28 



Letters on the Diseases of Plants. 



6. See to it that the drainage is good. Bad drainage is a prolific source 

 of disease among orange trees. 



7. Where the use of too mucli organic nitrogenous 

 manure has already caused a rank soft growth of 

 foliage particularly subject to disease, this objection- 

 able material may be absorbed from the soil, according 

 to Webber and Swingle, by allowing the weeds to grow. 



8. If possible, induce your neighbours to adopt 

 these precautions. Tour fortune is more or less 

 bound up in theirs, so far as contagious diseases are 

 concerned. 







2§ 





2. Mai di Goma. 



This is a disease of the bark of citrus-trees which 

 rarely extends more than 18 inches above the ground, 

 and hence is sometimes also called collar rot. It 



III 





2 o § 



&3 



Fig. 31.— Spores of the fungus found attacking the bark of young 

 orange-trees, and causing the appearance shown in Fig. 30. 

 These ellipsoidal, colourless, smooth-walled spores arise 

 from minute dark pustules occurring singly or in groups 

 hereand thereon the bark. The spores measure 3 -3"5x 3- 6yu.. 



attacks trees of all ages, and is more particularly found 

 where the drainage is bad. The bark first of all turns 

 yellowish, and has a gummy consistency and a sour 

 or fermenting odour. The disease continues to spread 

 until, in some cases, the tree is girdled and dies. 



^'r; 



li 





Fig. 32. — Cross-section, nativral 

 size, of the young orange-tree 

 stalk shown in Fig. 30. This 

 section was taken near c, and 

 is in a fairly healthy condition. 

 The bark is shown dark, and 

 the wood radiated. Compare 

 with Fig. 33. 



Fig. 33.— Cross-section, natural size, 

 of the young orange-tree stalk 

 shown in Fig. 30. This section was 

 taken near b Fig. 30, and shows the 

 effect of the disease. One of the 

 cracks in the bark is shown at a. 

 At b the effect of the disease is 

 shown on the newly formed wood, 

 which has become thickened. 

 Compare with Fig. 32. 



doubt 



Occasionally half girdled trees recover themselves, 

 and go on growing and bearing, though the trunk 

 does not heal over. 



Observations have been made tending to show 

 that this disease is caused by a fungus, but some 

 to exist as to the real cause. The illustrations herewith. 



