6 BULLETIN 1038, U. S. DEPAETMEN'T OF AGEICULTURE. 



(a) The dwarfing effects of unfavorable soil or climatic conditions. 

 (&) Starvation plienomena due to insufficiency or absence of essential 

 nutrients. 



(c) Chloroses caused by the absorption of toxic amounts of mineral or 

 organic soil constituents. 



(d) The general chloroses due to insufficient or to overabundant water supply, 



(e) Restrictions in chlorophyll development due to reduction of light. 



(f) Finally there are chloroses due to lowering of temperature. 



All these reactions, however, are rather general effects which are 

 more or less comparable to starvation, overfeeding, or direct poison- 

 ing. There are no profound or strictly localized derangements in 

 both metabolic and anatomical development, and it is a question 

 whether in the restricted sense some of these phenomena should be 

 regarded as diseases at all. 



INFECTIOUS CHLOROSES. 



As opposed to ithe general chloroses caused directly by soil or 

 climatic conditions are those specific chlorotic diseases of infectious 

 nature and obscure origin in which are simultaneously brought about 

 fundamental derangements in both physiological and structural de- 

 velopment. Concomitant with the rise of plant pathology as a 

 science there have come to light an increasing number of diseases of 

 tliis type until now it seems apparent that almost every plant group 

 may have one or more infectious chloroses. 



Reports of early scientific investigations upon two of the principal 

 types of infectious chlorosis appeared at nearly the same time — those 

 upon tobacco mosaic by Mayer (52) and Beijerinck (17) and upon 

 peach yellows by Penhallow (58) and Erwin F. Smith (69, 70). 



In peach yellows the sign often first to appear is a red blotching 

 of the fruit on one or more branches, with the color extending 

 through the flesh to the pit. A yellowing of the foliage always oc- 

 curs at some stage of the disease. Another characteristic feature con- 

 sists in the premature development of the buds of several series into 

 spindling depauperate shoots with dwarfed and linear and often 

 curled or inroUed leaves. A premature ri]3ening of the fruit also 

 usually takes places. The disease ordinarily affects one or more 

 branches at first, but may develop signs at once over the whole tree. 

 Penhallow (58) found an abnormally loose cellular structure in the 

 bark, but a reduction in the size of the cells and an abnormally dense 

 structure of the wood. Assimilation is profoundly affected and trans- 

 location of starch is delayed. The leaves become gorged with starch, 

 and excessive storage occurs in the cortex rather than in the inner 

 bark and the wood, as in normal trees. The oxidizing enzyms are 

 increased in the diseased leaves, and a larger tannin content-has been 

 found in the diseased fruit. In one instance (18) delayed starch 

 translocation was also found in an apparently health}^ branch con- 



