12G 



THE AGRICULTURAL NEWS. 



Apeil 11, 1914. 



FUNGUS NOTES. 



LEAF-OUT, OR TOMOSIS, A DISORDER 

 OF COTTON SEEDLINGS. 



In view of the occurrence in St. Croix {Agricultural 

 Keus, Vol. XII, p. 302) of a serious affection of cotton 

 closely resembling that which has recentlj- been described in 

 the United States as Tomosis, we reprint in full below an 

 article on the subject by Mr. O. F. Cook, published in 

 Circular 120 of the Bureau of Plant Industry, United States 

 Department of Agriculture. 



Seedlings and young cotton plants are subject to 

 a peculiar disorder that results in extensive injury to the 

 leaves and frequent abortion of the terminal bud. While 

 these injuries are seldom fatal, they undoubtedly impede the 

 growth of the plants, delay the period of production, and 

 reduce the crop. Though not taken into account hitherto, 

 the losses occasioned by the leaf cut disorder, though most 

 severe in the South-western States, seem to be very general 

 and must amount to millions of dollars every year. With 

 the advance of the boll weevil this form of injury to the 

 young plants acquires a more serious aspect, because it is of 

 the utmost importance to shorten the period of production in 

 order to avoid damage by the weevil. The leaf-cut handicap 

 can be reduced by improved cultural methods, as stated in 

 previous publications of the Bureau of Plant Industry. 



Leaf- cut is suggested as a popular name because mutila- 

 tions of the leaves are the most characteristic symptom of 

 the disorder. The name 'juvenile haf-curl' has been applied 

 in previous publications, but is inconveniently long and not 

 sufficiently distinctive. Moreover, the leaves of cotton 

 seedlings are subject to another malformation, induced by 

 plant lice, for which the name 'leaf-cutl' is more appropriate. 



Leaf-cut is very widely distributed and familiar to 

 planters, though generally confused with the leaf-curl caused 

 by plant lice. Though both forms of injury are likely to be 

 found in the same field, or even on the same individual plant, 

 they are easily distinguished. The leaf-curl is a crumpling or 

 arching up of the leaf between the veins, but without perfora- 

 tions or rents. Even in cases of great distortion by leaf curl 

 the tissues of the leaf are left entire, without being punctured or 

 torn. Leaf cut injuries, on the other hand, represent an actual 

 destruction of some of the tissues of the leaf, leaving 

 irregular holes or marginal incisions. In single words, leaf- 

 curl may be described as distortion of the leaves, leaf-cut as 

 mutilation. In allusion to this distinction the word 'tomosis' 

 is proposed as a technical name for the leaf-cut disorder, 

 while the distortion caused by the plant lice may be called 

 'hybosis'. The insects inhabiting the badly distorted upper 

 leaves were identified by Mr. Th. Pergande, of the Bureau 

 of Entomology, as Aphis gosii/pii, Glover. 



And in addition to the open wounds that result from 

 leaf- cut there are usually some that have healed, giving 

 a characteristic torn-and mended appearance. Such scars, 

 like other leaf cut wounds, often lie in a somewhat radiating 

 position between the principal veins. Healing of wounds and 

 regeneration of lost parts show that the injuries are liable to 

 occur at a very early stage in the development of the leaf. 

 Sometimes an extensive new growth or regeneration takes 

 place, resulting in a curious doubling or overlapping of lobes 

 of injured leaves. The power of the injured tissues to heal is 

 also responsible for adhesions between parts that lie folded 

 together in the bud. These adhesions are usually respon- 

 sible for failure of normal expansions of the blade. None of 

 these secondary symptoms occur with the leaf curl induced 

 by plant lice. 



Leaf-cut is hardly to be reckoned as a disease unless the 

 word is used in its most general application that includes any 

 departure from normal structure or function. Neither of 

 the two general classes into which diseases are usually 

 divided, constitutional and parasitic, will include the leaf-cut. 

 Though some of the cells are destroyed, the remaining tissues 

 of the plant do not become abnormal in any way, and there 

 is no indication that parasitic organisms of any kind — 

 bacteria, fungi, insects, mites, or worms— are involved. 

 Another class of ecological disorder.s may need to be recog- 

 nized, intermediate between physiological diseases and 

 mechanical injuries or traumatisms. Leaf-cut is a disease 

 only in the sense that frostbite, snow blindness, and other 

 environmental injuries are to be considered as diseases. 



Young cotton plants are often subjected to extreme con- 

 ditions during the early stages of growth, when the leaf-cut 

 injuries occur. The leaves and roots are still close to the 

 surface soil, where they can be chilled at night and scorched 

 in the daytime. Cold nights are sometimes looked upon as 

 the cause of the injury, and may be an intensifying factor, 

 but the sudden heat of a bright morning .sun seems more 

 likely to kill the cells of the young leaves than low tempera- 

 tures during the night. Leaf-cut often affects late plantings 

 long after the night temperatures have ceased to approach 

 the freezing point. It has been noticed that exposure to 

 a bright morning sun after a cold night will throw cotton 

 seedlings temporarily into a wilted state, doubtless because 

 the leaves lose water by transpiration faster than it can be 

 absorbed by the chilled roots Leaf-cut seems to be especially 

 prevalent under such conditions. 



That leaf-cut is in some way connected with exposure or 

 wilting of the delicate tissues is also shown by the fact that 

 the injuries are most severe and occur most frequently along 

 radiating lines midway between the principal veins. These 

 lines of greater susceptibility represent the most exposed 

 parts of the ui)per surface of the young leaf as it lies folded 

 in the bud. The only suggestion for explaining the very 

 irregular manner in which the cells are killed is that some of 

 them may be unable to complete their divisions and nuclear 

 readjustment during the night and may thus be left in an 

 unusually susceptible condition. Sections of injured leaves 

 prepared by Dr. Albert Mann, of the Bureau of Plant Indus- 

 try, show that nuclear and protoplasmic disintegration are 

 the earliest symptoms. The damage often begins with the 

 death of a single cell, which results, of course, in increased 

 exposure for the neighbouring cells. 



Plants protected by partial shade suffer less than those 

 that are fully exposed, but, on the other hand, full exposure 

 does not induce leaf-cut when the plants are growing on wet 

 lands where the surface remains moist and is kept cool by 

 evaporation. The moist atmosphere and partial shade 

 afforded by ordinary green-house conditions also afford 

 complete protection from leaf- cut. 



Even in parts of the same field there may be obvious 

 differences in the extent of leaf-cut injury. Plants that 

 stand close together often show much less injury than 

 more scattering plants in the same rows. Where the 

 soil is too dry to germinate all the seed the leaf cut 

 injuries are more extensive. Such differences indicate the 

 possibility of avoiding or reducing the damage from leaf-cut 

 by giving better attention to the seed bed and to method 

 and times of planting and thinning. 



Though mutilation of the leaves is the most frequent 

 and familiar symptom of the leaf-cut disorder, abortion of 

 terminal buds is a more serious injury. In severe cases of 

 leaf-cut, from 30 to 60 per cent of the plants have been 

 found with their terminal buds aborted. When the leaf-cut 

 injuries are confined to the individual leaves the effect is 



