SOME RELATIONSHIP BETWEEN PLANT NUTRITION, FUNGAL POPULATIONS, AND INCIDENCE OF FUSARIUM WILT OF MUSKMELON

The rate of muskmelon wilt was shown to be a function of the amount of nitrogen available to and absorbed by the plant. Since NH4NO3 nitrogen did not increase the size of the soil population of either F. oxysporum or the wilt component, F. oxysporum f. melonis, it is concluded that its effect is through an increased host susceptibility. The effectiveness of nitrogen on wilt is expressed at all population levels of the wilt fungus, highest rates of wilt being found in soils with highest populations. Calcium and potassium retarded wilt in less highly fertilized soils and caused a reduction in population of the wilt fungus. Interactions of nitrogen, potassium, phosphorus, calcium, and magnesium are discussed.

STUDIES ON THE RELATIONSHIPS BETWEEN NEMATODES AND OTHER SOIL MICROORGANISMS. IV. INCIDENCE OF NEMATODE-TRAPPING FUNGI IN THE VICINITY OF PLANT ROOTS

A study was made of the occurrence of nematode-trapping fungi in the rhizosphere and on the root surface of different plants. Arthrobotrys oligospora was the predominant predaceous fungus isolated. It was almost completely absent from plant roots but occurred in varying frequency in rhizosphere soil and in root-free soil. The incidence of this fungus was consistently greater in the soybean rhizosphere and lower in the wheat rhizosphere than in corresponding soil devoid of roots, whereas for other plants, red clover, flax, etc., there was no obvious rhizosphere effect. Spore germination tests and growth of A. oligospora in root extracts of soybeans and wheat failed to account for the differences observed. However, bacterial isolates from the wheat rhizosphere were, on the whole, more antagonistic to this fungus than those from the soybean rhizosphere, whereas isolates from the latter appeared to exert a favorable effect.

NODULATION FACTOR FOR RHIZOBIUM-LEGUME SYMBIOSIS

Valera, Concepcion L. (Cornell University, Ithaca, N.Y.), and M. Alexander. Nodulation factor for Rhizobium-legume symbiosis. J. Bacteriol. 89:1134-1139. 1965.-Nodulation of excised roots of Medicago sativa was enhanced by an extract of alfalfa seeds but not by several other substances. Coconut water exerted a similar influence upon the formation of nodules on excised roots of Glycine max and Phaseolus vulgaris. Direct evidence for a host factor concerned in nodule genesis was obtained by demonstrating that, although P. vulgaris seedlings developing from embryos devoid of cotyledons nodulated in the presence of an infective Rhizobium strain, nodules did not appear on explants derived from hypocotyls obtained from ungerminated seed. The legume factor, when examined by a soybean hypocotyl bioassay procedure, could be replaced by a water-soluble, thermostable, dialyzable fraction of coconut water, but not by meso-inositol, scyllitol, sorbitol, yeast extract, or a number of amino acids.

THE CONTROL OF TRICHOSTRONGYLE LARVAE (NEMATODA) BY FUMIGATION IN RELATION TO THEIR BIONOMICS. 3. FIELD TRIALS WITH METHYL BROMIDE

In a small-scale trial in which boxes of grass infested with Trichostrongylus colubriformis were treated with methyl bromide, a dose of 100 mg. hr./l. prevented guinea-pigs picking up the infection when they grazed the grass at a later date. A practical method is described for fumigating small areas of pasture. In a field trial, sheep were used to test plots of pasture fumigated with doses which did not kill the grass but the results were inconclusive. These experiments are discussed and also the possible value and applications of this technique.

I am indebted to Professor B. G. Peters and to the other members of the Imperial College Field Station, particularly Dr A. B. Page and Dr O. F. Lubatti, for their help and criticism during the work. Thanks are also due to Dr William Davies, the Director of the Grassland Research Institute, Hurley, for permission to carry out the field experiment there, and also to Dr C. R. W. Spedding and Mr T. Brown for their co-operative assistance and advice throughout this work.

THE CONTROL OF TRICHOSTRONGYLE LARVAE (NEMATODA) BY FUMIGATION IN RELATION TO THEIR BIONOMICS. II. LABORATORY TOXICITY TESTS WITH METHYL BROMIDE

A standard culture technique is described for obtaining the various developmental stages of Trichostrongylus colubriformis, and a suitable technique for assessing the effect of methyl bromide fumigations is outlined.

Doses of 10, 25 and 150 mg. hr./l. were required to give an LD50 for eggs, pre-infective and infective larvae respectively. The age of infective larvae, the relative humidity and the temperature all influenced the toxicity of the fumigant. Soil, and more especially faeces, sorb the fumigant and probably protect larvae from its full effect. Infective larvae of T. axei, T. retortaeformis and H. contortus were also found to be susceptible to methyl bromide.

The activity of larvae surviving fumigation was considerably reduced, but these larvae were still capable of infecting guinea pigs.

Methyl bromide killed common pasture grasses at doses of 150–250 or more mg. hr./l., depending on the temperature. Chloropicrin was found to be highly toxic to grass and unsuitable where the pasture is required undamaged.

The results are discussed in the light of current theories of the mode of action of methyl bromide on insects and what is known at present of the physiology of nematodes.

It is concluded that under certain conditions, methyl bromide might be used to eliminate infestations of trichostrongyle larvae on pasture, without killing the pasture.

I am indebted to Professor B. G. Peters and other members of the staff of the Imperial College Field Station and Dr C. W. R. Spedding of the Grassland Research Institute, Hurley, for their help in this work and their advice and criticism.