In an effort to determine the relationship between minor symptoms of nematode galling sometimes observed in the seedling readings and subsequent behavior of such plants a series of twenty-five tile beds were planted with individual plants which showed either swollen root tips or tiny side galls but no heavy galling at the time seedling readings were made on month-old plants which had been heavily inoculated when sown. In each tile bed susceptible seedlings which had been started in sterilized soil were used as controls. Each tile bed was inoculated with galled tomato roots with subsequent heavy galling of all the controls. The resistant plants which had shown these "minor symptoms" in the seedling tests were completely free from anything which could be called real galling in every case when mature plant readings were made two and a half months after planting in the tile beds. A few very small side galls were found in a small percentage of the plants. These minor galls were so small that it is difficult to see how such plants could be classed as susceptible in view of the very complete, heavy galling of the controls. Seedlings from thirty individual plant selections were represented in this test.
The classification of seedling tomatoes segregating for resistance or susceptibility to nematode root knot has been complicated here in some of our tests by the appearance of numerous individuals with swollen root tips but no severe galling such as observed in the true susceptibles. In some cases the rootlets would present a stubby appearance due to first swelling of the tips and then abrupt termination of their growth. These symptoms in otherwise resistant seedlings appeared to be related to the presence of damping-off fungi and possibly other organisms in some of the old infested tomato roots used as nematode inoculum. In experiments with the control of damping-off organisms in partly rotted tomato root knot material it was found that a yellow cuprocide suspension (50 gr. CuO per 500 ml. water) gave fair control if the chopped roots were immersed for 24 hours and then drained and allowed to stand with the copper residue on the material for two or three days longer. If the condition of the root knot inoculum quite bad with respect to decay of the galls and danger of seedling losses from, damping-off, it was found that the yellow cuprocide was less effective than a heat treatment in making the inoculum safe. This heat treatment consisted of a 4-hour immersion of the inoculum in water at 38 deg. C. or a longer period at the same temperature if the chopped roots are held under humid conditions but not in a water bath.
Further experiments with this heat treatment of old galled roots are now under way but the evidence to date indicates the existence of a wide enough margin between the heat tolerance of our locally more dangerous damping-off fungi and of the more resistant stages of the life cycle of Heterodera marioni to allow good control of damping-off in badly infested root knot innoculum without killing all the nematodes present. Susceptible seedlings exposed to such heat treated inoculum showed uniform but delayed galling with no damping-off and no swollen root tip symptoms. Controls showed heavy damping-off losses with greatly reduced growth in surviving plants. An instant dip in the yellow cuprocide suspension gave poor control of damping-off fungi in the innoculum compared with longer dips or with a combination cuprocide dip and heat treatment run simultaneously.