DEPARTMENT OF AGRICULTURE. 



insects carrying the sticky ooze on their bodies, or by incompletely disinfected tools. 

 Bacteria have not the same power of directly entering the tissues of plants which 

 many fungi possess, the cuticle or protecting membrane which envelops the greater 

 portion of a plant being suflicient to keep them out. They can only obtain entrance 

 where this has been broken by wounds, insect-punctures, etc., or where it is naturally 

 absent, as at water-pores, breathing-pores (stoinatai. or % delicate structures like 

 nectaries or growing points. Once inside the plant, however, they usually show their 

 effects rapidly, either killing the host-cells outright as in fire-blight, or plugging up 

 the sap-tubes with their masses, thus causing various " wilts." In some cases, how- 

 ever, as in crown-gall, the cells are not killed, brt stimulated to increased division. 

 forming : abnormal tissue. Owing to the way in which infection takes place, 



spraying has not the same direct effect in con trolling bacterial diseases that it has 

 in many fungus mies. on the other hand, spraying and other measures for the reduc- 

 tion of ii are of th importance indirectly, owing to the part played 

 by insects in spreading such : The complete and prompt destruction of 

 affected plants or parts ..f plants which would other as centres of infection 

 - > of the utmost imjMjrtance. 



IMMUNITY AND RESISTANCE. 



It is well knwn that some varieties of a plant are much more subject to certain 

 diseases than are others ; in other words, the former are more susceptible and the 

 latter more resistant. Among apples, for example, the Mclntosh Red is much more 



-:ant to fire-blight than is the Spitzenberg. This relative resistance, however, 

 may be completely reversed if taken with reference to some other dis< 

 Mclntosh Red is one of the most susceptible of all varieties with respect to apple- 

 :nilarly. different individuals of the same variety under identical conditions 

 may show differei: - - - :ice to a dis -a-.-, while the condition as regards 



growth, nutrition, etc.. may make a vast amount of difference. Trees making an 

 y growth are much more susceptible to fire-blight: roses subjected to 

 sudden chills are more liable to be attacked by mil. B, therefore, the 



resultant of a complex set of factors, including the nature and constitution of the 

 host-plant, the nature and vigour of the parasite, and the various factors which may 

 modify one or the other. A parasite may be grown under artificial conditions in such 

 a way that it almost or quite loses the power of attacking a living plant. On the 

 other hand, it may be cultivated in such a way that it becom-- -ingly virulent. 



By immunity we mean a resistance so complete that the diseases cannot develop 

 at all in the " immune " plant. Complete immunity of one cultivated variety of a 

 plant to a disease affecting other varieties of the same kind of plant is rare; e.g., 

 while there is much difference amongst va: . pples in the resistance they show 



to fire-blight, there is no variety known which -ely immune. On the other 



hand, the less closely related plants are. the less likely are they to suffer from the 

 saiii- Fire-blight, for i; - only known to attack plants be!-- _ 



the family Rosace:?. Plants outside this family are immune. The fungi causing 

 apple and pear scab resemble each ot! - - be practically indistinguish- 



able, but the apple-scab fungus will not produ- ir. and r 



The Ithizoctonia fungus, however, which sometimes causes serious injury to the 

 potato-crop, is apparently able to attack a wide variety of plants, ranging from 

 potatoes to garden ir is probable that in future the greatest advances to be 



made in the control of plan: \ ill be in the development of resistant vari- 



by means of selection and hybridization. At the same time, it is to be borne in mind 

 that the type of plant desired by the consumer, and consequently by the cultivator, 

 is usually one far removed from the " normal " as seen in the natural plant, and in 

 the nature of things more susceptible to : The so-called " improved '' plants 



are usually only such from the consumer's point of view, and not from the standpoint 

 of the vitality of the plant. The juicy, high-flavoured fruit or vegetable usually 



