Graduate Thesis Or Dissertation
 

Studies of northern root-knot nematode (Meloidogyne hapla, Chitwood 1949) biotypes and some factors associated with their biology and pathogenicity

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  • Root-knot nematodes were among the earliest plant-parasitic nematodes to be recognized. Before 1949, they were considered as the one species Heterodera marioni (Cornu 1887) Goodey 1932. In 1949 Chitwood completed a taxonomic study of the root-knot nematodes and placed them in a separate genus Meloidogyne. Economically they are one of the most important group of nematode pests and their distitibution is cosmopolitan. As in many other parasitic organisms, biotypes or physiological races exist within various species of root-knot nematodes. Most frequently pathogenic characters on given hosts have been used in making distinctions of biotypes. Meloidogyne hapla, Chitwood 1949 is a root-knot nematode species found mainly in the temperate regions. It is widely distributed in the northwestern USA on numerous hosts, but it is also known that some populations of this nematode have different host preferences. In the biotype study, 14 of the 15 populations of M. hapla were collected from northwestern USA; one population came from eastern USA. These populations were exposed to various hosts in the greenhouse at temperatures averaging 73.4 F day and 62.6 F night. Five biotypes were established through pattern of infection on various hosts. The resistance expected of two graminaceous plants, corn and oat, was broken by two M. hapla biotypes. A test was made to cross males of M. haplabiotype Five (Quincy #1) and females from M. haplabiotype Three (Ontario). Larvae that hatched from eggs of this "bisexual union" did not show a physiological action different from larvae that hatched from eggs of the female parent alone. A few larvae from the "two parents" and from the female parent alone, penetrated roots of lettuce (Iceberg) but did not develop into egg laying females. It did not appear that root-knot nematode males altered the physiological character of the larvae. Tests were made to determine the influence of host age on resistance and susceptibility to M. hapla (biotype Five, Quincy #1). Results indicated that in a resistant alfalfa (65-298), newly germinated seedlings showed evidence of resistance to infection. This resistance increased with age in one-week and two-week seedlings. In susceptible alfalfa('Lahontan'), newly germinated seedlings, one-week and two-week seedlings were equally disposed to infection. Tests were made to determine the influence of pH ranges 4.7, 5.9 and 7.8 on resistance and susceptibility of plants inoculated with M. hapla (biotype Five, Dayton #1). All plants (resistant and susceptible alfalfa and susceptible Rutgers tomato) grew best in pH 7.8, good in pH 5.9 and poorly in pH 4.7. These pH ranges did produce marked differences in plant growth but did not affect their basic nematode resistance or susceptibility. Influence of pH on nematodes appeared to be indirect via the host plant in which the nematodes thrive.
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