This study on the interactions between various cultivars of the black pepper plant (Piper nigrum L. ) and three of its pathogens, Meloidogyne Incognita (Kofoid & White) , Radopholus similis (Cobb) , Thorne and Fusarium solani f. sp. piperi Albuquerque was initiated to search for a biological cause of "yellow disease" of black pepper.This disease was described already in 1932 by Van der Vecht as "geelziektel" and its symptoms on the aerial parts of the pepper plants were yellowing of the leaves, always associated with a stiff droop. Its cause was suspected to be the nematode R. similis. Later, the data in the literature were inconsistent. Some authors thought a number of organisms to cause this disease, others suspected mineral deficiencies to induce the above mentioned symptoms.In the field, the disease is spreading from a few infected plants and gradually the patches with diseased plants become larger. This points towards a biological cause of this disease. Based on literature and field observations the three pathogens mentioned above were selected for this study.Some additional studies were necessary particularly with respect to R. similis.A method to rear R. similis under sterile conditions is described. It appeared that in 35 days an over 400 times multiplication of this nematode is possible when reared on sterile carrot discs at about 27 °C. An inoculum of good vitality and without contamination was obtained (Chapter 3). R. similis was already found in the roots of pepper plants 24 hours after inoculation. The nematode laid eggs inside the roots within five days. One month after the inoculation a clear destruction of tissues in the roots was found, and "gumlike" substances appeared to obstruct xylem vessels (Chapter 4).Only R. similis appeared to induce the typical symptoms of yellow disease. This was more pronounced, when M. incognitaand/or F. solani were also present.Infected plants were seriously reduced in growth when infected with R. similis, much more than when infected with either M. incognita or F. solani (Chapters 4, 5 and 8).When both R. similis and M. incognita, were present in the roots, the populations of both species developed less well than when they were inoculated separately. An even further reduction in nematode numbers was obtained when F. solani was inoculated also (Chapters 5 and 8). One of the causes might have been the inhibition of hatching of juveniles of M. incognita. A culture filtrate of F. solani appeared to inhibit this hatch of M. Incognita (Chapter 6).M. incognita appeared not to induce the typical symptoms of yellow disease. When this nematode was inoculated, alone or in combination with F. solani, flaccid wilting and yellowing of the leaves was found, but not the stiff droop without loss of turgor, typical for yellow disease.When the plants were inoculated with M. incognita two weeks prior to the inoculation with F. solani wilting and early death of the plants was observed. F. solani alone did cause wilting, but much less than in plants, also inoculated with M. incognita. The typical symptoms of yellow disease were not induced by F. solani (Chapter 5).Four cultivars of black pepper, viz. Kalluvalli, Kuching, Jambi & Cunuk, were tested on their response to all three pathogens. All cultivars appeared susceptible for these pathogens, but cv. Kuching was hardly affected by M. incognita. It appeared to possess tolerance to this nematode (Chapter 7).The soil moisture level appeared not only to influence the growth of the pepper plants, and nutrient levels in the leaves, but also to affect the yellowing of the leaves.Plants grown at 60% of field capacity showed less growth, compared to those grown at field capacity. Yellowing of the leaves was observed earlier under the drier conditions. At both soil moisture levels, there were sufficient minerals in the leaves. The positive effect of "mulch" on the growth of pepper can be understood since it maintains soil moisture at high levels. Yellowing could not have been the result of mineral deficiency.In the discussion the results found in this study were further compared to observations made in various parts of the world.The clear result in this study, that only R. similis induced the symptoms typical for yellow disease, also at nonlimiting mineral levels in the leaves, agrees with the findings in the field on Bangka. The patches and the gradual increase in size of these patches is consistent with the activities of R. similis. in the soil. The symptoms tended to be more severe under conditions of drought and low mineral levels. In our studies similar results were obtained. R. similis appears the prime cause of yellow disease of pepper, while other biological or non biological factors aggravate the symptom expression.The apparent tolerance of cv. Kuching to M . incognita gives a possible explanation for the wide spread and successful growth of this cultivar in Sarawak, where R. similis was not reported so far.The yellowing of leaves found in pepper in Brazil, where R. similis also is not reported to occur, can now be attributed to an interaction of M. incognita. and F.solani. Both pathogens are known to occur in pepper plantings in these regions. Field observations in Brazilian pepper plantings suggest a less severe yellowing than on Bangka. This is in agreement with the results found in our studies.The apparent differences in symptoms induced by various pathogens justify a distinction between yellowing of leaves associated with flaccid wilting, and yellowing of leaves associated with a stiff droop.The cause of the yellowing of pepper leaves in Brazil is different from the one on Bangka. It is not correct to call a disease of pepper with symptoms involving yellowing of leaves and flaccid wilting the "yellow disease of pepper".This term, "yellow disease" or "geelziekte", first used by van der Vecht in 1932, should be exclusively reserved for the disease caused by R. similis .