There appear to be two main causes of “ blight,” namely, the attacks of the froghoppers (Tomaspis saccharina) and the attacks of root fungi (Marasmius and Odontia).

The susceptibility of the cane to the attacks of these root fungi has been shown to be largely due to unsatisfactory conditions as regards the health of the plant and particularly to conditions of drought—physiological or actual. Moreover, this root disease tends to intensify the effect of the water-shortage. It is suggested that a careful analysis of the evidence might also show that the susceptibility to froghopper attack is due to causes which disturb the water balance of the cane, a disturbance which would be even more marked where root disease was present.

The balance in the water content is dependent upon the simultaneous working of the absorption that takes place through the roots and the transpiration that takes place through the leaves ; if transpiration is excessive the plant makes too great a demand upon the supply of available water in the soil. The humid climate of Trinidad is, therefore, particularly favourable to the growth of the sugar-cane. A favourable climate can, however, be offset by unfavourable soil conditions. The distribution of the rainfall, the conditions of the soil, the degree of atmospheric humidity, the effect of the disturbance of the water-balance upon the nature of the cell-sap, etc., all need to be taken into account. As regards rainfall and blight, it would appear that : (1) Prolonged periods of drought, or of excessive rainfall ; and (2) periods of drought following on spells of excessive wet weather, are both particularly harmful.

In view of the part played by root absorption in the maintenance of the water-balance, a detailed consideration of the root system of the sugar-cane in connection with blight might possibly throw fresh light upon the subject.

A disturbance of the water-balance may also affect the cane's susceptibility to the attacks of insects through its effect upon the physiology of the cane. Is it possible that drought (physiological or actual) or other unfavourable climatic and soil conditions may affect the concentration or constitution of the sugars in the sap in such a way as to render them more attractive to sap-feeding insects ?

The problem is extremely complex. We have seen that root disease is dependant upon the weakness and debility of the cane, resulting from unfavourable environmental conditions and from shortage of water ; that the root disease thus caused intensifies the effect of this water-shortage by lowering the plant's capacity for root absorption ; that the plant, susceptible in any case to attacks of froghoppers as the result of any disturbance in the water-balance, is rendered even more susceptible as a result of root disease. On the other hand, root disease is said to be due to a lack of vitality in the plant ; a plant attacked by froghopper is greatly reduced in vitality, and thus the tendency to root disease is increased.

We have considered the interrelation between the absorption of water by the roots and the transpiration of water through the leaves, and the effect of the water content on the physiology of the plant. We must also consider the interrelation between the attacking froghopper and root fungus, and the cane which is attacked.

Can we decrease the number of attacking organisms, or is it more feasible to increase the cane's resistance to their attack ? In the past the economic entomologist has directed his attention almost entirely to answering the first question ; it is now realised that an answer to the second question is quite as, perhaps even more, urgently demanded. Unhealthy canes are undoubtedly more susceptible to attack than healthy canes, that is, if we use the term “ health ” in its rightful sense as denoting vigour and the capacity to resist adverse environmental conditions, and not merely luxuriant growth.