DISEASES OF ROOT CROPS 333 



hundred square feet, and constant in position from year to year. 

 The symptoms do not make themselves apparent until the 

 rhizomes are nearly ripe for digging. At this time the affected 

 plants have fewer leaves than the healthy ones growing in their 

 vicinity, and these leaves are often rolled up and somewhat 

 wilted. Extension of the patch is very slow, but the fungus is 

 very persistent, and the disease was reported to have reappeared 

 in the first crop on a field replanted after remaining in bush 

 fallow for twenty-five years. 



In 1915 the present writer visited a field in which the rhizomes 

 had recently been dug and were lying in small heaps on the 

 ground. The material from a diseased patch was conspicuous 

 owing to the black discoloration of the rhizomes. Towards the 

 base of the heaps sufficient moisture had been retained for the 

 fungus to continue growing, and there were woolly tufts and 

 strands of greenish grey mycelium, undoubtedly that of a Rosel- 

 linia, and an abundance of the typical conidial fructifications. 

 Allowing for the difference in structure of the two plants, the 

 appearance of the mycelium in the rhizome corresponds exactly 

 to that of R. bunodes in lime roots, that is to say, the mycelial 

 strands are identical and their distribution throughout the 

 relatively soft tissue of the arrowroot rhizome is similar to that 

 seen in the bark of the lime. South records the presence of 

 perithecia of R. bunodes on other plants in close proximity to a 

 diseased patch. 



On the exterior of the arrowroot rhizomes as seen when they 

 are dug there is little or no loose external mycelium. When 

 such does occur, the hyphae are of the usual " varicose " type. 

 On the surface of both rhizome and scale leaves there is a dense 

 scatter of shiny raised dots and ridges (less than i to about 3 mm. 

 in diameter) consisting of compact aggregations of dark hyphae 

 in the superficial tissue. From these the slender compact branch- 

 ing strands, black with a white core, radiate through the paren- 

 chyma both of the scale leaves and of the rhizome. 



In the latter they mostly follow a radial course, and are con- 

 spicuous to the naked eye as black dots and lines in a section. 

 At this stage the rhizomes are still full of starch. At an early 

 time on the scale leaves, and later on the rhizomes, the black 

 spots tend to run together until the whole surface is black and 

 shining, and about this time the parenchyma of the rhizome 

 may be found generally infested with hyaline hyphae and depleted 

 of its starch. 



The special characteristics of the disease on arrowroot are : 

 (fl) that the disease occurs in patches which are reported to show 

 little or no observable increase from year to year ; {b) that in such 

 patches the disease persists for a long and apparently indefinite 

 period of years. 



The explanation of these features is probably to be found in 

 the methods pursued in arrowroot cultivation. The fields are 



