206 



THE AQRICVLTURAL NEWS. 



June 22, 1912. 



FUNGUS NOTES. 



GUMMOSIS OF PRUNUS AND CITRUS. 



An interesting paper by O. Butler was published a short 

 time ago in the Annals of Hot an!/, Vol. XXV, No. XCVII. 

 p. 107, under tbe title: 'A Study on Gummosis of Prunus 

 and Citrus, with observations on Squamosis and Exanthema 

 of the Citrus.' It is proposed in this article to give some 

 account of the results arrived at in Butlers paper so far as 

 gummosis is concerned, and to leave the consideration of that 

 part dealing with squamosis and exanthema for a subsequent 

 number of the Agricultural News. 



DISTRIBUTION AND HISTORY. Gummosis of Piunus and 

 Citrus occurs in France, Germany, Great Britain, Italy, Por- 

 tugal, Spain and America, as well as in fcicily, the Azores, 

 and in India. The disease has never assumed serious pro- 

 portions on Prunus but has been responsible for a large 

 amount of damage in the case of Citrus. In St. Michael, in 

 "the Azores, it first appeared in 1S34, and was at its height 

 in 1840, when it caused the destruction of a large proportion 

 of the citrus trees of the island. By the year 1873 it was 

 no longer feared, though it still occurred. It subsequently 

 made its appearance in Portugal, Sicily, Spain, Corsica and 

 Algeria. It seriously affected the groves of new South Wales 

 between 1860 and 1870^ and appeared in California in 1875, 

 in Florida in 1876. 



DESCRIPTION. The first external signs of the disease 

 are the occurrence of raised places on the epidermis or of 

 drops of gum on its surface. As the disease progresses, the 

 swollen spots on the epidermis burst and allow the gum to 

 tlow out, while in those oases where the first sign was the 

 occurrence of small drops on the surface, the progress of the 

 disease simply increases the flow of gum. At the stage 

 when gum has appeared on the .surface of the bark, the inner 

 bark, or cortex, will be found to be permeated to a greater or 

 ^ess extent. Tlie infiltrated bark subsequently dies, cracks, 

 curls and sloughs off. Severe cases of gummosis are always 

 accompanied by chloiosis, that is the disappearance of the 

 green colouring matter of the plant, whose green parts 

 "become yellow; this probably results from a decrease in 

 the absorption of mineral nutriment b_v the roots, brought 

 about by the partial destruction of the bark which entails 

 a partial starvation of the roots as regards the food-supplj' 

 from the leaves. On cutting a slightly affected branch, two or 

 more years old, througli a diseased area, it will be found that 

 the damage first arises in. the young wood which has just 

 ieen formed by the cambium. 'The diseased area is fusoid 

 in form from the greater development of the diseased tissues 

 near the point of gum accumulation and their lesser and 

 lesser development as one proceeds further away. The gum 

 exudes eii masse from the centre of the sickle, but as one 

 proceeds towards its extremities it will appear in droplets of 

 decreasing magnitude and separated with larger intervening 

 spaces of apparently healthy tissue. The gum in the centre 

 of the sickle is more or less tinted yellow, depending on its 

 age, whereas that pearling from the tissues at its extremities 

 is always colourless. 



' If sections are now cut at various distances above or 

 below the centre of disease, it will be found that gum form- 

 .ation proceeds downward to a much less extent than 

 upward. Furlherniore, if we imagine a line drawn through 

 the middle of the pathognomic tissues, it will be found, as 

 one proceeds upwards .tnd downwards, that the gumming 

 sickle. diminishes in size.' 



Gumming may attack the stems, small branches, leaves 

 or fruit, and may affect the tree locally or generally, accord- 



ing to external circumstances and the species attacked. It 

 may arise autogenously, that is to say, without direct outside- 

 cause, or may be induced by the attacks of insects or fungi, 

 or by any other form of damage such as may result from, 

 wounds caused accidentally by instruments or brought about 

 by frost, sunburn, or the application of acids and poisons. 



As has been Stated already, the gum pockets may be- 

 formed among the embryonic cells of the xylem (the cells of 

 the wood that have been formed most recently from the- 

 cambium). .The cell walls of this tissue swell up and give- 

 rise to the gum This breaking down of the cell wall may 

 proceed until all its layers are disorganized, when the cell 

 contents become mixed with the gum; or it may be arrested 

 when only the two outer layers of the walls of certain of the 

 cells have broken down, so that freed cells contained by the 

 third layer are left floating in the gum. In mild cases this- 

 disorganization is confined to the embrj-onic xylem cells, but 

 in more serious attacks it may extend to the cambium, the 

 medullary rays, and the older cells of the xylem. When> 

 gummosis is arrested, the young xylem cells become lignifiecl 

 (provided with woody walls), and the cambium proceeds to- 

 lay down normal tissue so that the gum pockets are enclosed 

 in healthy wood 



c.^usE. For an outbreak of the disease the simultaneous 

 occurrence of two factors is necessary, namely, active growtli 

 and a free supply of moisture. Butler believes tb\t the 

 formation of gum is due to the hydrolysis of the cell walls of 

 the young xylem. Exactly how this is brought about is 

 uncertain, but he is inclined to think that it is not due to the' 

 action of an enzyme. He puts forward good reasons for 

 believing that gumming is not produced by the oxidation of 

 the cell walls, nor. by the direct action of a cellulosedissolr- 

 ing enzyme: two theories which have been suggested by 

 previous writers. 



It will be seen from this that gumming is likely ta 

 occur when trees are grown in heavy, badly drained soils, or 

 in suitable soils underlaid by an impermeable subsoil. Fur- 

 thermore, it may be encouraged by excessive irrigation, or 

 by high fertility of the soil combined with want of drainage- 

 or excessive irrigation; or by the too great application of 

 manures, especially of nitrogenous manures; and, lastly, hy. 

 continuous wet weather at the commencement of the growing 

 period. These factors may either induce the disease or 

 encourage its development on wounded trees or on those 

 attacked by insects or fungi. 



PREVENTIVE AND REMEDI.VL ME.iSURES. AVhen the 



disease is traceable to wounds, or to the action of insects or 

 fungi, preventive and remedial measures must deal with these 

 agencies, and need not be discussed at length here. Preven- 

 tive measures in the case of autogenous outbreaks of tba 

 disease should aim at attention to drainage, irrigation, and 

 manuring. The use of resistant stocks and of high budding 

 also possesses a preventive effect. The maximum resistance 

 is exhibited by Citrus tri/oliata and C. amara, the bitter 

 orange. The rough lemon, shaddock, orange and citron 

 exhibit intermediate resistance in the order given; while the 

 lemon is highly susceptible. Applications of salt at the rate 

 of from 2 lb to 3 tt) per tree are suggested as being likely t» 

 prove of value in preventing gumming. Certain other minor 

 preventive measures are also mentioned. 



The most important remedial measure is attention 

 to drainage. Slitting of the bark, crosswise, longitudi- 

 nally, or spirally, .gives some alleviation when trees are 

 subject to gummosis irregularly or accidentally. The ex- 

 cision of all tissues affected, both bark and wood infiltrated 

 the gum, cannot be recommended; excision should be cots- 

 fined to the dead bark only. 



