DISEASES NOT DUE TO PARASITES, OR OF UNKNOWN ORIGIN I03I 



after the eruption of the Taal volcano in January, 1911, many growers 

 believed that the eruption, which defoliated the trees, is responsible for the 

 outbreak;- 



The mandarin [Citrus nobilis Lour.) appears extremely susceptible to 

 the disease. The " calamondin " (C mitis Blanco) is also attacked, but 

 with much less severity. The sweet orange (C. Aurantmm L.) and the po- 

 melo (C. decumana L.) are much more resistant than the mandarin to 

 barkrot, being seldom attacked to a seri6us degree. 



Barkrot has much in common with gummosis, with the difference that 

 the disease is an exudation of sap instead of gum, and that young trees 

 and seedlings are immune. The characteristic of barkrot in which it is 

 similar to gummosis is that in some cases badly infected trees mature their 

 fruit prematurely, the fruit showing a bright yellow colour while still very- 

 small. This is particularly noticeable in the mandarin, which usually does 

 not colour well in the Philippines. 



The disease makes its presence known by the oozing out of sap from 

 the bark, which softens and forms a putrid sore, varying in size from one- 

 half to 3 ^/g centimetres on the trunk and branches, generally at the height 

 of ^/.2 to I 1/2 metres from the ground, though the disease occurs as high 

 as 3 to 4 metres on the larger branches. Where the outbreak is se- 

 vere these spots occur tliickly and finally unite, with the result that the 

 tree or branch is quickly girdled. Numbers of insects are attracted to 

 the sores by the putrid sap. and the spots are usually' found to be infected 

 with the larvae of various insects which keep up the irritation. As the tree 

 strives to overcome the disease, the spots dry up. the edges of the sores be- 

 gin to heal over, and the bark curls or scales. On brushing off the dead 

 bark the new wood is seen forming on the edges of the sores, giving to badly 

 affected branches a queer distorted appearance. 



The term " barkrot " is to some extent misleading, as the disease does 

 not originate in the bark. If the bark and cambium layer are cut away 

 and the underlying wood carefully examined, a dark-coloured area, usually 

 of a brown to a reddish tinge, is found. This extends into the wood to 

 different depths. As in gummosis, the sap collects in small pockets be- 

 tween the wood and cambium laj^er until the bark is separated from the wood 

 and finally split by the pressure within, so that the sap oozes out. 



The Philippine Bureau of Science has carried on investigations on bark- 

 rot, but failed to find an organism to which the disease might be attribut- 

 ed. Apparently it is a physiological disease produced by unfavourable 

 soil and cultural conditions. The irregularity of the water supply is also a 

 factor. 



Proper ploughing and cultivation varying in depth each year, and the 

 planting of leguminous cover crops during the rainy season, are apparently 

 essential in the prevention and cure of barkrot. The cover crop should be 

 cut and left as a mulch during the dry season. The treatment of the tree 

 itself is a matter of secondary inqjortancc. 



The earth round the tree should be thoroughl}' loosened and stirred. 



The diseased spots should be cut out down to the healthy wood, at once 



