Occurrence of Pratylenchus sp. and the successive growing failure of chrysanthemum were examined at 3 locations in Shizuoka. Plant symptoms and the nematode population level were different depending to the location as described below. In the Hamamatsu area, chronic growth retardation and acute wilting after a heavy rainfall were apparent. This wilting seldom occurred in the fields which had been tilled deeply and/or in those where population level of Pratylenchus sp. had been kept low. Negative correlation as well as a high regression coefficient was obtained between the logarithm of the nematode population in early season and the plant growth at harvest time, except for the plants affected by other causal agents. In the east area, growth retardation was comparatively indistinct, and, instead of it, leaf wilting and discoloration upward from the base was evident in late season. Negative correlation and regression coefficient between the nematode population and plant growth were found not so high as those obtained in the Hamamatsu area. In the Shida area, neither growth retardation nor successive growing failures were observed. This is possibly due to the practice, in this area, of submerging fields to leach out salt for approximately 20 days before cropping which effectively lowers the population level of Pratylenchus sp.

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Aboveground symptoms of the potato plants were recorded in a field heavily infested with the potato cyst nematode, Heterodera rostochiensis Wollenweber, in July and August, 1973. Wilting and stunting were observed first early in July, and were followed by general discoloration and yellowing of the lower leaves in the middle of July. Overall yellowing of the plants and degeneration of the lower leaves were observed late in July and early in August. In the middle of August, the feather duster symptom appeared on those plants which still held some leaves on the upper part. Most of the heavily infected potato plants were dead subsequently.
Growth and yield of potato were severely reduced; 29% yield reduction in the heavily infested field plot in comparison with that treated with D-D 40l/10a. Correlation coefficient between the plant height (or tuber weight) and the population of cysts in the root-zone soil was -0.813 (-0.774) in the field A, and -0.796 (-0.642) in the field B.

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Studies were made to establish techniques for disinfecting bacterial flora of the pine wood nematode, Bursaphelenchus lignicolus. All our trials by the methods so far described by other authors for surface sterilization of nematodes were found unsuccessful. Three bacterial species, Brevibacterium sp., Pseudomonas sp., and Bacillus sp., were always detected from nutrient agar plates where treated nematodes had been transferred to test for contamination. It appears that there are close relationships between these bacteria and this nematode species. Following procedures have been found successful, and can be applied to a large number of this nematode.(1) Nematodes are immersed in 0.1% Merthiolate for 3 hr, and then washed twice in sterilized distilled water.(2) PDA plate (15112, ) in a 90 mm petri dish is flooded with 1 me, of 1000 ppm tetracycline hydrochloride solution which is absorbed by the relatively dry plate. Botrytis cinerea is inoculated to the plate, and incubated for about 2 weeks at 25-30°C.(3) A drop of water suspension of surface sterilized nematodes is added to B. cinerea which has grown on the plate containing tetracycline hydrochloride. Reproduced nematodes on the plate are completely free from bacteria. Success of this method was confirmed by the incubation of the treated nematodes on nutrient agar plate.

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A kind of disease of unknown etiology, which belongs to “black scurf-like” syndrome, has long been recognized in a Chinese yam-producing area at Matsushiro, Nagano. In order to clarify the cause of the disease, the potted young seedlings grown from the bulbils were inoculated with each of three nematode species having been recovered from the diseased yam fields. Of these three nematodes, the characteristic symptoms of the tubers identical with those naturally occurring were reproduced only by the inoculation of Trichodorus porosus Allen in the duplicated experiments conducted under different temperature and soil conditions.

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Both of African marigold, Tagetes erecta L., var. Gold Smith, and French marigold, T. patula L., var. Fantango, were cultivated separately or together in a field infested with the reniform nematode, Rotylenchulus reniformis Linford et Oliveira, 1940, at Tokorozawa, Saitama, where great burdock, Arctium lappa L.(one of the important vegetables specific to Japan), had been grown with an occurrence of a soil borne disease called “Yake”. Soil samples were collected from each experiment plot at 30-40 day intervals for one year after sowing of plants in May, 1961, and nematodes in 100 g soil were extracted with Seinhorst elutriator. The African variety did not cause any reduction in population but played as a moderate host, being in contrast to the French marigold which markedly reduced nematodes, compared with that of fallow soil. Mixed cultivation of both plant varieties revealed that the nematicidal effect of French variety could not affect parasitism nor reproduction of the nematode on the roots of the other one. No detection of males from the field would suggest a parthenogenetic reproduction of this population. Great burdock was a good host. Saprozoic nematodes were not affected by the plants tested.

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