6 BULLETIN 1038, U. S. DEPARTMENT OF AGRICULTURE. 
(a) The dwarfing effects of unfavorable soil or climatic conditions. 
(&) Starvation phenomena 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. 
(/) 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 the 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 
this type until now it seems apparent that almost every plant group 
may have one or more infectious chloroses. 
Keports 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 (IT) 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 inrolled leaves. A premature ripening 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 healthy branch con- 
