 101, a more susceptible genotype tha K82, a relatively less susceptible genotype. The pathogen was not recovered from cocoa tree bark although other fungi, such as Lasiodiplodia sp. and Colletotrichum sp. were commonly isolated during the study period, probably indicating that the pathogen was not present or that conditions necessary for its establishment and development were missing during the study period. A major and important finding from this study is that Scolytid and Nitidulid beetles bore holes into infected cocoa pods. The study revealed for the first time compelling evidence that these beetles have an important passive role as pathogen vectors in the canopy. The longer an infected cocoa pod is left on the tree in the canopy, the more beetles were attracted to ot. Not only was (1), inoculum supply potential from these infected pods prolongedbut (2), the longer an infected cocoa pod remained, the more inoculum it supplied. Field trials showed that removal of path and tent building ants and termites reduced the levels of Phytophthora pod rot. It further showed that where these insects were eliminated from the field, the removal of infected cocoa pods and healthy pod husks from the field was not necessary to effectively control cocoa pod rot (in a remote sense). Since rainplash can only take inoculum to 75 cm above the ground, and since primary infection still appeared in very clean fields where ants and termites were removed, it can be hypothesized that the flying beetles do have a very important role in the transmission of the pathogen. This thesis suggests that the flying beetles could account for some of the sources of inoculum which were placed in the "unknown" category by many past studies. Results from field trials using integrated disease management shows the benefit of combining cultural control methods (promoting mulch or litter under cocoa) with potassium phosphonate injection. The results further shows that the disease cycle o P. palmivora on cocoa is complex and numerous infection routes and sources exists. It indicates the need for combiningmany different methods of controlling the disease into a single integrated management package instead of applying different control methods in isolation. A series of strategies for use in integrated disease management are proposed.
Incluye referencias bibliográficas.
Cocoa is a very important cash crop in Papua New Guinea comprising about 17 percent of the national agricultural revenue each year. Diseases of cocoa caused by P. palmivora are very serious wherever cocoa is grown with losses ranging from 10 to 90 percent worldwide. In Papua New Guinea, losses to Phytophthora palmivora diseases range from 20 to 40 percent, but estimates could be higher in different areas and at times of high rainfall. This thesis reports the findings of studies carried out to investigate the infection of the cocoa roots and the survival of P. palmivora on infected cocoa tissues under grass and natural leaf litter ground cover types. Cocoa pods which were used as baits for bioassay were the most desirable selective medium in the absence of artificial media. It was found that the pathogen attacks cocoa roots in PNG without causing any apparent external symptoms. Different strains of the same pathogen showed different levels of infectivity under different root germinating media. Relatively younger cocoa roots were more susceptible than relatively mature seedling roots. Infection of cocoa root spread relatively more rapidly towards the root tips than the stem. The investigation on survival of the pathogen on infected cocoa tissues remaining in the field illustrated that survival of the pathogen declined with time. The survival of the pathogen was longest on cocoa bark and shorter on pod and leaf respectively. The pathogen's survival on these infected host tissues declined more rapidly in leaf litter ground cover than grass ground cover. Infected cocoa pods in the cocoa canopy had the potential to supply infective inoculum until they were removed. The main survival structure of the pathogen in these mummified cocoa pods appeared to be chlamydospores which could still be recovered and remained viable six months later. There were different levels of recovery of the pathogen from infected cocoa pods of different genotypes. Higher levels of the pathogen were recovered from the clone KA2 - 101, a more susceptible genotype tha K82, a relatively less susceptible genotype. The pathogen was not recovered from cocoa tree bark although other fungi, such as Lasiodiplodia sp. and Colletotrichum sp. were commonly isolated during the study period, probably indicating that the pathogen was not present or that conditions necessary for its establishment and development were missing during the study period. A major and important finding from this study is that Scolytid and Nitidulid beetles bore holes into infected cocoa pods. The study revealed for the first time compelling evidence that these beetles have an important passive role as pathogen vectors in the canopy. The longer an infected cocoa pod is left on the tree in the canopy, the more beetles were attracted to ot. Not only was (1), inoculum supply potential from these infected pods prolongedbut (2), the longer an infected cocoa pod remained, the more inoculum it supplied. Field trials showed that removal of path and tent building ants and termites reduced the levels of Phytophthora pod rot. It further showed that where these insects were eliminated from the field, the removal of infected cocoa pods and healthy pod husks from the field was not necessary to effectively control cocoa pod rot (in a remote sense). Since rainplash can only take inoculum to 75 cm above the ground, and since primary infection still appeared in very clean fields where ants and termites were removed, it can be hypothesized that the flying beetles do have a very important role in the transmission of the pathogen. This thesis suggests that the flying beetles could account for some of the sources of inoculum which were placed in the "unknown" category by many past studies. Results from field trials using integrated disease management shows the benefit of combining cultural control methods (promoting mulch or litter under cocoa) with potassium phosphonate injection. The results further shows that the disease cycle o P. palmivora on cocoa is complex and numerous infection routes and sources exists. It indicates the need for combiningmany different methods of controlling the disease into a single integrated management package instead of applying different control methods in isolation. A series of strategies for use in integrated disease management are proposed.
Keywords: Enfermedades de las plantas.; Phytophthora.