222 



THE AGRICULTURAL NEWS. 



July 6, 1912. 



FUNGUS NOTES. 



EXANTHEMA 



AND SQUAMOSIS 

 CITRUS. 



OF 



The following article contains in a condensed form the 

 information given by O. Butler on these two diseases in his 

 paper in The Annals of Botany, Vol. XXV, p. 107. It may 

 be looked upon as a continuation of the article on Gummosis 

 that appeared in the last number of the Agricultural News. 



SYMPTOJi.s OF EXANTHEMA. This disease is at present 

 only known in the United States, where it affects all varieties 

 of citrus of all ages that are growing on dry permeable soils 

 or on light soils overlying an impermeable subsoil. The 

 parts characteristically affected are the small branches, shoots, 

 and occasionally the fruits. Frequently, the definite symptoms 

 are preceded by a luxuriant growth and dark colour of the 

 foliage, and by the presence of thick-skinned fruit. Then 

 the shoots become more or less stained beneath the epider- 

 mis by a yellowish brown substance, and begin to die back. 

 If this symptom is not observed, another conclusive indica- 

 tion is that the fruit becomes stained, and its epidermis is s© 

 hardened that it cracks and splits on account of the pressure 

 of the growing pulp cells. In more advanced stages of the 

 disease the young shoots swell at the nodes — occasionally 

 also at the internodes; while similar swellings may form on 

 the fruit. Then linear erumpent pustules break out on the 

 internodes as the shoots grow older; in some cases the pus- 

 tules may alternate with nodal swellings. On the older 

 branches nodal swellings are not produced, but the pustules 

 may become very numerous and a small amount of gum may 

 be observed in them. Gum may also exude through the 

 bark in small amounts. When swellings and pustules are 

 not formed on the shoots and branches, an excessive number 

 of buds may be produced which ma}" develop into short 

 branches with yellow foliage; so that a witches' broom effect 

 is brought about. 



Cross sections of the diseased shoots show that the swell- 

 ings are due to an accumulation of gum formed from the 

 young xylem cells, as in gummosis; that the pustules are 

 caused by excessive growth of the cortical tissues accom- 

 panied by a sickle-shaped patch of young xylem containing 

 gum pockets; and that the rings of growth are well marked 

 and are too numerous to be considered as variations in growth 

 due to seasonal changes. The malady is very similar to 

 gummosis, and is only differentiated from it by the occurr- 

 ence of the erumpent pustules. These are possibly caused 

 by the circumstance that the epidermis first becomes inelastic 

 and checks normal growth and then bursts; after this vigor 

 ous or excessive growth recommences. 



CAUSE OF EXANTHEMA. This discase is so similar in 

 most respects to gummosis that its occurrence is in all proba- 

 bility dependent on the same conditions, namely vigorous 

 growth coinciding with an abundant supply of moisture. At 

 the same time the abnormally large number of rings of growth 

 indicates clearly that there has been a fairly rapid alterna- 

 tion of favourable and unfavourable conditions for develop- 

 ment. This might be brought about by a quick alternation 

 of climatic conditions, or by frequently recurring changes in 

 the available supply of water. The last is clearly the cause 

 in this case. The light soils on which plants with the 

 disease are found become readily saturated with water, which 

 they as readily lose. 



It has been discovered that excessiveapplications of organic 

 nitrogenous manures increase the severity of exanthema, in 

 Florida; while inorganic manures have a less marked effect. 



Butler suggests that this is due to the fact that nitrogenous 

 manures stimulate growth, and that organic manures in which 

 the nitrogen is only rendered available by the process of 

 nitrification are only so altered when the soil is sufficiently 

 damp. Thus the growth stimulus due to the nitrogen takes 

 effect at the very time when an ample supply of water is 

 available and when the presence of that water has already 

 induced a vigorous growth in the trees. 



Cultivation is another factor that increases the suscepti- 

 bility of citrus to the disease. Swingle and Webber suggest 

 that this is because the surface roots are destroyed and the 

 remainder are forced to grow down into an unsuitable subsoil. 

 Butler believes, however, that this is not the true explanation, 

 but that cultivation increases nitrification, particularly in 

 humid climiites; while it prevents excessive evaporation from 

 the soil. 



REMEDIAL MEASur.E.s. The object to be aimed at in 

 preventing or remedying the disease is to produce a uniform- 

 ity in the water- and in the nitrogen-supply, which will give 

 rise to regular, and not to fluctuating, growth. Thus drain- 

 age, especially on soil overlying an impermeable layer, is 

 a matter of importance, as is the correct regulation of irriga- 

 tion water where irrigation is necessary. Care should be 

 taken to increase the humus content of light soils, and appli- 

 cations of nitrogenous manures should be carefully regulated. 

 Heavy fertilisation with phosphatic and potassic manures is 

 also frequently found to be useful, as these have a restraining 

 influence on growth. It may also be desirable to discontinue 

 excessive cultivation, especially in soils underlain by an 

 impermeable subsoil; and merely to apply mulches of different 

 kinds to the soil, according to the material available. 



SYMPTOM.^ Of squamosls. This is at present limited in 

 distribution to Florida and Southern California, and is 

 a disease confined to the orange tree. The conditions which 

 favour its occurrence are prevalent only infrequently, so that 

 the disease is rare; moreover it develops extremely slowly on 

 affected trees. The chief characteristic is a scaling of the 

 bark which may occur on the limbs or on the trunk, where it 

 first appears as a single scale or group of scales. When the 

 outbreak is on the smaller branches, the leaves turn yellow 

 and eventually the branches are ringed and die, so that much 

 dead wood is present. When the attack commences on the 

 trunk or main limbs, the leaves do not turn yellow until 

 much later and the tree may live for fifteen or twenty years. 

 In fact in these instances, an attack is often hard to identify, 

 as scaling of the bark may arise from other causes, while 

 there are no indications of disease in the foliage and there is 

 no definite correlation with environmental conditions. 



Squamosis commences as a rounded or an irregular 

 sloughing of the bark about an inch or somewhat less in 

 diameter. The detached bark soon dies and mils more or 

 less. This curling reveals the underlying cmin-al tissues 

 which are warty, white, or white with a yellow tinge, and 

 somewhat mealy. When a piece of bark falls, a pustule will 

 be found beneath it: this pustule has a depression in the 

 middle, from which gum frequently oozes. In some cases 

 the pustular outgrowth becomes less prominent, especially in 

 advanced cases, and is replaced by a general swelling of the 

 cortex lying below the epidermis. This causes the bark to 

 flake off in large strips. On shoots and water sprouts, shield- 

 shaped, raised spots are formed which are discoloured 

 beneath the epidermis. These become hard, and the 

 epidermis ruptures round their margins. In more serious 

 attacks the spots are larger and less definite in outline. 

 Cracks parallel to the axis of the shoot occur in them, and 

 the disease-producing tissues form distinct swellings on the 

 shoots 



