.i.956>.] FROGHOPPEB BLIGHT OF SUGAR-CANE. 15 



receiving tlie reply that while the agreement in appearance was very 

 close the absence of discoloured vascular bundles at the nodes prevented 

 a decision that the disease was actually Sereh. In specimens collected 

 ■by Mr. Williams and myself not only was the correspondence with 

 the appearance of Sereh complete but the character mentioned by 

 Professor Went as essential was also present. There is no difficulty in 

 deciding, however, that the resemblance is one of symptoms only, since 

 of the character from which Sereh derives its destructiveness— the 

 steady and invariable degeneration of the affected cane stool, proceeding 

 year by year until a grass-like condition is reached — there is no trace 

 whatever. 



21. Mosaic Disease. A mosaic or mottled-leaf disease has for several 

 years been causing widespread losses in Porto Rico, and has recently 

 been found to exist in Cuba, Florida and Louisiana, where the prospect 

 of its spread is viewed with apprehension. It is of special interest in 

 that there are indications that the infection may be distributed by a 

 sucking insect allied to the froghopper. The primary symptom is the 

 appearance of various degrees of light and dark-green spottings or 

 streakings in the leaves. The disease is cumulative, and in the second 

 year white opaque spots and streaks appear in addition to the markings 

 already mentioned. At the same time or later canker of the stalk 

 appears and severe stunting occurs. 



22. The unidentified leaf-spotting on plots of D 3596 and B 1753 at 

 St. Augustine, recorded by Williams, Bulletin Deimrtment of AgricuUare 



' xviii, pp. 77 and 80, simulates the first symptom of the mosaic disease 

 and its nature is under investigation. In the period 1917-1919 no 

 development in the direction of more serious symptoms was detected but 

 a recent report is to the effect that the affection, whatever its nature may 

 prove to be, is spreadmg to neighbouring plots. As the mosaic disease 

 is easily introduced by cuttings great cave should be exercised in 

 importing new planting material, and introductions from places at all 

 doubtful in this respect should be carried out only through the Depart- 

 ment of h griculture. There is no indication that the disease exists in 

 connexion with froghopper blight. The need for caution is illustrated by 

 the figures regarding the worst infested area in Porto Rico, in which ten 

 mills have shown an average reduction of 40 per cent, in output of sugar. 

 The disease can be controlled by elimination methods and by the 

 selection of healthy material for planting, since fortunately the infection 

 does not reside in the soil or in dead plant material. 



23. EooT Disease : Type A. The common type of root disease (as 

 considered apart from complications with froghopper blight) arises from 

 the invasion and destruction of the roots at any stage of their growth, 

 accompanied or not, as the case may be, with the cementing of the 

 dead leaf-sheaths round the shoots. It is prevalent in soils lacking in 

 fertility or in which aeration is obstructed by poor tilth or saturation 

 with water. Since each of these factors produces its own direct eilect on 

 the growth of the cane it is usually difficult to estimate the amount of 

 the contribution of root disease to the general result. The belief that the 

 effects produced by the fungus are considerable is based mainly on the 

 constancy of its association with particular types of failure, during 

 observations carried on fur a long number of yeavs and i.i many 



