No. 6. DEPARTMENT OF AGRICULTURE. 501 



begins to braucti out and continues growing upon the nurface of the 

 leaf or fruit, leaving its mycelium exposed. These are known as ex- 

 ternal fungi, and are by far the easiest class of fungous diseases to 

 combat. (See Fig. 1.) For it is obvious that the fungicide can be 

 applied directly to the mycelium. In this way the fungus can be de- 

 stroyed and its further development prevented. To this class of 

 fungi belong the powdery mildews of the grape, the gooseberry and 

 the rose. 



In the other class of fungi, the internal fungi, the germinating tube 

 penetrates the skin and there branches. Thus the mycelium is de- 

 veloped within the tissues of the parts of the plant attacked. (See 

 Fig. 2.) It is obvious, then, ttat the mycelium is wholly out of reach 

 of any spraying compound, and, therefore, once the fungus has 

 gained entrance it is practically impossible to arrest its development. 

 The remedy can be only preventive in its action. The fungus must 

 be killed before the germinating tube enters, otherwise all effort is 

 lost. This is so fundamentally important that it will bear the strong- 

 est emphasis. In addition to this it must be pointed out that the most 

 comujouly used fungicide, Bordeaux mixture, does not destroy the 

 spores themselves. It is the little germinating tube only which is 

 destroyed by the fungicide, and, therefore, before the remedy can be 

 effective, the spore must germinate. Hence, the necessity of having 

 the remedy applied in time becomes doubly important, for the germi- 

 nating tube must be destroyed before it penetrates. The de- 

 development of the disease will continue, despite the pres- 

 ence of the remedy applied too late, and complete its life 

 history by producing new crops of spores ready to spread the dis- 

 ease anew^ The downy mildew of the grape, the scab, the fruit rots, 

 the cankers — in fact, most of the fungous diseases which afflict trees 

 and plants in this region of the United States come under the head 

 of internal fungi, and must be dealt with accordingly. Remedies 

 should be applied before the fungi of this class have gained a firm 

 foothold. Spraying should be begun upon the first appearance. It 

 is not necessary to vrait for it to make a "showing." Otherwise, the 

 spores may become so extremely abundant that, relatively, a large 

 proportj *n of them may escape the fungicide and thus complete their 

 mis^ior of destruction. 



Classes of Fungi Summarized. 



To summarize briefly, therefore, regarding fungi, it is seen that 

 there are: 



External fungi. — powdery mildews of grape, rose and gooseberry 

 — which develop on the surface or outside and which are thusi 

 comparatively easily killed at almost any stage of their 

 growth by fungicidal mixtures coming in contact with their 

 exposed mycelium. (Fig. 1.) 



