L5 



Propagation by grafting is also found to render the tree suscep- 

 tible al the junction of the scion and stock, and, as will be shown 

 m ire fully later, certain varieties and certain stocks are found to be 

 highh resistanl to the disease, [t is generally stated that the use of 

 lemon stock renders the tree specially liable to this disease 



There is a totally distinct disease, known as " ; root rot," which i- 

 often confounde 1 with " collar rot." commencing at or near the extre- 

 mities ot the roots and proceeding upwards until the tree is killed. 

 The malady IS due to quite a different cause, and will be discussed 

 afterwards. 



How the Disease may be spread.— It* the gum exuding from an 

 affected tree is examined it is found to contain innumerable reproductive 

 bodies of the fungus, or conidia, as they are technically termed; and 

 not only so. but these bodies are actively germinating, and so capable 

 of growing on any suitable medium and reproducing a similar fungus 

 to that from which they originated. Suppose a little portion of this 

 gum. containing these conidia at the stage when they are ready to 

 germinate, is conveyed by any means to a citrus tree under favourable 

 conditions for the growth of the fungus, then we may expect that the 

 disease may appear. It is thus highly probable that the tools used for 

 purposes of cultivation may carry the germs of the disease from 

 infected to healthy trees, or from diseased to healthy groves. Besides, 

 the gum may act as a preservative for the conidia, and thus prolong 

 their vital activity. During irrigation, gum with conidia might 

 easily be carried from diseased to healthy trees, more especially when 

 the water is allowed to flood up to the trees. It is not maintained 

 that this is the only way in which the disease is spread, but it is 

 sufficiently evident to be worthy of being guarded against. 



Treatment. — This will be based upon a knowledge of the im- 

 mediate cause of the disease— in this case a fungus — and of those 

 favouring conditions which have been found by experience to pre- 

 dispose or render the trees susceptible. 



1. There is a general remedy which has been found applicable to 

 citrus trees affected with diseases due to fungi, and that is to cut away 

 the diseased portion, taking care to remove every trace of it, and. to 

 make sure, a little of the apparently healthy tissue surrounding it as- 

 well. This has been found successful in bark diseases generally, and 

 of course every such diseased portion should be burnt. The wound 

 will generally heal, and the further progress of the disease be stayed. 

 2. In order to prevent infection from stray conidia. or from fila- 

 ments of the fungus which may still be in the surrounding tissue 

 without external signs, some antiseptic solution should be used to dress 

 the wound. 



( a.) It has been found that painting with carbolic and olive oil. 

 mixed in equal proportions, will prevent any furthei 

 extension of the disease. 



As the result of the Florida experiments, the following solutions 

 are also recommended :— 



(b.) Sulphurous acid in the proportion of 15 to 85 parts of 

 water. This may be used to wash or paint the wound, or 



it may lie sprayed on the exposed roots. 



