POTATO HAULM DESTRUCTION WITH SPECIAL REFERENCE TO THE USE OF TAR ACID COMPOUNDS
Authors: WILSON A. R. , BOYD A. E. W. , MITCHELL J. G. , GREAVES W. S. 
Published: February 2008

The chief aim of the investigation was to find an effective non‐corrosive spray for killing potato haulm.

Certain tar acids were nearly as effective as sulphuric acid, but as some samples of such compounds were highly irritant, only those proving both effective and relatively harmless to the human skin were passed as suitable.

The chemical composition of a sample of tar acids is largely dependent on the method of carbonization employed. The samples examined were from horizontal‐retort, coke‐oven, vertical‐retort and low‐temperature processes. At present, suitable compounds which can be produced commercially have been obtained from the two last‐named only. Fractionation at intervals of 10° C. indicated that, as a rule, high‐boiling fractions (250–290° C.) were more effective haulm‐killing agents and less irritant to the skin than low‐boiling fractions (190–250° C.).

Various methods of emulsification are discussed. An emulsifier, consisting of a mixture of gum arabic and casein, was developed, yielding a stable concentrate, easy to dilute with water of any degree of hardness. The spray when applied formed a tenacious rain‐resistant film on the leaves. The action of sulphuric acid was considerably reduced by rain falling shortly after application.

Sodium chlorate gave a fair leaf kill but a poor stem kill and is, therefore, far less effective than sulphuric acid. Restricted trials carried out with calcium cyanamide dust indicated that this compound is even less efficient than sodium chlorate.

Di‐nitro‐cresol derivatives were only moderately effective and have objectionable staining properties.

Of the copper compounds tested, copper chloride and nitrate proved the most effective but were not as good as sulphuric acid and are relatively expensive. A mixture of copper sulphate and salt was moderately effective on senescent haulm only. Like sodium chlorate it has the advantage of being inexpensive.

Tar‐oil winter wash proved effectìve, but is expensive and slow in action.

Young haulm is readily killed but becomes more resistant as the season advances and finally again susceptible during senescence. Individual, leaves on a plant show the same age/susceptibility relation. Varietal differences in susceptibility appeared to be largely the result of varietal differences in maturity.

Control of blight in tubers was related to the extent of haulm kill obtained by spraying. Unless the fungus was actually wetted by the spray, it continued to sporulate freely on the leaves until the tissues died. For this reason slow‐acting compounds, such as tar‐oil winter wash and those which are ineffective on stems, provide less adequate control than quickacting, highly effective compounds such as sulphuric acid and tar acids.

‘Setting’ of the skin of tubers is accelerated by killing the haulm, but the kill must be virtually complete. Therefore, to obtain maximum resistance to lifting injury, the crop should be left in the ground as long as possible after spraying. Methods of assessing ‘skin setting’ are described.

Cooking tests on the produce of plots where the haulm had been killed by various tar derivative and other compounds showed no deleterious effect on quality. Neither was there any indication of detrimental residual effects in the soil following spraying with various tar‐acid and copper compounds, tar‐oil winter wash, sodium chlorate (3 %) or sulphuric acid.

A low‐temperature tar‐acid compound, T.A.C. 36 c, incorporating a gum emulsifier, gave satisfactory results in large‐scale tests made by War Agricultural Executive Committees in 1945. This compound, with a slightly improved emulsifier (gum arabic + casein), reference T.A.C. 46, and a vertical retort tar‐acid compound, T.A.C. 42, proved equally successful in a restricted field trial and are regarded as satisfactory for commercial use. Because of present cost of materials (40s. per acre for a 100 gal./acre application of 10% spray) their use is likely to be restricted. Comparative costs of the more effective compounds are given.

The most effective non‐corrosive substitutes for sulphuric acid on vigorous haulm are certain tar‐acid sprays. For killing haulm at the end of the season some of the less effective and cheaper compounds, such as sodium chlorate or copper sulphate and salt, may prove reasonably satisfactory in most years.