FIELD TRIALS WITH D‐D MIXTURE* AGAINST POTATO‐ROOT EELWORM

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Abstract
A D‐D mixture has been tested against Heterodera rostochiensis at seven 2‐acre sites on sands, silts and blackland soils. Apart from a pilot trial where soil was injected in spring, injections were carried out in the autumn, and potatoes were grown the following year. Factors investigated were rate of application (0, 200, 400 and 800 lb. D‐D/acre), depth of injection (4 or 8 in.) and the effect of rolling after injection. At the most responsive of the sites (Wainfleet), increases in yield, kill, and the post‐crop eelworm population were all roughly proportional to the rate of application. Under favourable circumstances a 50% increase in yield and something like a 50% reduction in eelworm population 4 weeks after injection can be expected from 800 lb./acre, but the reduction is more than made good during the growth of the subsequent crop; accelerated multiplication of eelworm on the treated plots leads to their having a larger population than the untreated controls at lifting time. Of the sites tested, the blackland soils gave a lower eelworm kill and a much lower yield increase from D‐D than silts or sands. After autumn injection the nematocidal, and probably the phytocidal, effects of D‐D persist in the soil for many weeks. The hypochlorite method of ‘hatching’ eelworm larvae for counting has proved unreliable.