DISEASES OF PLANTS. In addition to being of great scientific interest. of arc of immense economic importanee. Losses running into millions of dollar, have been caused in .1111Crent ct by certain plant di,ea-es. In ra I ia the wheat-rust often greatly depreciate, the wheat crop; phylloxera has ravaged the vineyards of France: potato-rot held partly responsible for some of the famines in Ireland: mildew and black rot :t no seriou, enemies to grape culture in .kmerica. and in part, of California grape growing has been aban doned of While the oc eurrence, though not the valise,. of many plant 11 i has long been known, their definite study (phytoprithology) ha, liven devel oped since about Since that (late our knowledge of the tames of diseases and of the means of preventing them has been with remarkable rapidity. and no country ha, added more to this information than the United States, through the plant of the Depart ment of Agriculture and of the several agri• cultural experiment stations.
For ordinary purposes, plant may he according to their causes. into four categories—namely: I 1 ) Those caused by fungi; (2) those cawed by bacteria (3) those due to nematode, and insects: and (4) those due to physiological Taking these in the order of enumeration, the diseases due to fungi are given first rank. and they are perhaps the most spread and destructive of all. There is hardly a crop or a species of plant that is not liable to attacks of fungi through either its roots. stem, loaves, or fruit. The spores of fungi are exceedingly mi nute, and through them disease Is Spread. They are carried everywhere by the wind. by and other agencies. Finding lodgment upon a plant. if the conditions are favorable. it is at tacked. Plants not in perfect physical and physiological condition offer 1111. entrance to the germ lobe sprouted from the spore, and the thread-like mycelium spreads the disease through the host plant, as it is called, the fungus being referred to as a para,ite. All plant, are not ,ilbject to the attack of the same fungi, since certain conditions render a given plant immune to and susceptible to others: nor are all fungi parasitic. since s111111• live use
fully as saprophytes 1111.111 (lead :11111 vegetation. Some fungi gain entrance to plants by sending their germ tulles through the minute breathing pores of the leaves; other, dissolve their way through the cuticle by means (if ferments which they secrete. Still other, known a, wound fungi cam gain entrance only through some wound 4011 the 1141,t plant. entranee having to be prepared for them. Once the host is attacked and the conditions of temperature and moisture favorable. the disease progrcs,es with great rapid ity• from individual to individual. and veritable epidemic. The host plant is injured in n min her of way,. snob as being robbed of its nouri,liment. having its w•lier-supply -lint off. etc. SOM.• parasites stimulate the host to un usual growth, as mu ay be seen in the g ills.
knot-, and smelling,. some of which are of gro tesque shape_: still others produce stunted growth in destroy the host plant entirely. Alany fungi whi•ii cause plant di-ease, are interest ing on :14.1.1111t of the fact that they exist in two form-. each of which live, upon ft different plant. The phases which belong to one cannot be made to grow upon the other, but the phase, of each host inocu late the other. This i, trite of the fungus causing apple-inst. the alternate form of which is found in the so-ealleil apple, of the nil• cedar or juniper. Different stage-. of nue of the wheat• rusts 1 Puccini:1 giaminis I are found upon wheat and upon the barberry-lcav( s. A lisea,c of pine occur- in another f, rill upon currant•leaves, and 111:111y Other are well known. Familiar examples of disease. causal by fungi are the rust, :Ind smut, of grain, potato-rot, grape-rot, clubroot. t ron-gall. plum-knot. n thra•nose of n ;An• lea f•spots, wilt mildew.. all more fully described under the mann , of their respective hosts.