Experiences with the breeding of red clover resistant to the stem eelworm

Summary The methods used in collecting stem eelworms from diseased plants, in raising and inoculating plants, in assessing resistance and in crossing selected plants in cages using bumble-bees for pollination are described in detail.Research showed that in swollen seedlings the eelworms had multiplied (susceptible plants) whereas this is not the case in non-swollen plants with a stunted growth and brown stripes (resistant plants).The increase of eelworms in greatly swollen plants of Roosendaal red clover took place at a greater speed than in slightly swollen plants. In healthy-looking plants mostly no or few eelworms had invaded. It seems that these plants have escaped a degree of infection necessary to judge their resistance.It appeared that varieties of red clover which are susceptible or resistant to stem eelworms of a given origin from the Netherlands were susceptible or resistant in about the same degree to eelworms derived from a quite different part of this country. A good conformity between laboratory and field resistance was found.If in spring bumble-bee boxes are hung in trees according to the method described, in about 10 to 20% of the boxes a good bumble-bee nest is found in the beginning of July. These populations are suitable to use for the crossing of clover plants in cages. Inside the cages seed production per plant was generally much lower than outside.By intercrossing resistant plants of Dutch local varieties and selection in the progenies, fairly resistant families were obtained within some years.