Host-parasite relationships of the root-knot nematodes,meloidogyne incognita and m.javanica, on beans (phaseolus vulgaris)

PhD Thesis

Field, laboratory, and greenhouse experiments were carried out to study e££ect o£ nematicide application on bean (Phaseolus vulgaris) yields in Kenya; penetration o£ larvae ot:' Meloidogyne incognita and M. javanica in 6 bean varieties, and the subsequent development ot:' the nematode parasites within the hosts; and histopathology o£ the roots ot:' the hosts in£ected with the two parasites. The nematicides were applied to a dark-red t:'riablecLay soil heavily in:f'es ted with the two roo tknot nematod'e species, at Kenya Canners Plantation Ltd., and Kuraiha Estate, Thika. It was t:'oundthat, in the t:'irstt:'ieldexperiment at Kenya Canners Plantation Ltd., application ot:' D-D as a broadcast (overall) and a row treatment, and EDB applied as a broadcast treatment, signit:'icantlyreduced the number ot:' M. incognita and~. javanica larvae in the soil. EDB row treatment did not reduce larval populations. Signit:'icantyield increases (45-60%) were obtained with Kikara and Mexico 142 with broadcast and row application ot:' D-D, and broadcast application ot:' EDB. This is consistent with the larval population decreases in these treated plots. Nematicides did not increase the yield ot:' Marathon green bean. Yields ot:' this variety were unit:'ormlylow, presumably because the soil type was not conducive to good growth. In the second field experiment at Kuraiha Estate, D-D in the planting row did not significantly increase the yield of Kikara but a significant yield increase (32%) was obtained with Mexico 142. These results indicate that root-knot nematodes, in large numbers can reduce bean yields in Kenya by as much as 60%. In the preliminary laboratory and greenhouse experiments, non-freshly-hatched larvae of M. incognita and M. javanica, at inoculum .levels of 100 larvae, failed to penetrate root systems of Kikara, Mexico 142, Canadian Wonder, Masterpiece, Marathon, and Premier bean varieties after exposure periods of 12, 24, or 48 hours. When the exposure period was increased to 48 hours, penetration of 5 of the 6 bean varieties by freshly-hatched larvae of the nematode parasites occurred at inoculum .levels of 100, 150, 200 and 300. In Mexico 142, even at the inoculum level of 300 larvae, there was no penetration. Although larvae of the nematode parasites penetrated root systems in all 6 bean varieties, results showed that, when the experiment was conducted under .lowand variable room or greenhouse temperature conditions, no significant differences in penetration percentage existed between the varieties and the species. Results of a similar experiment conducted under controlled growth chamber conditions at higher temperatures indicated that highly signiricant dirferences at 0.1% level occurred between the two species. At temperatures of 2Jo and 29°C ror night and day respectively, M. incogni ta .larvae penetrated root systems in a.LL the 6 bean varieties in signiricantly greater numbers than~. javanica larvae. When the temperatures were raised to 250 and JOoC ror night and day r-e spect.Lve.Ly , significantly higher numbers or M. incognita larvae penetrated root systems of Kikara, Canadian Wonder, Masterpiece, Marathon, and Premier than M. javanica larvae. There were no significant differences between ~. incognita and Mo javanica larvae in penetration of root systems of Mexico 142. Within the hosts, it was found that rates of development rrom larvae to sexually dirferentiated nematodes occurred more rapidly in Mexico 142 for M. incognita and M. javanica, and in Masterpiece for M. javanica only, than in other bean varieties inoculated at the same time. However, the period taken for larvae of both nematode species to develop to the adult stage was identical in all 6 bean varieties. At growth chamber ambient temperatures of 25°C and JOoC night and day respectively, a high percentage or both species developed into females which laid eggs within 20 days after inoculation; these females completed their life cycle and a second generation was developing within 30 days in all varieties. A low percentage or the nematode species developed to males. It is concluded rrom this study that M. incognita and M. javanica can develop and reproduce rapidly on Kikara, Mexico 142, Canadian Wonder, Masterpiece, Marathon, and Premier bean varieties; hence, the varieties are highly suitable hosts or both nematode species. Under optimum temperature conditions ror rapid development or both species, at least 3 generations are possible on these bean varieties in any or the growing seasons in East Af'rica. -Results' or the histopathological studies revealed extensive damage to the cortical and vascular tissue or roots of' the 6 ,bean varieties inrected with M. incognita and M. javanica. These studies showed that all 6 bean varieties were highly susceptible to both nematode species; the nematodes red in the vascular area and caused the f'ormation or 3-20 multinucleate giant cells around a single f'emale. It was also shown that disruption of' the stele by the giant cells, prolirerated parenchyma cells, and the parasites themse.lves caused the conducting tissues to be scattered, so that, in cross section, the phloem and xylem occurred in irregular patches rather than in one continuous column. While most or the giant cells were f'ound in the vascular cylinder in all 6 bean varieties, giant cells were also observed in the cortex or between the pericycle and endodermis in Mexico 142, Canadian Wonder, and Premier bean roots. A type of' tissue designated "abnormal xylem", .which is characterized by secondary wall thickenings of annular, reticulate, or pitted types, was observed associated with giant cells. No nuclear division was observed in giant ce.lls in ·this study in any roots of' the bean varieties; it was assumed that nuclear division in giant cells was either not occurring at the time the tissues were killed and fixed, or did not exist in the root cells of' the bean varieties.